Temp Compensation

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Temperature Compensation

+he ideal strain gauge would change resistance in accordance with stress*

producing deformations in the structural surface to which it was onded and for no

other reason. /nfortunately, gauge resistance is affected y many other factors,

out of which temperature is very important.

+he total indicated strain occurring at a point in a structure is made up of

mechanical strain and apparent strain. +he mechanical strain is that produced y

e0ternal forces. +he apparent strain is the portion of the total indicated strain

induced y thermal effects including e0pansion of the ase metal, e0pansion of the

gauge metal and change in electrical resistance of the gauge. +hus, when the

amient temperature increases say, then

1. +he gauge grid will elongate so that

l.T.

l

∆= α ∆

2. +he ase material on which the gauge is mounted will elongate so thatl.T.

l

∆= β ∆

3. +he resistance of the gauge metal will increase ecause of the influence of

the temperature coefficient of resistivity of the gauge material so thatR

.T.R

∆ = γ ∆



+he comined effect of these three factors will produce a temperature

induced change in resistance of the gauge,T

R

R ∆

∆ ÷

with may e e0pressed as:

( )T

RT.F .T

Rβ α γ

∆

∆ = − ∆ + ∆ ÷

here α thermal coefficient of e0pansion of the gauge material

β thermal coefficient of e0pansion of the ase material

γ temperature coefficient of resistivity of the gauge

material

5 gauge factor

6 resistance of gauge

T∆

rise in temperature.

7quation holds only for small values ofT∆

, where α,β and γ can e

considered constant. 5or large values ofT∆

, average values of these factors might

e used without introducing large errors.

8fα β ≠

, then the gauge will e su)ected to a mechanical strain,

( ) Tβ α − ∆,

which does not occur in the specimen.



8f α β, then this component of apparent strain vanishes. However, the

gauge will still register a change of resistance with temperature if γ is not 9ero. 8n

order to prevent significant errors due to this effect, some form of temperature

compensation; is usually employed when strain gauges are used in applications

where the steady state or static component of strain must e measured. <urrently

availale methods of compensation for the apparent strain include the use of a

dummy or compensating strain gauge, self*temperature compensating =+<

gauge, compensation y dissimilar or similar gauges in the heatstone ridges and

compensation y computation.

1. Compensating Dummy Gauge:

+he earliest form of temperature compensation ma%es use of the electrical

ridge circuit in which the active gauge is connected to alance out unwanted

temperature induced resistance change. +his is usually called the compensating

dummy; arrangement. +he dummy gauge; identical to the active gauge in type

and lot numer, is mounted on an unstressed piece of the specimen material and

placed in the same thermal environment as the active gauge. +he active and

compensating gauges are then connected as ad)acent arms of the ridge circuit in

the readout instrument. 7ffects common to oth gauges will preserve ridge >

alance conditions, and no output signal results. =ince only the active gauge is

e0posed on mechanical or thermal strain caused y specimen stress, ridge

unalance is proportional to the magnitude of specimen stress producing strain.

+he method fails entirely if the temperature does not vary in an identical fashion at

oth gauge locations.

2. Self-temperature Compensated Gauge:

+he terms ;temperature compensated; is applied to strain gauges in which

the resistance change due to temperature is equal to 9ero. =elf*temperature

compensated gauges will perform properly only when used on materials having



the specific value of thermal e0pansion coefficient for which they are designed.

=+< gauges can e otained for use on materials having thermal e0pansion

coefficients from 9ero to 2! ppm?o<.

+wo method are used for otaining self*temperature compensation. 8n the

first method, self*temperature compensation is created y altering the temperature

coefficient of resistance of the grid material so that, when mounted on materials

having a certain thermal e0pansion coefficient, the apparent strain will e a

suitaly low value. +his is done, in most cases, y special selection or thermal

processing of the grid alloy. +he two principal classes of strain*gauge alloys

susceptile to such treatment are <onstantan and @arma. +he second method

includes forming a grid with two lengths of gauge wires )oined suitaly in seriesso that the resultant apparent strain is 9ero.

Dual-element self temperature Compensation by dissimilar

gauges.

3. Compensation by Dissimilar Gauges:

<ompensation of the temperature effect in a ridge networ% is accomplished y

putting dissimilar gauges into ad)acent ridge arms as shown in 5igure. +he gaugein the first arm should have a relatively small temperature effect in the same

direction. ith proper, fi0ed series and shunt resistances for the gauge in the

second arm, it is possile to otain an overall temperature effect for the second

arm, that is equal to that of the first arm. Hence, the temperature effects of the two



arms will cancel each other with a relatively small loss in the strain sensitivity of

the networ%.

+his method would appear to have a etter chance of success than the self*

temperature compensated gauge ecause the relative resistance of the filament isnot critical. 8f will always e possile after a gauge has een made, to select the

fi0ed resistance for proper compensation. 5urthermore, compensation over a

greater temperature increases. in this case, temperature would not have to e

%nown very accurately.

4. Compensation by Similar Gauges:

est possile temperature compensation is otained for unpredictale effects

as for predictale effects with two similar gauges in ad)acent arms of a heatstone

ridge. However, this circuit arrangement eliminates the hydrostatic component of

stress from the reading and only the shear component of stress is reflected. Hence,

the gauges should e arranged so as to pic% up the greatest signal from the shear

component of stress. +his means that one gauge should e positioned in the

direction of the ma0imum principal strain, the other in the direction of minimum

principal strain. +his method is li%ely to give est results when the direction of the

principal strains is %nown.

. Compensation by Computation:

y %nowing the temperature characteristics of a strain gauge and the ase

metal, and if the temperature can e oserved separately, a correction can e

calculated theoretically from 7quation and applied to the oserved strain.



!igure: Compensation by similar gauges.

Temp Compensation

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