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