Hygrometry

Methods for Measuring Humidity

Removal of Water Vapor from Moist Air

Addition of Water Vapor to Air

Electric Hygrometers

Mechanical Hygrometers

Spectroscopic Hygrometers

Removal of Water Vapor from Moist Air

Separation or removal of water vapor from moist air ca be accomplished by using a desiccant to absorb water vapor, by freezing out water, or by separation of moist air constituents using a semi-permeable membrane.

These are standard lab techniques that operate on a sample of moist air.

After removal of water vapor, the mass of the water vapor and remaining air sample are determined, humidity is then calculated.

Humidity can be determined by measuring the amount of water vapor that must be added to a sample of moist air to achieve complete saturation (lab technique, but variation of method is suitable for field).

Psychrometry is a method of adding water vapor to moist air where complete saturation is not achieved. The humidity is determined from cooling of a wet bulb relative to the ambient air temperature.

Psychrometer comprises two temperature sensors exposed to the ambient air flow. One sensor, called the dry bulb, measures the ambient air temp, the other, wet bulb, is covered with a wick moistened with water and measures a lower temperature.

Addition of Water Vapor to Air: Psychrometry

Addition of Water Vapor to Air: Psychrometry

The wick of the wet bulb can be moistened intermittently or continuously by capillary flow through the wick material.

Forced ventilation is normally required for optimum performance; natural ventilation may be adequate only when the temperature sensor and wick are very small or ambient wind speed is high.

Addition of Water Vapor to Air: Psychrometry

Sources of error in psychrometry:

1. Sensitivity, accuracy, matching of temperature sensors-a psychrometer is less sensitive to the absolute error in the temperature sensors than to the relative error, or matching error between wet and dry bulb sensors.

2. Ventailation rate- typically, the ventilation rate should be at least 3 m/s to maximize heat transfer by convection and evaporation and to minimize heat transfer by conduction and radiation.

-sensors made from small thermocouple wire with a fine cloth wick have been successful without forced ventilation.

Addition of Water Vapor to Air: Psychrometry

Sources of error in psychrometry:

3. Radiation incident on the temperature sensors: sensors must be shielded from direct and reflected solar radiation and from long-wave, earth radiation.

4. Size, shape, material and wetting of wick: specially prepared wick material should be used! Cotton is often treated with hydrophobic chemicals.

5. Relative positions of wet and dry bulbs: air must not flow from cooled wet bulb to dry bulb.

6. Purity of water used for wick: only distilled or deionized water should be used.

Addition of Water Vapor to Air: Psychrometry

Addition of Water Vapor to Air: Psychrometry

Electric Hygrometers

Electric hygrometers are sorption sensors that take up water which causes a change in an electrical parameter such as resistance or capacitance.

A capacitive sensor comprises an optional glass substrate for mechanical strength, a thin gold layer(one plate of cap), a thin layer of polymer (sorption layer), and a very thin layer of gold(other cap plate). Must be thin enough to permit water vapor transport.

Polymer has a low dielectric constant, ~4, Water has a dielectric constant of 80 (temp sensitive), so sorption of small amounts of water substance between the plates will affect capacitance.

Electric Hygrometers

Electric Hygrometers

Electric Hygrometers

Electric Hygrometers

Electric Hygrometers

Anderson (1995) also showed that the temperature sensitivity of the water film thickness is very low, so the observed temperature sensitivity of capacitance sensors is most likely due to temperature sensitivity of dielectric constant of water.

Drift can affect the sensor in many ways:

Dust can accumulate on sensor surface, sorb some water. Cleaning in pure water can restore calibration.

A fine-pore filter is used to prevent dust accumulation.

-However, if water condenses on filter, will moisten air and cause a temporarily high reading until it dries out.

Mechanical Hygrometers

Are made from dimensionally variable materials mechanically coupled to an indicator or transducer.

Human hair, goldbeater’s skin, cotton, silk, nylon, paper and wood have been used as the sensing element.

Main defects are drift, hysteresis and large lag times.

Mechanical Hygrometers

CS215Temperature and Relative Humidity Probe

Sensirion SHT75, a combinedrelative humidity and temperature element

•Energy consumption: 80uW (at 12bit, 3V, 1 measurement / s) 

•RH operating range: 0 – 100% RH

•T operating range: -40 – +125°C (-40 – +257°F)

•RH response time: 8 sec (tau63%)

Sensirion SHT75

Sensor ChipSHT75 V4 – for which this datasheet applies – features aversion 4 Silicon sensor chip.

Besides a humidity and a temperature sensor the chip contains an amplifier, A/D converter, OTP memory and a digital interface.

Material ContentsWhile the sensor is made of a CMOS chip the sensorhousing consists of an LCP cap with epoxy glob top on anFR4 substrate.

Pins are made of a Cu/Be alloy coated with Ni and Au.

Spectroscopic Hygrometers

Measures the attenuation of certain bands in the spectrum due to water vapor absorption.

These bands occur mostly in the UV and Infrared.

Lyman-alpha line in the UV has been used.

The fraction of incident radiation transmitted through an atmospheric path is given by beers law:

τ = I/Io = exp(-κλdvx)

X = path length, dv= absolute humidity in kg m-3, reduced to standard atmosphere, κ = absorption coefficient, Io and I represent source intensity and intensity of light after passing through absorbing atmosphere.

Spectroscopic Hygrometers

To apply this technique, we need a known Io or source strength in a known wavelength band, x, fixed path length, and, I, detector with a known sensitivity.

Also need absorption coefficient, k, in wavelength interval determined by bandwidths of the source and detector.

Ideal wavelengths would be where no other atmospheric gas is an absorber, where sources and detectors are readily available, and where transparent window materials (to enclose source and detector) are available.

Spectroscopic Hygrometers

The KH20 sensor uses a krypton lamp that emits two absorption lines: major line at 123.58 nm and minor line at 116.49 nm. Both of these lines are absorbed by water vapor, and a small amount of the minor line is absorbed by oxygen. The KH20 is not suitable for absolute water vapor concentration measurements due to its signal offset drift.

Spectroscopic Hygrometers: CSI KH20