moisture measurement guide for building envelope applications

Moisture Measurement Guide for Building Envelope

Applications Said, M.N. Research Report #190

Date of Issue: August 20, 2004

http://irc.nrc-cnrc.gc.ca/ircpubs

http://irc.nrc-cnrc.gc.ca/ircpubs

Moisture Measurement Guide for Building Envelope Applications

M. Nady Saïd, Ph.D., P.Eng Building Envelope and Structure Program

Institute For Research in Construction National Research Council Canada

Abstract Moisture measurement is an important consideration in building envelope investigations because of moisture impact on the performance and durability of buildings. This document reviews literature and describes moisture measurement methods for field monitoring applications of building envelopes with emphasis on continuous monitoring applications. Example measurements and guidance on applications of moisture measurement methods are also presented. Reviewed measurement methods are grouped according to measurement principles (resistance-, voltage-, capacitance-, microwave-, or thermal-based methods). Moisture measurement methods have various capabilities. Some moisture measurement methods are used to warn of excessive moisture conditions in the building envelope particularly in hidden or difficult to access areas. Other methods can quantify moisture content for some materials such as timber, while providing comparative moisture measurements for other building materials. Calibration data and temperature correction factors are readily available for various timber species. For other building materials, calibration data are quite limited, and in this case, sensors could only indicate changes in material wetness. Resistance and voltage-based sensors are most suitable for continuous monitoring applications. They can be readily connected to a data logging system. Voltage-based moisture sensors are usually used to measure time-of-wetness of surfaces. Their main weakness is durability, which can be quite short in outdoor applications. Resistance-based sensors are used to monitor changes in wetness level within materials as well as time-of-wetness of surfaces. They are durable and can be fabricated in-house. Their challenge is for an instrumentation system that can measure a wide range of electrical resistance from few ohms to several hundred MΩ. Alternatively, electric resistances can be measured indirectly in terms of voltage using a half-bridge electric circuit. Keywords: moisture sensors, field monitoring, literature review

Moisture Measurement Guide for Building Envelope Applications

M. Nady Saïd, Ph.D, P.Eng.

Abstract ........................................................................................................................ 1 1. Introduction .............................................................................................................. 3 2. Moisture Measurement Methods.............................................................................. 3

2.1 Resistance-Based Methods.................................................................................................................. 3 2.1.1 Moisture-Pins Sensor .................................................................................................................... 4 2.1.2 Brick-Ceramic & Stone Moisture Sensors ............................................................................. 7 2.1.3 Duff Probe (Sensor)....................................................................................................................... 9 2.1.4 Moisture-Measuring-Dowel and Disc ................................................................................... 10 2.1.5 The Wood-Disc Moisture Probe.............................................................................................. 11 2.1.6 Gypsum-Block Moisture Sensors ........................................................................................... 11

2.2 Voltage-Based Methods..................................................................................................................... 12 2.2.1 Sereda Moisture Sensor (Electrochemical cell).................................................................. 12 2.2.2 Printed Circuit Condensation Sensor ..................................................................................... 13 2.2.3 WETCORR Moisture-Temperature Sensing System ....................................................... 15

2.3 Thermal-Based Methods.................................................................................................................... 15 2.3.1 Thermal Heat-Sink Method ...................................................................................................... 15 2.3.2 Thermal Conductivity Method................................................................................................. 15

2.4 Moisture Detection/Alarm Methods............................................................................................... 16 2.4.1 Nicked-Wire Sensor .................................................................................................................... 16 2.4.2 Moisture Detection Tape/Cable............................................................................................... 16

2.5 Moisture Diagnostic Methods .......................................................................................................... 16 2.5.1 Moisture Meters............................................................................................................................ 16 2.5.2 Electromagnetic Wave Methods.............................................................................................. 19 2.5.3 Infrared Thermography Method .............................................................................................. 20

2.6 Moisture Measurement in Concrete Structures .......................................................................... 21 3. Example Moisture Measurements.......................................................................... 23

3.1. Example Measurements – Resistance-Based Sensors............................................................. 23 3.2. Example Measurements – Voltage-Based Sensors .................................................................. 27

4. Conclusion.............................................................................................................. 29 5. References .............................................................................................................. 30

Saïd Moisture Measurement Guide for Building Envelope Applications 2

1. Introduction Moisture measurement is frequently used in investigating moisture-related issues in building envelopes. It could also be used for quality control to ensure that the building envelope’s moisture control measures are functioning effectively. Handheld moisture meters are usually used for spot measurements to assess changes in moisture content of building envelope components or determine surface wetting patterns in order to determine sources and extent of wetness. Moisture sensors and a data-logger system are usually used in continuous monitoring applications of building envelope assemblies. Moisture sensors and an alarm system could be used to warn of excessive moisture conditions in the building envelope particularly in hidden or difficult to access areas in order to ensure long-term durability of the building envelope.

This document reviews literature and describes moisture measurement methods for field monitoring applications of building envelopes. The focus of the document is on continuous monitoring applications. Handheld moisture meters are briefly discussed because they are usually used to identify measurement locations and also verify the operation of moisture sensors. The document also provides example measurements and guidance on applications of moisture measurement methods, which demonstrate measured values and data interpretation options of moisture measurements.

2. Moisture Measurement Methods Moisture measurement methods are grouped according to the measurement principle (resistance-, voltage-, capacitance-, microwave-, or thermal-based methods) or function. Resistance-, voltage-, and capacitance-based moisture measurement methods utilize the electrical properties of materials that vary with the materials moisture content (i.e. electrical resistance, dielectric constant, and radio frequency (RF) power-loss). Microwave-based methods work on a similar principle to the capacitance methods, but at a much higher frequency. Thermal-based moisture measurement methods utilize the change in the temperature of materials caused by the change in moisture conditions. Capacitance-based methods are commonly utilized in pin-less-type handheld moisture meters and the microwave-based methods are usually utilized in specialized equipment for laboratory and field investigations. The resistance-based methods are utilized in probe-type handheld moisture meters as well as in a variety of moisture sensors that are used in continuous monitoring applications of building envelopes. Resistance-based moisture sensors can quantify moisture content for some materials such as timber and provide comparative moisture measurements for other materials. Other moisture measurement methods make use of an electrochemical cell, which is activated when the cell gets wet and generates a voltage potential across a known resistor and a capacitor. The output voltage varies with changes in the degree of the material wetness. Voltage-based sensors are usually used to indicate time-of-wetness of surfaces. This section reviews literature and describes moisture measurement methods for field monitoring applications of building envelopes. Methods described are equally applicable to laboratory applications. 2.1 Resistance-Based Methods Resistance-based methods measure moisture levels in the materials in terms of their electric resistance or dielectric property, which varies as a function of moisture content. As the material moisture content increases, its electrical resistance decreases and conductance increases. The magnitude of the resistance can vary between several hundred kΩ when wet to over several hundred MΩ when dry. Examples of resistance-type moisture sensors include moisture-pins, brick-ceramic, stone, Duff moisture sensor, moisture measuring dowel & disc, nicked-wire, and moisture detection tape/cable. The moisture-pins and brick-ceramic sensors have been used


extensively at the Institute for Research in Construction, National Research Council. Thus, these two sensors are discussed in detail including application considerations and example measurements. 2.1.1 Moisture-Pins Sensor Figure 1 shows various types of moisture- pins: insulated pins (A), non-insulated pins (B), and a pair of stainless-steel screws (C). The insulated-pins (except the tip is not insulated) type is the most common and is available commercially1 in various lengths (3/4 to 9-1/8 inches or 19 to 232-mm). A pair of stainless-steel screws with required length can be used as an alternative to commercial moisture-pins. The pair of stainless-steel screws shown in Figure 1 includes stainless-steel wire brazed to each screw for connection to a data logger. In addition, heat-shrinkable tubing is used to insulate the length of the screw that will be extending outside the material being measured. Stainless-steel screws provide good grip in mortar applications.

A B C

Figure 1. Various moisture-pins sensors, insulated pins (A), non-insulated pins (B), and stainless-steel screws (C). Shown at the top is a pair of insulated moisture pins inserted in a wood block

Moisture-pins sensors measure moisture indirectly by measuring the electrical resistance of the material between the two probes (or between the tips of the probes for the insulated pins type), which makes measured resistance dependent on the dielectric property of the material being measured in addition to possible temperature effect. Moisture-pins can quantify moisture content for a variety of timber species with an accuracy of ±2% (Garrahan 1988). For other materials, moisture-pins provide comparative moisture profiles indicating changes in moisture content of the material and time of wetness and drying. Moisture-pins sensors respond quickly to changes in wetness conditions of the material. They are also durable and require minimum maintenance. For timber applications, temperature adjustments

1 Delmhorst Instrument Co. http://www.delmhorst.com/


http://www.delmhorst.com/

are necessary because the temperature change affects the dielectric properties of the timber material. James (1975 & 1994), Garrahan (1988), and Pfaff & Garrahan (1985) reported correction factors for timber species. Their temperature and species correction factors were developed for resistance- or probe-type moisture meters but they could be applied to moisture-pins application in timber. For insulated moisture-pins, care should be taken not to damage the insulation when inserting the pins into the material. The following is a recommended procedure for installing insulated moisture-pins sensors:

1. Two 3.175- mm diameters holes, 25.4-mm apart, are drilled in the material (timber, mortar joint, concrete, etc.) to the required depth. The 25.4-mm spacing corresponds to the spacing between the probes of some common moisture meters. Hint: A steel block (about 25 x 50-mm and 25-mm thick) is used as a drilling guide to maintain the 25.4-mm spacing between the two holes and ensure that the holes are normal to the surface.

2. The moisture pins are gently tapped into the holes. 3. The pre-soldered wires on the pins are protected with heat-shrinkable tubing (UV resistant

Teflon is recommended for outdoor applications) or covered with high strength epoxy. 4. The wire-cables are secured to the surface at a distance of about 40-mm away from the

moisture-pins using a screw and a cable-tie. A drainage loop (U-bend) is made in the cable to prevent water from running down into the moisture-pins (see Figure 2 for an example).

5. The location of the moisture-pins should be selected such that it is not in the line of water drips from other sensors.

Figure 2. Shows an example installation of a pair of stainless-steel screws (MP52) in a

mortar joint. Notice the drainage loop in the cables attached to the screws.


Electric resistance output of moisture-pins can be measured directly or indirectly in terms of voltage. Figure 3 shows the electric circuit diagram for converting resistance to voltage output measurements. In order to minimize errors due to the effect of polarization and electrolysis at the electrodes, the excitation voltage applied to the sensor is preferred to be an alternating voltage (2.5-volts is usually used). Forrer & Vermaas (1987) used a DC voltage excitation with low frequency alternating polarity and presented good analysis of electrical resistance measurement in terms of voltage.

Rg sensor Rm (moisturesensor)

Excitation volt( 2.5 V AC)

Rf Vo output

Where: Vo is measured output voltage

Rf is a fixed resistor for output signal (usually low value, about 10 kΩ) Rg is a fixed resistor in parallel with the moisture sensor (usually high value 1 to 10 MΩ)

Rm is the resistance output of the moisture sensor, which is calculated by:

Rm = Rf / [(Vo/(1-Vo) – Rf / Rg]

Figure 3. Electric circuit diagram to convert resistance measurement to voltage output measurement

As a variation of moisture-pins sensor, Cunningham (1985) proposed a technique for measuring timber moisture content (mc) over the range of 10 % to 50%. His method consisted of 10-mm square gold plated parallel plates, which is embedded in the timber. Cunningham’s motive was that timber moisture content above 25%-30% could not be measured reliably using the DC electric resistance property of timber because the DC electric resistance is almost constant in this range. In addition, possible polarization might affect the reading. His approach was, below 30% mc, the moisture content is determined by measuring the DC electric resistance across the plates. Above 25% mc, The AC impedance would be used to determine the moisture content. In the overlap range 25%-30% mc, the weighted mean of the DC and AC measurements is used to determine the moisture content. Cunningham (1985) reported the electric circuit, calibration data, and the computer algorithm for the data logger.


2.1.2 Brick-Ceramic & Stone Moisture Sensors Brick-ceramic and stone moisture sensors, Figure 4, measure moisture indirectly by measuring the electric resistance across a small block of ceramic or stone material. For brevity, the brick-ceramic will be referred to as “ceramic”. When the ceramic or stone material becomes wet, its electrical resistance decreases and vice-versa. The use of ceramic and stone moisture sensors eliminates the variation of the dielectric property of the material being monitored because measurements are based on the dielectric properties of the ceramic or the stone material of the sensor’s block. This is in contrast to the case when moisture-pins sensors are imbedded in the material. The sensors are fabricated by attaching two wires to the opposite sides of a ceramic or stone block (see Figure 4 for dimensions) using conductive silver epoxy2. The ceramic block may be cut from a clay brick using a low speed wet saw (Figure 5). The stone block is usually cut from a stone material similar to that of the wall being monitored. The ceramic moisture sensor is usually used in surface mount applications to monitor time of wetness of surfaces. To determine the wetting and drying pattern of a surface, ceramic sensors are placed at various locations on the surface. The ceramic sensor could also be imbedded in the material in order to determine changes in moisture content of the material at a required depth. The stone moisture sensor is usually used in stone masonry-wall applications to monitor changes in moisture condition at a specified depth in the wall. The small size of the stone moisture sensor allows it to be embedded through the typical 10-mm mortar joint. The ceramic and stone moisture sensors respond quickly to changes in wetness conditions of the surface or the material. They are also quite durable. They require limited maintenance, e.g. a check on the adhesion of the wires to the sensor’s block. For surface mount, the sensor is glued to the surface using a fast curing 5-minute epoxy. Similar to the moisture–pins installation, the sensor cables are protected with heat-shrinkable tubing (UV resistant Teflon is recommended for outdoor applications). The sensor’s wire-cables are secured to the surface at about 40-mm away from the sensor using a screw and a cable-tie. For outdoor installations, a drainage loop (U-bend) should be made in the cable to prevent water from running down into the sensor. The location of the sensor should be selected such that it is not in the line of water drips from other sensors. To insert the stone moisture sensor in a masonry wall, for the dimensions given in Figure 4, a 6.35-mm diameter hole is drilled in the wall (usually a mortar joint). The debris is removed from the hole using a vacuum cleaner. Water is sprayed into the hole to wet the stone/mortar. Mortar, similar to that used in the wall, is packed into the hole around the sensor cables using steel rods of various diameters. It is important to ensure that there are no air cavities around the sensor. 2 Circuit Works, Chemtronics #2400


(a)

(b)

(a)

(b)

Brick-ceramic moisture sensor Stone moisture sensor

Figure 4. Typical size and configuration of the brick-ceramic (a) and stone (b) moisture sensors

The temperature effect on measured resistance remains to be determined in a laboratory study. At temperatures below freezing, the output from the moisture sensors is not valid because the formation of ice increases the measured resistance, which leads to the erroneous result of dry conditions. As well, salts dissolved in moisture will lower measured resistance.


Low speed wet saw

Close up of cutting a stone moisture sensor block

Figure 5. Low speed wet saw and a close up of cutting a stone moisture sensor block 2.1.3 Duff Probe (Sensor) Duff (1966) describes the probe fabrication and lists data for moisture content vs. electric resistance for the sensor. The Duff probe consists of two wires attached (with conductive silver epoxy) to opposite sides of a 2-mm square and 20-mm long wood block. The wood block should have a straight and close-grained structure in order to minimize inter-probe variation. Good measurements were obtained using sugar maple, yellow birch, and American beech to fabricate Duff sensors. Duff noted that the sensor could be fabricated in various sizes. Similar to the brick-ceramic and stone moisture sensors, the Duff moisture sensor measures moisture content indirectly by measuring the resistance across a small wooden block that is in equilibrium with the humidity of the surrounding environment. Similar to the ceramic and stone


moisture sensors, the Duff probe eliminates the effects on measured data due to the variation of the dielectric property of the material being tested with moisture content and temperature. The Duff moisture sensor can be used in applications requiring a surface mount or embedded into the material. Installation procedures and application considerations would be similar to that of the ceramic and stone moisture sensors. Moisture measurements using the Duff probe require temperature corrections in order to account for the affect of temperature on the probe’s electric resistance. Correction factors reported by James (1975) and (1994) can be used; Garrahan (1988) and Pfaff & Garrahan (1985) also provide temperature correction factors for timber materials. Duff (1966) noted that the sensor was developed for accurate determination of moisture content of wood products for research applications. In addition, the Duff probe is capable of measuring at the lower range of moisture content. He reported that the accuracy of the Duff sensor readings is within 1%. The probe’s small size allows the probe to respond quickly to changes in moisture content. Duff also suggested that the moisture probe could also be used to determine average atmospheric moisture conditions (relative humidity). Carll & TenWolde (1996) later investigated this application. They evaluated the application of wood-resistance-type sensors (the Duff probe) to measure air relative humidity. They indicated that the sensor could measure air relative humidity but with considerably low accuracy of ±10%, which they noted was due to the sorption hysteresis and sensor memory. However, the wood sensor is inexpensive to fabricate. Carll and TenWolde (1996) measured the sensor’s electric resistance indirectly using a conditioning circuit (reported in their paper). They used a relationship similar to that of Altmann (1974) to present the relation between measured RH and the sensor moisture content and additional relationships between the sensor’s electric resistance, moisture content, and the temperature. The later includes correction for the temperature effect. Similar to the Duff probe, Dai & Ahmet (2001) described the development of a wood moisture sensor (resistance-type) with stringent accuracy requirements (1%) for long-term monitoring of timber moisture content below the fiber saturation point. The sensor consisted of two pairs of parallel electrodes that are embedded in a cylindrical wood block that is 8-mm diameter and 25-mm long. They used silver-painted brass screws as electrodes, which they argued were better than pin-type electrodes because they ensured effective and consistent long-term contact with wood. They suggested that the contact resistance of pin-type electrodes changes with time. A data-acquisition system (block diagram shown in the paper) was developed specifically to log data from the sensor in order to measure a wide range of moisture contents corresponding to an electric resistance range of 1 kΩ to 100-GΩ. The data-acquisition and sensor system was calibrated and its long-term performance over 2 years was reliable. 2.1.4 Moisture-Measuring-Dowel and Disc Brandt & Hansen (1999) developed two resistance-type moisture sensors that operate on the same principle as the Duff probe and moisture-pins. They called the moisture probes “moisture-measuring-disc” and “moisture-measuring-dowel”. The moisture-measuring disc consisted of two electrodes inserted in a 50-mm diameter and 12-mm thick plywood disc (using the same plywood for which moisture content is to be measured). The moisture-measuring dowel consisted of two electrodes glued to a 10-mm diameter beech wood dowel. The probes also included a thermocouple to measure temperature to use for correcting the measured resistance for the temperature effect. The probes were calibrated using controlled humidity conditions. Brandt & Hansen (1999) discussed the long-term stability, accuracy, advantages, and drawbacks of the two moisture probes. They also discussed the effect of the shape, dimension, electrode material, and


probe fabrication on the moisture probe performance. The application of the two moisture probes has been included in the Nordtest (1993) method. Nordtest (1993) describes the Nordtest method for measuring the long-term moisture content of wood using wooden probes with embedded electrodes. The test method is applicable to various wood species as well as plywood with known absorption curves. The document describes the principle, fabrication, calibration procedure, accuracy, and expression of measured results for wood-type moisture probes 2.1.5 The Wood-Disc Moisture Probe The wood-disc moisture probe was developed for measuring moisture content in straw-bale walls (Fugler (1996) and Gonzales (1998)). Similar to the Duff probe, the sensor is constructed of 3-mm thick and 18-mm diameter wood block of balsa or white pine timber. Balsa was chosen because it has a uniform grain, is easy to cut, and conveniently available. Two small stainless-steel screws are inserted in two holes placed 15-mm apart in the wood block for connecting to electrical wires. The wood-disk is then placed at the end of a perforated PVC tube. The tube length varies according to the depth of the measuring location in the wall. The electric wires are connected to another small stainless-steel screws on the cap at the inlet of the tube. The screws are spaced to line up with the tips of the probes of a moisture meter (calibrated for wood) that is used for taking the moisture measurements. Fugler (1996) discussed the performance of the wood-disc moisture probe. There was a good correlation between the electric conductivity of the wood-disc moisture probe, relative humidity, and the straw moisture content. The slow reaction of the probe to reach equilibrium matched the slow reaction of the straw to the uptake of moisture. The equilibration rate was slow below 55% relative humidity. The sensor’s block readily loses moisture when the straw bale was at a lower humidity, but was slow to pick up moisture when the humidity level in the bale increased but stayed below 55%. Fugler noted that there may be some effect of temperature on measurements, but the influence was small and difficult to determine for the range of temperatures, 13ºC to 28ºC, studied. Goodhew et al. (2004) further investigated the performance of the wood-disc moisture probe for measurements in cold climate winter conditions of Plymouth, UK. They also investigated the effect of the length of the connecting wires used between the wood-disc and the connection to the moisture meter. A different wood block size was used, which they noted that exact dimension was unimportant. The sensor was constructed from a 5-mm thick and 5.6-mm diameter wood block, and the two holes for the stainless-steel screws were 5-mm apart. They concluded that the wood-disc sensors were easy to use and reasonably accurate. The accuracy was ±2% when compared to oven drying method measurements. An increase in temperature of the straw surrounding the sensor lead to a small increase in measured moisture content. 2.1.6 Gypsum-Block Moisture Sensors Gypsum-block moisture sensors are resistance-type moisture sensors. Their operation is similar to the ceramic and Duff moisture sensors. Gypsum is, however, a hygroscopic material. They were developed for soil moisture measurements in order to determine the need for irrigation. The gypsum-block sensors indicate relative changes in soil moisture and do not quantify soil moisture content. The service life of gypsum-block sensors is short, 1 to 5 years and depends on how often they become saturated (Larsen (2004)). Larsen (2004) used gypsum-block sensors to diagnose moisture conditions in masonry walls. He noted that the range of electric-resistance measurements responded to the actual moisture


condition of the masonry and they were not sensitive to the effect of salts because the gypsum pores contain a saturated solution of calcium and sulfate.

2.2 Voltage-Based Methods Moisture sensors using voltage-based methods measure moisture in terms of a DC voltage across a known resistor. The output voltage varies with changes in wetness such that the voltage increases as the wetness increases and vice versa. Examples of voltage-based methods include the Sereda moisture sensor (an electrochemical cell), printed circuit condensation sensor, and the WETCORR monitoring system. 2.2.1 Sereda Moisture Sensor (Electrochemical cell) The Sereda moisture sensor is an electrochemical-cell sensor. The sensor is named after Peter Sereda who developed it at the Institute for Research in Construction, National Research Council Canada (Sereda et al. 1982). The Sereda sensor is usually used to indicate time-of-wetness and drying of surfaces. Placing sensors at various locations on the façade can determine the façade’s wetting and drying pattern. The Sereda moisture sensor reacts rapidly to wetting and drying of surfaces. The sensor, Figure 6, consists of alternate copper (35-µm thick) and gold (1-µm thick) electrodes deposited about 200-µm apart on a glass-reinforced polyester substrate to form a galvanic cell (Sereda et al. 1982). The Sereda miniature moisture sensor model SMMS-013 has overall dimensions of 11-mm by 18-mm. When the sensor is wet, the electrochemical cell is activated and generates a voltage potential across a 10-MΩ shunt resistor and 0.068-µf capacitor (see Figure 6 for the wiring diagram). When wet, the output voltage usually exceeds 80-mV and cannot be related directly to the quantity of moisture. The output voltage may also vary between sensors, which are likely due to manufacturing variations as well as the size of the wet area of the sensor surface. To select a group of Sereda sensors with similar voltage output for a monitoring application, the sensors are immersed in a cup of water and the voltage output of each sensor is measured across a 10-MΩ shunt resistor and 0.068-µf capacitor. It is also common that the Sereda moisture sensors produce 2 to 4-mV noise signals when dry.

Sereda moisture sensor Wiring diagram of Sereda sensor

Figure 6. Sereda moisture sensor and its wiring diagram

3 Manufactured by: Compass Electronics Inc, Epitek Microelectronics Division, 100 Schneider Road,

Kanata, K2K 1Y2


ASTM G84.729-89 (re-approved 1999) describes the application of the Sereda moisture sensor. Yamasaki et al. (1983), in a study of metal corrosion, described the application of the Sereda moisture sensor to determine the time-of-wetness due to condensed moisture. Yamasaki (1984) described the use of Sereda moisture sensors in an outdoor exposure study to monitor the time-of-wetness of plastic surfaces. Sereda moisture sensor is usually mounted on the surface of materials. For masonry surface applications, sensors are glued to the surface using a fast curing 5-minute epoxy. For glass surface applications, an industrial grade RTV silicone sealant is usually used. The Sereda moisture sensor also reacts to high humidity and is more sensitive than the ceramic moisture sensor. The Sereda moisture sensor, however, is not durable. Its service life is short particularly when used on the façade and is exposed to the elements. The sensor tends to corrode in about a year depending on the air quality. Indoor applications may last longer, 3 to 4 years, depending on the exposure conditions. Unsuccessful attempts were made to revive corroded sensors using various cleaning materials such as Brasso metal cleaner, very fine steel wool, and alcohol. The alcohol cleaned the surface but did not remove the oxidation. The gold plated layer is very thin and was easily removed, which affected the sensor output voltage. Another drawback is that the sensor must be ordered in large quantities. 2.2.2 Printed Circuit Condensation Sensor The printed circuit condensation sensor (PCCS) is a business-card size sensor that consists of copper films deposited in an interlaced pattern onto an epoxy-fibreglass plate (Figure 7-a). A 5 DC volt excitation is applied to the circuit and the output is 0 to 3 volts depending on the size of the sensor’s wet area. The main application of this sensor is to detect time-of-wetness of surfaces. When exposed outdoors, the copper film is not durable, which makes the service life of this sensor is quite short. Within 6-months, the sensor becomes fully corroded and not usable (see Figure 7-b. This moisture sensor is suitable for short 4 to 6 weeks duration investigations. However, the low cost of the PCCS moisture sensor is an advantage. Various types of this sensor are available; examples include the “Condensation probe4” and the “Leaf wetness sensor5”. The installation of the PCCS is similar to the Sereda moisture sensor. The surface should be smooth and clean. For masonry surface applications, sensors are glued to the surface using a fast curing 5-minute epoxy. For glass surface applications, an industrial grade RTV silicone sealant is usually used. Similar to the printed circuit condensation sensor, Gillespie & Kidd (1978) described an electric circuit and the so-called mock-leaf sensor, which consists of a grid of solder-coated copper fingers mounted on a 1-mm thick epoxy-fiberglass 25-mm x 100-mm board. The sensing grid was coated by latex paint. They investigated the effect of various colors, white, 4-gray shades, and dark green on the duration of wetness retention. They used latex paint because of its ability to transmit moisture through to the sensing copper grid. The electric circuit was powered by a low AC voltage in order to avoid electrolytic depositions on the grid. The impedance of the sensor varied from about 20 kΩ when fully wet to over 1 MΩ when dry.

4 Lakwwood Systems Ltd., 5 Leaf wetness sensor www.frostproof.com/catalog/t7846.html


http://www.frostproof.com/catalog/t7846.html

a) Printed circuit condensation moisture sensor

b) PCCS condition after 6-month on a façade

Figure 7. Printed circuit condensation moisture sensor (PCCS)


2.2.3 WETCORR Moisture-Temperature Sensing System The Norwegian Institute for Air Research developed the WETCORR moisture monitoring system to map humidity as well as temperature conditions on surfaces and within building materials (NILU 1994). The WETCORR moisture sensor is similar to the Sereda moisture sensor, but it includes a thermistor temperature sensor. The WETCORR sensor consists of gold electrodes in an interlaced pattern plated onto a ceramic substrate. The output of the WETCORR sensor when wet is an electric current. The sensor is available as a system consisting of up to 64 sensors and a system controller that includes a control unit, excitation power for sensors, and sampling data-logger. NILU (1994) describes the WETCORR moisture-temperature system and some of its applications to measure humidity and temperature on and within timber materials.

2.3 Thermal-Based Methods Thermal-based moisture measurement methods utilize the change in the temperature of materials caused by the presence of moisture. Because of the high thermal capacity of water, a wet material has a lower temperature than when it is dry. Moisture flowing through materials in the building envelope assembly cools or warms surfaces by direct contact and evaporation. 2.3.1 Thermal Heat-Sink Method Hagemaier (1970) described the development of a thermal heat-sink test method to determine moisture content in thermal insulation panels used on space vehicles. The method is based on the heat-sink principle that is a wet material acts as a heat sink, which affects the temperature of the material such that a wet material would have lower temperature than when it is dry. Thus the temperature difference, ∆T, between the material surface and the ambient air can be related to the moisture content of the material. Hagemaier presented calibration data of moisture content versus ∆T for the space vehicles insulating panels. 2.3.2 Thermal Conductivity Method Lucas (1974) described the development of moisture sensors that were intended for permanent installation in highway substructures. His work resulted in two moisture sensors, a capacitance sensor and a thermal conductivity sensor. The thermal conductivity sensor was based on the principle that soil moisture content affects its thermal conductivity, which could be measured and calibrated in terms of soil moisture content. The thermal conductivity sensor consisted of a thick-film heater and thermistor fabricated on opposite sides of an alumna substrate. When a fixed power is supplied to the heater, the sensor temperature increases to a steady-state value that depends on the thermal conductivity of the surrounding soil. The increase in the sensor temperature was calibrated in terms of soil moisture content. The capacitance sensor consisted of two sets of platinum gold electrodes deposited 0.007 inches apart on an alumina substrate and covered with a layer of clear glass. The operation of the capacitance sensor was based on the principal that moisture changes the dielectric constant of the soil, which could be measured and calibrated in terms of soil moisture content. Lucas (1974) reported that the thermal conductivity sensor was more difficult to use than the capacitance sensor. The capacitance sensor required a minimum of instrumentation and was easily adapted for remote operation.


2.4 Moisture Detection/Alarm Methods The main function of these methods is to alert for the presence of wet conditions. A change in the electrical resistance of the moisture detector due to the presence of water triggers an alarm. Hutton (1996) described a monitoring system that remotely monitors moisture in timber and other materials in building structures in order to warn of excessive moisture conditions. His monitoring approach included imbedding timber moisture sensors (similar to the Duff probe) in critical structure locations during survey or renovation. 2.4.1 Nicked-Wire Sensor This sensor consists of a side-by-side wire pair whose insulation had been removed "nicked" at regular intervals - approximately 1-mm (1/32 in.) is nicked in a 5-mm (3/16 in.) interval. The length of the wire is selected to cover the space to be monitored in a wall cavity, roof space, etc. The resistance between the wires decreases when moisture is present between the exposed sections of the wire-pair. A resistance drop therefore indicates the presence of moisture. The nicked-wire moisture sensor can be connected to an alarm to alert for the presence of moisture. 2.4.2 Moisture Detection Tape/Cable Moisture detection tapes or cables are a water sensing length of tape or cable. Their main function is to alert for the presence of water. The tape/cable is connected to an alarm, which transmits an alarm signal when triggered by a change in the electrical resistance of the tape or cable due to the presence of water. The moisture detection tape/cable could be placed around the perimeter of a roof space, basement, living space, or in suspected water entry locations in a wall. Moisture detection tapes and cables are available in a variety of sizes, materials, technologies, and commercial names. Examples:

“Floodline detection tape & cable”, Andel Ltd. http://www.andel.co.uk/pdfs/two.pdf

“Lineal leak detection tape”, Hydro-Temp Inc. http://www.hydro-tempinc.com/leakdetc.htm

“Moisture detection tape”, Norscan Instruments Ltd. http://www.norscan.com/web/cable-monitor-zoom.shtml?pfl=products-mon-single.param&op2.rf1=22

“Moisture detection Alarm tape”, KT Industries Ltd. http://www.ktg.com/industrial.html

“Water alert sensor cable”, Dorlen Products Inc. http://www.wateralert.com/sensor.htm

“Water leak detection cable”, Proline Protection Systems Ltd. http://www.lineardetection.com/moisture.htm

2.5 Moisture Diagnostic Methods Moisture diagnostic methods are in-situ measurement methods that are usually used to identify areas with moisture anomalies in the building envelope. These tests are often followed with an invasive investigation and measurement using moisture meters and/or continuous monitoring to further investigate areas with moisture anomalies. 2.5.1 Moisture Meters Handheld moisture meters are briefly discussed because, in field monitoring applications, moisture meters are usually used to help determine measurement locations and also to check the operation of moisture sensors. Handheld moisture meters, in general, are best used for conducting comparative measurements to:

• Assess changes in moisture condition in order to determine whether the material is drying or not.


http://www.andel.co.uk/pdfs/two.pdf

http://www.hydro-tempinc.com/leakdetc.htm

http://www.norscan.com/web/cable-monitor-zoom.shtml?pfl=products-mon-single.param&op2.rf1=22

http://www.ktg.com/industrial.html

http://www.wateralert.com/sensor.htm

http://www.lineardetection.com/moisture.htm

• Determine surface wetting pattern in order to trace moisture sources. To measure the moisture level in materials, moisture meters utilize the electrical properties of materials (electrical resistance, dielectric constant, and radio frequency power-loss), which vary with the material moisture content. Moisture meters using the electrical resistance are known as resistance-type or probe-type meters, those using dielectric constant are called capacitance-type, and those using the radio frequency (RF) power loss are called RF power-loss-type. The resistance- and capacitance-type are the most common moisture meters. They are briefly discussed and closely related literatures are reviewed below. The resistance-type meters include one or more pairs of electrodes (insulated moisture-pins) that are available in various lengths for a range of building envelope applications. The pins are inserted in the material to the required depth. The electric-resistance of the material between the tips of the pins varies with the moisture content of the material. These meters, historically, have been used to quantify moisture content for timber species. For other materials, resistance-type moisture meters provide comparative moisture profiles indicating changes in moisture content of the material. Some moisture meters provide multi scales. The numerical values of one scale represent the actual percent moisture content of wood (% MC) and other scales represent the wood moisture content equivalent for masonry, gypsum wallboard, and other materials. Capacitance moisture meters use radio frequency signals to penetrate the material being tested. They include surface contact electrodes, a transmitter, and receiver electrodes. The transmitter sends a radio frequency signal into the material underneath the electrodes. The attenuation change in the signal received back by the receiver electrode indicates the wetness degree of the material. The readout scales of capacitance moisture meters are usually in arbitrary units, which are used to compare or determine changes in moisture conditions. Some capacitance moisture meters are calibrated to determine moisture content in timber. However, the reading is not species-specific. Timber density and temperature affect the reading of the dielectric-based meters. A key advantage of capacitance moisture meters is that they are non-destructive. They do not damage the building element being tested, as there is no need to puncture the surface, as is the case for the resistance-type moisture meters. In addition, surface contamination or salts do not affect the reading. Moisture meter measurements depend on the skills of the meter operator. Capacitance moisture meters should be pressed firmly against the surface such that the electrodes are in contact with the surface, which should be smooth and no water presence. For the probe-type moisture meters, the reading should be taken as soon as the probes are inserted in the material in order to avoid erroneous reading due to the polarization effect. ASTM D4444-92 (re-approved 2003) describes the use and calibration of hand-held moisture meters. James (1975) and (1994) thoroughly discussed fundamentals and use of hand-held moisture meters including factors affecting the readout of timber moisture content. Discussed are the history, fundamentals, types and their advantages, accuracy, error sources, and maintenance of moisture meters. Factors affecting readings of moisture meters included timber species & grain distribution, temperature, and chemical treatments as well as the skill of the operator. The document also included tables of moisture content and electric resistance for various timber species as well as graphs for the temperature correction of measured moisture content. The listed tables and graphs are also presented in the Wood Engineering Handbook (1982). Dill (2000) provided guidance on the selection of appropriate moisture test methods for a variety of applications.


Pfaff (1974) in a study to develop correction factors for 2-pin resistance-type moisture meters for eastern Canadian lumber species, quantified the following factors affecting the use of resistance-type moisture meters:

- Using a meter scale to measure moisture in a lumber species other than for which the scale was calibrated could produce significant error in measured moisture content. For instance Pfaff noted that using correction factors developed for general spruce lumber for eastern white spruce resulted in underestimation of moisture content by 5%. There was no practical difference in correction factors between regions for white pine and white spruce. For balsam fir, correction factors differ significantly between regions.

- It is important to specify the number and type of electrode used in measurements. Using a 2-pins meter resulted in lower moisture reading than a 4-pins meter by about 1.6%.

- Temperature correction must be made whenever the wood temperature deviates from the temperature used in determining the species correction factor.

Pfaff & Grarrahan (1985) reported temperature correction relations as well as combined temperature-species correction tables for probe-type moisture meters application in 14 timber species. Their work was motivated by the inappropriateness of temperature correction factors for low temperature applications in the Wood Engineering Handbook (1982). Their investigation confirmed that moisture content of frozen lumber was over estimated by 3 to 7%. Pfaff & Grarrahan provided correction tables for 10 to 23% moisture content and -29 to 49ºC (-20 to 120ºF) ranges. Forrer & Vermaas (1987) discussed the development of an improved moisture meter for continuous mode operation for dry kiln control applications. Their work was driven by the need to improve upon some of the limitations of available moisture meters. Limitations included limited accuracy, inadequate temperature compensation, improper timber species calibration, and the inability to function in a continuous mode. The later limitation is caused by the effect of polarization and electrolysis at the electrodes. Their solution for this limitation was to use the so-called constant voltage method for measuring of electrical resistance, in which a DC voltage excitation with low frequency alternating polarity was used. Forrer & Vermaas presented a detailed analysis of the electrical resistance measurement in terms of voltage. They also developed an equation for resistance versus temperature-corrected moisture content. Grarrahan (1988) reported an empirical correlation for temperature correction for probe-type moisture measurement in black spruce, jack pine, and balsam fir timber species. He stressed that moisture meters provide an estimate of moisture content because, in his opinion, there is no meter available that can give a direct reading on the amount of moisture in lumber. This is because the electric property being measured varies with timber species and the environmental conditions. Grarrahan recommended that each moisture meter be calibrated for each application. Correction factors must be applied to readings taken at conditions for which the meter was not calibrated. He also pointed to a problem related to measuring moisture content of timber that was dried at temperatures higher than 88ºC (190ºF). The high temperature affects the relationship between the moisture content and the electric resistance of timber, which result in overestimated moisture content by as much as 4%. Oxley & Gobert (1983) described a system of mapping moisture in masonry walls using a moisture meter calibrated for wood. His approach was not to quantify moisture content of the masonry but to make a series of comparative readings to determine the extent and severity of moisture in the masonry and its possible source. Clancey et al. (1995) used the measurement


approach described by Oxley & Gobert (1983) and a probe moisture meter calibrated for wood to map rising damp levels and areas of water infiltration through brickwork of a historic building in Georgia, USA. They reported that the 0-30 reference scale on the meter was sensitive enough to allow the mapping of contours from normal (they determined to be in the 8-12 range) to very damp (30 reading). The meter was less sensitive in the upper ranges where the masonry wall was excessively damp. Williams et al. (1998) evaluated various moisture meters to measure the moisture level in walls with an exterior insulation finish system (EIFS) cladding. Their objective was to find a tool to screen EIFS-cladding for moisture. They discussed the accuracy and effectiveness of intrusive (probe-type) and non-intrusive moisture (capacitance-type) meters. They concluded that intrusive meters were effective in detecting moisture throughout the mockup wall assembly. The use of non-intrusive meters was supplemented with intrusive meters. Larsen (2004) described his experience using various types of moisture sensors to diagnose salt efflorescence on decoration and masonry of an eleventh century church. He noted that the capacitance-type moisture meter gave erroneous results because of the effect of salt in the plaster. It also did not provide a true picture of moisture level in the masonry walls deeper than 50-mm. The neutron-type probe moisture meter6 provided more reliable results. To obtain the moisture distribution in the masonry wall, Larsen inserted gypsum-blocks and wooden-dowel moisture sensors (described in Subsection 2.1.4) in various locations in the wall. The range of the electrical resistance measurements of the gypsum-blocks responded to the actual moisture condition of the masonry and they were not sensitive to the effect of salts because the gypsum pores already contain a saturated solution of calcium and sulfate. The variation of the moisture content over the masonry wall cross-section indicated the direction of water movement and aided in identifying the source of moisture. Kininmonth & Williams (1972) described methods for determining moisture content of wood in New Zealand with emphasis on the probe type moisture meters. Methods described included the standard oven-drying method (method commonly used in North America as described in the Wood Engineering Handbook (1982), chemical, and distillation methods. 2.5.2 Electromagnetic Wave Methods These moisture diagnostic methods require specialized equipment and expertise to conduct the test and interpret results. Microwave moisture sensing techniques work on a similar principle to that of the capacitance technique, but at a much higher frequency. The microwave method is claimed to give a much better accuracy (Dill 2000) because the high frequency makes it less susceptible to impurities in the material such as salt in masonry walls. Many commercial microwave moisture-measuring systems are based on the Time-Domain-Reflectometry (TDR) technique [Examples: http://www.campbellsci.com/cs616-l, http://www.mesasystemsco.com/,http://www.tews-elektronik.com/english/pdf/1000e.pdf, http://www.hf-sensor.de/]. Radar and sonar techniques are early forms of the TDR technique. TDR technique measures the dielectric constant of a material, which changes as the volumetric water content of the material changes. TDR emits short high-frequency (up to 1 GHz) pulses of electromagnetic waves and processes reflected waves. The wave velocity in the material when

6 The neutron probe meter emits slow-moving neutrons into the masonry wall. The fraction of the neutrons

reflected by hydrogen nuclei is a measure of average moisture content in the wall. This meter is not sensitive to salts and measures average moisture content up to 250-mm depth. It does not measure moisture distribution in the wall.


http://www.campbellsci.com/cs616-l

http://www.mesasystemsco.com/

http://www.tews-elektronik.com/english/pdf/1000e.pdf

http://www.hf-sensor.de/

normalized with the speed of light in air determines the dielectric constant of the material. Thus, these methods are sometimes called dielectric measurement methods (Făcăoaru (1974) and Scheurer (1989)). TDR-based methods have been used to monitor changes in volumetric water content in soil, concrete and masonry structures, and a variety of other applications. Menke et al. (1995) presented a procedure of a microwave test method for moisture measurements in masonry walls. Menke et al. reported that the accuracy of their measurements was in the order of 1%. Rainwater et al. (1999) used a TDR system to measure moisture conditions in the stone and soil subgrade. They reported that the TDR system was able to directly measure water movement and detect small changes in the subgrade water content, which correlated with rainfall. They concluded that the TDR probes used, while designed for low-density agriculture soil applications, have proven to be effective moisture sensors in compacted soil, granular material, and asphalt material. Morey and Kovacs (1977) investigated the use of an impulse radar technique to monitor the curing of concrete and to detect moisture variations in a built-up roof. Morey and Kovacs concluded that impulse radar could be used to detect wide variations in roof moisture associated with surface deterioration of built-up roofs, and that TDR technique has the potential of providing a non-destructive test method for measuring the strength of concrete during curing. Andrews (1994) discussed the principles and reviewed the stat-of-the-art of TDR methods with emphasis on environmental, infrastructure, and mining applications. O’Connor (1996) also discussed geotechnical, environmental, and infrastructure applications. Dill (2000) and Făcăoaru (1974) provide a primer on microwave sensing techniques. Dill (2000) prepared a guide for testing for moisture in building elements for the Construction Industry Research and Information Association (CIRIA) in the UK. The guide covers a wide range of moisture measurement methods that are mostly used for diagnostic applications of moisture issues in building envelope components. It also provides guidance on the selection of the appropriate measurement method for various applications. Făcăoaru (1974) discussed the advantages and drawbacks of non-destructive test methods for determining the changes in moisture content of building materials and elements both in the laboratory and in-situ. Methods discussed included dielectric constant method, attenuation of microwave method, atomic methods (neutron thermalisation and magnetic nuclear resonance methods), and ultrasonic pulse velocity methods. 2.5.3 Infrared Thermography Method Infrared (IR) thermography is a non-destructive method in which electromagnetic radiation emitting from surfaces is captured in a visual image by an infrared camera. Thermography has commonly been used to assess insulation effectiveness, detect thermal bridges, and air tightness of building envelopes. Thermographic technologies have advanced significantly, equipment size is much smaller, and costs have been reduced. This has led to a wider application of thermography in building envelope inspections. Attempts have been made to use thermography to identify probable areas of moisture anomalies in building envelopes. The IR camera can only detect variations in surface temperature. It does not sense moisture or see inside the wall. The camera detects the thermal effects and temperature change caused by the presence of moisture. Moisture creates temperature depressions on the building envelope because wet material acts as a heat sink due to the high thermal capacity of water. Hence, as discussed earlier, a wet material has a lower temperature than when it is dry. The main advantage of thermography is the ability to assess moisture conditions over large areas of the building envelope. The scan is often followed with invasive investigation or a moisture measurement system such as moisture meters to further investigate areas identified having


moisture anomalies. However, thermography requires a qualified professional to interpret and carry out thermographic scans, as these must be made under appropriate conditions. Relevant standards include: ASTM C1153-97 (2003), ASTM E168-99 (2004), and ISO 6781-1983. The following are example literature on IR thermography applications to identify areas of moisture anomalies in building envelopes. Balaras and Argiriou (2002) reviewed potential applications of IR thermography to inspect and perform non-destructive testing of building elements. Examples include locating missing or damaged insulation, thermal bridges, air leakage, moisture damages, and detecting cracks in concrete structures. They noted that an IR roof inspection could locate water-damaged areas with “good accuracy” and provide information to possible sources of the problem. Grinzato et al. (1998) presented a quantitative methodology for processing IR images to map defects in buildings. They reported example application results including mapping of moisture content. Rosina and Spodek (2003) presented a case study on IR thermography application to detect moisture distribution in historic masonry walls. Moisture distribution was obtained by comparing thermal images to observed moisture damage in the walls. They concluded that IR thermography is a promising nondestructive testing method that can be used to map damp areas in building envelopes. Jenkins et al. (1982) evaluated in a laboratory the application of IR thermography for detecting moisture in a roofing system specimen. They assessed two performance parameters: the thermographic data versus moisture content and the threshold moisture content. They reported that surface temperatures determined by the IR thermography correlated well with those measured by thermocouples. The threshold moisture content measurements were 30% or less for specimen containing glass fiber insulation and 15% for a specimen containing polystyrene insulation. Korhonen & Coutermarch (1982) used a hand-held infrared camera to determine its effectiveness in detecting damp and wet cellular plastic insulation in roofs. They concluded that the IR camera could be an effective inspection tool for wetness in insulating material and emphasized the importance of core samples for verification afterwards. Gayo et al. (1993-1 and 1993-2) demonstrated the application of IR thermography as a tool for studying the movement of water through some building materials (gypsum, mortar, and cement).

2.6 Moisture Measurement in Concrete Structures This section is devoted to moisture measurements in concrete materials because of the considerable interest in the literature on this subject. Measurement and processing methodologies discussed could be applicable to other moisture measurement methods. Scheurer (1989) and Assenheim (1993) briefly reviewed moisture measurement in the concrete industry and discussed their advantages and disadvantages. Methods discussed included electric resistance, heat conductivity, neutron moderation, infrared radiation, and microwave methods. Germann Instruments7 developed two resistance-type sensors called HUM-Meter. One sensor is a Duff-like wood probes that measures the humidity of the concrete in terms of the electric resistance across a cylindrical wood probe. The probe is placed in a drilled hole at the required depth, after which the hole is sealed. The wood probe will reach equilibrium with the humidity of

7 WWW.germann.org


http://www.germann.org/

the surrounding concrete after about 24 hours. The other sensor utilizes the electric resistance of the concrete material measured between two graphite probes that are installed in drilled holes. The factors affecting the electric resistance of the concrete include moisture content, chloride content, porosity, and the temperature of the concrete. For a given temperature, a change in electric resistance indicates a change in the concrete moisture content and/or the chloride content. Vaisala8, a known manufacturer of humidity sensors, developed a sensor (model HM 44) for measuring relative humidity in general, but has been optimized for measurements in concrete structures. The sensor is placed in a drilled hole at the required depth, and the manufacturer’ installation procedure is followed to seal the hole properly. The Vaisala HM 44 sensor will reach equilibrium with the surrounding concrete material considerably faster (about an hour) than a wood probe. Altmann (1974) described a resistance-type moisture sensor for continuous monitoring of moisture content of concrete construction elements. The sensor could either be embedded in fresh concrete or inserted later in drilled holes. The sensor consisted of two platinum electrodes implanted in a porous ceramic cylindrical block that is 50-mm diameter and 100-mm long (a 100-mm diameter block was recommended for lightweight concrete). Altmann noted that using platinum electrodes would eliminate the possible effects of chemical reaction. At equilibrium, the moisture content of the sensor would be the same as its surrounding concrete. The following relation between measured electric resistance (R) and the moisture content (f) was used: Ln (R) = a – b • f (3) Where a and b are coefficients that depend on the material and were determined by calibration. Equation 3 gives a straight line when plotted on a semi-logarithmic scale. Altmann (1974) recommended using 12 V AC voltage to eliminate errors due to polarization. He reported the electric circuit used for measuring the electric resistance. He also presented a relationship for correcting sensor readings due to temperature effect. Altmann’s presentation approach, Equation 3, could be followed for resistance-type moisture sensors in other applications. Lundberg (1997) described the development of a system, called HUMI-Guard-System for measuring concrete humidity based on measuring the conductance and temperature of a polypropylene sensing material. The system also included a temperature sensor for measuring ambient temperature. The sensor is placed in a drilled hole at the required depth and the hole is sealed. The reported sensor accuracy is 1.5%. Hedenblad (1997) presented results of humidity measurements in high performance concrete. He concluded that measuring humidity in high performance concrete was more difficult than measurements in ordinary concrete. He investigated various sealing approaches and quality of the measurement hole, which highlighted the significance of properly sealing the measurement hole. Hedenblad’s recommendations suggested that the distance between the sensing element and the surrounding concrete should be minimized, the evaporation surface of the concrete around the sensing element should be large, and at least one week should be allowed between measurements to ensure equilibrium. 8 www.vaisala.com


http://www.vaisala.com/

3. Example Moisture Measurements This section presents examples of moisture measurements that demonstrate data presentation options and values. The examples are taken from field monitoring studies by the author (Saïd et al. (1997, 1999, and 2003) and Maurenbrecher et al. (2000)). Examples presented include measurements using moisture-pins, ceramic, and the Sereda moisture sensors. 3.1. Example Measurements – Resistance-Based Sensors Measured data from electric-resistance-type moisture sensors can be presented in terms of electric-resistance or wetness (electric-conductivity) scales.

1. Electric-resistance scale: Measured data is presented in electric-resistance vs. time graphs. The graphs will show changes in moisture conditions (wetness/dryness) such that a decrease in measured electric-resistance indicates an increase in wetness conditions and vice versa.

2. Wetness scale: Measured data is presented in wetness vs. time graphs. The wetness (electrical conductivity) is an inverse of the electric-resistance scale multiplied (magnified) by a constant value as shown in Equation 1.

RWetness 800= (1)

Where R is measured resistance in MΩ and the 800 is an arbitrary value.

The wetness scale is an arbitrary one. It illustrates relative changes in wetness conditions and hence cannot be directly related to the amount of moisture. On the wetness scale, a zero value indicates dry or same conditions and higher values indicate an increase in the wetness condition.

Hedlin (1965) developed a plotting technique and an empirical equation for resistance-type humidity sensors. He used logarithmic representation to describe the sensor’s electrical conductance in terms of relative humidity and temperature. Figures 8 to 10 demonstrate presentation of moisture measurements using resistance-type moisture sensors. Figure 8 shows an output example from two resistance-type moisture sensors, a ceramic sensor (BCS2) and a moisture-pins sensor (MP3). The ceramic sensor was mounted on the façade of a 330-mm thick brick masonry wall while the moisture-pins sensor was mounted on the interior surface of the wall. The sensors show the changes in wetness profile of the wall exterior and interior surfaces. In this example, the sensors illustrate the delayed reaction of the wetting & drying pattern of the interior surface of the wall (MP3) to the wetting & drying of the exterior surface (BCS2). This delay is associated with moisture diffusion and capillary suction processes through the brick-masonry wall.


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Figure 8. Example output from two resistance-type moisture sensors, a ceramic sensor (BCS2)

and a moisture-pins sensor (MP3). Figures 9 and 10 demonstrate the presentation of moisture measurements in terms of the wetness scale, which was calculated by Equation 1. Figure 9,Graph (a), shows electric-resistance measurements taken from a ceramic moisture sensor that was mounted on the façade of a building. The results, supported by the rainfall record show that the noted month started with a two days of dry period followed by about 10-days of wetness caused by heavy rainfall and ended with about a 2-week dry period. Graph (b), Figure 9, shows the same moisture profile but in terms of the wetness scale. As noted earlier, the wetness scale is an arbitrary scale that illustrates relative changes in wetness conditions. Figure 10, Graph (a), illustrates an example of moisture measurements using a pair of stainless-steel screws as moisture-pins sensor. The screws were inserted to a 10-mm depth in a mortar joint in a brick-masonry wall. The length of the screws outside the wall was covered with shrink tubing; the portion inserted in the wall was not covered (i.e. un-insulated). Thus, in this case, the screw moisture-pins indicate changes in moisture conditions from the surface to the 10-mm depth in terms of the electric-resistance of the mortar material (in this example) between the two screws. It is noted that from Graph (a) of Figure 10, measured resistance in this case was up to 12 MΩ, which is much lower than that measured by a ceramic moisture sensor (see Figure 9, Graph (a)) [Note: the brick-ceramic sensor measures the electric resistance of the ceramic material.]. Equation 1 was used to convert measured electric-resistances (Figure 10, Graph (a)) to wetness scale, but it was more suitable to use a smaller factor (6 instead of the 800). Wetness results are shown in Figure 10, Graph (b), which directly illustrates the wetting and drying pattern of the mortar material.


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(0-value indicates dry or same conditions, higher values indicate change in wetness) Figure 9. Resistance moisture measurements taken from a ceramic moisture sensor and its

presentation in terms of the wetness (electrical conductivity) scale


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(0-value indicates dry or same conditions, higher values indicate change in wetness) Figure 10. Resistance moisture measurements taken from a pair of stainless-steel screws as

moisture-pins sensor and its presentation in terms of the wetness (electrical-conductivity) scale


3.2. Example Measurements – Voltage-Based Sensors Figures 11 to 13 show examples of moisture measurements taken from Sereda moisture sensors, which were mounted on the façade of buildings. The figures show the sensor output voltage across a 10 MΩ shunt resistor and 0.068 µf capacitor according to ASTM G84.729 guidelines. As noted earlier, the Sereda sensor indicates time-of-wetness, and dryness. Measured voltage cannot be related directly to the amount of moisture on the surface. The following observations can be made from Figures 11 to 13:

• The Sereda moisture sensor reacts rapidly to wetting and dries quickly afterwards as indicated by the sharp changes in the output voltage in the course of a wetting/drying event.

• The output voltage (mV) of the Sereda sensors when wet varied between 70-mV and 250-mV. Occasionally, the voltage output may be as high as 350-mV (Figure 12).

• The voltage also varied from sensor to sensor. As noted earlier, the variation might be due to manufacturing variations as well as the size of the wet area of the sensor surface.

Rain Gauge RG3 and Sereda Moisture Sensor S5

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rainfall recorded by a neighboring rain gauge. The Sereda moisture sensor detected the presence of wetness on the façade quite well. In this case, the voltage output varied from 25 to 80 mV.

Figure 13 demonstrates the accurate response of the Sereda moisture sensor to diurnal condensation at two heights on a building façade. Condensation, verified by the surface and air dew-point temperatures, was formed on the wall surface during the nighttime when the surface temperature was equal to or lower than the air dew-point. The sensors showed that the façade dried rapidly in the morning when the sun rose and heated the wall.


Sereda # 42 & 95 (W & S-Façade)

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Figure 12. Example moisture measurements taken from two Sereda moisture sensors mounted on the

west and south façades of a building. In this case, the voltage output was up to 340 mV.

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Figure 13. Response of Sereda moisture sensors to diurnal condensation at two heights on the

façade of a building. Notice, in Figure 13, the variation in the output voltage of the two Sereda sensors. The output of the Sereda sensor at one-third wall height ranged from 3-mV (noise) when dry to 86-mV when wet, whereas the output of the sensor at the wall-top ranged from 0-mV when dry to 84-mV when wet. The 3-mV output is dry is typical measurement noise.


4. Conclusion Moisture measurement is an important consideration in building envelope investigations because of moisture effect on the performance and durability of buildings. The document reviewed literature and described moisture measurement methods for field monitoring applications of building envelopes. Example measurements and guidance on application and installation of various measurement methods have been presented with emphasis on methods used in field monitoring studies of building envelopes at the Institute for Research in Construction, National Research Council. Reviewed moisture measurement methods are grouped according to the measurement principle. Resistance, voltage, and capacitance-based measurement methods utilize the electrical properties of materials that vary with the materials moisture content (i.e. electrical resistance, dielectric constant, and radio frequency power-loss). Microwave techniques work on a similar principle to the capacitance methods, but at a much higher frequency. They make use of the dielectric constant of a material, which changes as the volumetric water content of the material changes. Thermal-based moisture measurement methods utilize the change in the temperature of materials caused by the presence of moisture. Moisture measurement methods have various capabilities. Some measurement methods are used to warn of excessive moisture conditions in the building envelope particularly in hidden or difficult to access areas. Other methods can quantify moisture content for some materials such as timber, while providing comparative moisture measurements for other building materials. Calibration data and temperature correction factors are available for various timber species. For other building materials, calibration data are quite limited, and in this case, sensors could only indicate changes in material wetness. Calibration of moisture sensors to quantify moisture content for other common building materials is needed. Thermal-based methods are used in special tests and in soil moisture applications. Infrared thermography and microwave-based methods are specialized tests that are used to identify areas with moisture anomalies in the building envelope. The tests are often followed with invasive investigations or continuous monitoring to further investigate areas with moisture anomalies. Resistance and voltage-based sensors are most suitable for continuous monitoring applications because they can be connected to a data logging system. Voltage-based moisture sensors are usually used to indicate time-of-wetness of surfaces. Their main weakness is durability, which can be quite short in outdoor applications. Resistance-based moisture sensors are used to monitor changes in wetness level within materials as well as time-of-wetness of surfaces. They are durable and can be fabricated in-house. Their challenge is for an instrumentation system that can measure a wide range of electrical resistance from few ohms to several hundred MΩ. Alternatively, electric resistances can be measured indirectly in terms of voltage using a half-bridge electric circuit.


5. References Altmann, K., 1974, “Measurement of moisture in construction components made of EPS-concrete”, 2nd International CIB/RILEM Symposium on moisture problems in Buildings, Rotterdam, Netherlands, Paper No. 5.1.1.

Andrews, J.R., 1994, “Time Domain Reflectometry”, proceedings of the symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, Illenoi, USA, September, pp. 4-13. Assenheim, J.G., 1993, “ Moisture measurement in the concrete industry”, Concrete Plant and Production Journal, September/October, pp. 129-131.

ASTM C1153-97 (2003) e1, “Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging”, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959 USA

ASTM D4444-92 (re-approved 2003), “Standard test methods for use and calibration of hand-held moisture meters”, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959 USA.

ASTM E168-99 (2004), “Standard Practices for General Techniques of Infrared Quantitative Analysis”, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959 USA.

ASTM G84.729-89 (re-approved 1999), “Standard practice for measurement of time-of-wetness on surfaces exposed to wetting conditions as is in atmospheric corrosion testing”, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959 USA.

Balaras, C.A. and A.A. Argiriou, 2002,”Infrared thermography for building diagnostics”, Energy and Buildings Journal, Vol. 34, pp. 171-183.

Brandt, E. and M.H. Hansen, 1999, “Measuring moisture content in wood with built in probes 20+ years of experience”, Proceedings Durability of Building Materials and Components 8, IRC/NRC, pp 669-679.

Carll C. and A. TenWolde, 1996, “Accuracy of wood resistance sensors for measurement of humidity”, Journal of Testing and Evaluation, ASTM, Vol.24, No 3, May, pp.154-160.

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