strain gages and humidity transducers

8/11/2019 Strain Gages and Humidity Transducers

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Strain Gages and

Humidity TransducersTed John G. Orante

Faye Camille P. Yturralde


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Strain Gages

Strain gage is one of the most important tools of the

electrical measurement technique applied to themeasurement of mechanical quantities

Invented by Edward E. Simmons and Arthur C. Ruge in

1938

Is a sensor whose resistance varies with applied force; It

converts force, pressure, tension, weight, etc., into a

change in electrical resistance which can then be

measured.


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They are used for the measurement of strain. As a

technical term "strain" consists of tensile and compressive

strain, distinguished by a positive or negative sign. Thus,

strain gages can be used to pick up expansion as well ascontraction.

The strain of a body is always caused by an external

influence or an internal effect. Strain might be caused by

forces, pressures, moments, heat, structural changes ofthe material and the like. If certain conditions are

fulfilled, the amount or the value of the influencing

quantity can be derived from the measured strain value.


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Is basically a piece of very thin foil or fine wire which

exhibits a change in resistance proportional to the

mechanical strain imposed on it. In order to handle such a

delicate filament, it is either mounted on, encapsulatedin, or bonded to some type of carrier material and is

known as the bonded strain gage.

Bonded strain gages are available in a wide range of sizes

and resistances. Unbonded strain gages, where the wire isfree, are rarely used because of their limited frequency

range and lack of sensitivity.


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Several types of strain gages depend on the proportional

variance of electrical resistance to strain: the

piezoresistive or semi-conductor gage, the carbon-

resistive gage, the bonded metallic wire, and foilresistance gages.

The three primary factors influencing gage selection are

operating temperature, state of strain (gradient,

magnitude, and time dependence) and stability required.


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The electrical resistance strain gage is the most versatile of many

devices to measure strains on the surfaces of machine componentsand structural members.

Resistance change in a strain gage is usually very small that it cannotbe measured accurately with an ordinary ohmmeter. Ideally, the strain

gage is the only resistor in the circuit that varies and then only due to

a change in strain on the surface.

This resistance change, usually measured using a Wheatstone bridge,

is related to the strain by the quantity known as the gauge factor.

The Wheatstone bridge is widely used in practice; one or more of the

four arms of the bridge are strain gages.


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Wheatstone Bridge

The Wheatstone bridge is a basic circuit employed tomeasure extremely small resistance changes in a strain

gage when it is subjected to a strain.

A constant-voltage Wheatstone bridge is normally used to

record strain gage outputs in static and dynamic

applications.


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The voltage drops across R1and R4denoted by Vaband Vadrespectively,

are given by the equations

=

+ 2 =

4

3 + 4

Where V is the applied voltage across the bridge.

The voltage output of the bridge E is represented by

= =3 24

+ 2 3 + 4

It is clear that the output voltage of the bridge is zero (i.e., the

bridge is balanced) when the term 3 24is zero or when3 = 24


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Strain Gage Transducer

A strain gage transducer is a device that uses strain gages

as the sensor to produce an electrical signal that is

directly proportional to such mechanical quantities as

force, displacement, pressure, torque, and acceleration.

Many different types of sophisticated strain gage

transducers are commercially available, such as load cell,

torque meter, accelerometer, and displacement

transducer.


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Sample application: Load Cell

Figure (a) shows a tension-compression load cell. To increase

sensitivity and accuracy (to eliminate possible bending or torsionaleffects), four strain gages are mounted on the central region of thebar with two gages in the axial direction and two gages in the

transverse direction and connected in a full bridge: two gages in each

set are bonded at diametrically opposite locations.


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Potential Error Sources

In a stress analysis application, the entire gage installation cannot be

calibrated as can some pressure transducers. Therefore, it is importantto examine potential error sources prior to taking data.

Some gages may be damaged during installation. It is important

therefore to check the resistance of the strain gage prior to stress.

Electrical noise and interference may alter your readings. Shieldedleads and adequately insulating coatings may prevent these problems.

A value of less than 500 M ohms (using an ohmmeter) usually indicates

surface contamination.


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Thermally induced voltages are caused by thermocouple effects at thejunction of dissimilar metals within the measurement circuit.

Magnetically induced voltages may occur when the wiring is located in

a time varying magnetic field. Magnetic induction can be controlled

by using twisted lead wires and forming minimum but equal loop areas

in each side of the bridge.

Temperature effects on gage resistance and gage factor should be

compensated for as well. This may require measurement oftemperature at the gage itself, using thermocouples, thermistors, or

RTDs. Most metallic gage alloys, however, exhibit a nearly linear gagefactor variation with temperature over a broad range which is less

than 1% within 100C.


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Prime Strain Gage Selection Considerations

Gage Length

Number of Gages in Gage Pattern

Arrangement of Gages in Gage Pattern

Grid Resistance

Strain Sensitive Alloy

Carrier Material

Gage Width

Solder Tab Type

Configuration of Solder Tab

Availability


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Strain gage resistance

The resistance of a strain gage is defined as the electrical

resistance measured between the two metal ribbons or

contact areas intended for the connection of

measurement cables. The range comprises strain gages

with a nominal resistance of 120, 350, 600, and 700 Ohms.


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Gage Factor (Strain Sensitivity)

The strain sensitivity k of a strain gage is the proportionality factorbetween the relative change of the resistance.

The strain sensitivity is a figure without dimension and is generally

called gage factor.

The gage factor of each production lot is determined by sample

measurements and is given on each package as the nominal value with

its tolerance. Reference Temperature The reference temperature isthe ambient temperature for which the technical data of the strain

gages are valid, unless temperature ranges are given. The technical

data quoted for strain gages are based on a reference temperature of

23C.


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Variations in temperature

Variations in temperature will cause a multitude of

effects. The object will change in size by thermal

expansion, which will be detected as a strain by the

gauge. Resistance of the gauge will change, and resistance

of the connecting wires will change.


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Humidity Transducers


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Humidity

Humidity is the presence of water in air. The amount of water

vapor in air can affect human comfort as well as manymanufacturing processes in industries. The presence of water

vapor also influences various physical, chemical, and

biological processes.

Humidity measurement in industries is critical because it mayaffect the business cost of the product and the health and

safety of the personnel. Hence, humidity sensing is veryimportant, especially in the control systems for industrial

processes and human comfort.


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Humidity measurement is one of the most

significant issues in various areas of applications

such as instrumentation, automated systems,

agriculture, climatology and GIS.


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Measurement Parameters for Humidity


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is defined as a ratio of the mass of water vapour in air to the volume of

air, with the unit of grams per cubic meter or grains per cubic foot (1grain = 1/7000 pound lb) and expressed as:

=

where ->AB

is the absolute humidity (/3

or /3

),->is the mass of water vapour (gram or grain)

->v is the volume of air (3or3)

Absolute Humidity (vapour density)


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is defined as ratio of the amount of moisture content of air to the

maximum (saturated) moisture level that the air can hold at a same giventemperature and pressure of the gas. RH is a temperature dependent

magnitude, and hence it is a relative measurement:

% =

100

where -> is the actual partial pressure of moisture content in air

-> is the saturated pressure of moist air at the same given temperature

(both in Bar or KPa)

Relative Humidity (abbreviated as RH)


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is defined as the ratio of the mass of water vapour at saturation to the

volume of air. The saturation humidity is a function of temperature and caprovide the maximum amount of moisture content (mass) in a unit volume

of gas at a given temperature:

=

where -> SH is the saturation humidity (g/3)

-> is mass of water vapour at saturation (g)

-> v is the volume of air (3)

SaturationHumidity


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Relative Humidity

can be represented in other way by calculating the ratio of absoluhumidity to saturation humidity as a percentage as follows:

% =

100


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Parts Per Million by volume (PPMv) & Per

Million by weight (PPMw)

PPMv is defined as volume of water vapour content pevolume of dry gas.

PPMwis obtained by multiplying PPMv by the mole weight owater per mole weight of that gas or air.

PPMv and PPMw are among the absolute humidit

measurements.


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Dew Point & Frost Point

Dew point is defined as a temperature (above 0 C) at which

the water vapour content of the gas begins to condense intoliquid water.

Frost point is the temperature (below 0 C) at which thewater vapour in a gas condenses into ice.

Both parameters are functions of the pressure of the gas, but

independent of temperature and are amongst the absolute

humidity measurements


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Humidity Sensors Classification

Amongst the various humidity evaluation terms and units,absolute humidity and relative humidity are the most prevalent.

Based on the units of measurement, humidity sensors are

subsumed in two main classes: Relative Humidity (RH) andAbsolute Humidity sensors (hygrometers)


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Table 1. The state-of-the-art of humidity sensors based on fabrication technologiesand sensing materials.


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Sensing Principle

Humidity measurement can be done using dry and wet bhygrometers, dew point hygrometers, and electro

hygrometers. There has been a surge in the demand of electro

hygrometers, often called humidity sensors.

Electronic type hygrometers or humidity sensors can be broadivided into two categories: one employs capacitive sens

principle, while other use resistive effects.


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Sensors based on Resistive Effect

Resistive type humidity sensors pick up changes in theresistance value of the sensor element in response to the

change in the humidity


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Figure 7. Sketch of a planar thick/thin film-based humidity sensor based on the inter-digitatstructure with the porous sensing element.

Basic structure of resistive type humidity sensor from TDK


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Sensors based on Capacitive Effect

Humidity sensors relying on this principle consists of ahygroscopic dielectric material sandwiched between a pair of

electrodes forming a small capacitor.

Most capacitive sensors use a plastic or polymer as the

dielectric material, with a typical dielectric constant ranging

from 2 to 15. In absence of moisture, the dielectric constant

of the hygroscopic dielectric material and the sensor

geometry determine the value of capacitance.


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At normal room temperature, the dielectric constant of water vaporhas a value of about 80, a value much larger than the constant of thesensor dielectric material. Therefore, absorption of water vapor bythe sensor results in an increase in sensor capacitance.

At equilibrium conditions, the amount of moisture present in ahygroscopic material depends on both the ambient temperature andthe ambient water vapor pressure. This is true also for thehygroscopic dielectric material used on the sensor

By definition, relative humidity is a function of both the ambienttemperature and water vapor pressure. Therefore there is arelationship between relative humidity, the amount of moisturepresent in the sensor, and sensor capacitance. This relationshipgoverns the operation of a capacitive humidity instrument


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Figure 10. Configuration of Humicape humidity sensor.

Basic structure of capacitive type humidity sensor

A li ti Ci it


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Application Circuits

strain gages and humidity transducers

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