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Campbell Scientific 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules User Manual

Page 41

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4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM)

BeginProg

'Program begins here

GF(1) = 2.1

:

GF(2) = 2.2

:

GF(3) = 2.3

'Initialize gauge factors for Strain( )

ZReps = 3 : ZIndex = 1

‘initialize cal reps and index pointer

For

I = 1

To

3

'Initialize adjusted gage factors to raw gage factors

GF_Adjusted(I) = GF(I)

Next

I

ZReps = 3 : ZIndex = 1

LoadFieldCal

(True)

‘Load prior calibration factors


Scan

(10,mSec,100,0)

'Scan once every 10 mSecs, non-burst

FieldCalStrain(

10,StrainMvperV(),ZReps,0,ZeromV_V(),ModeVar,0,ZIndex,1,0,Strain())

FieldCalStrain

(13,Strain(),ShuntReps,GF_Adjusted,0,ModeShunt,KnownRs,ShuntIndex,1,GF(),0)

BrFull

(StrainMvperV(),3,mV50,4,1,5,7,1,5000,True,True,70,100,1,0)

StrainCalc

(Strain(),3,StrainMvperV(),0,-1,GF(),0)

'Strain calculation

CallTable

STRAIN

CallTable

Calib

Next

Scan

'Loop up for the next scan


SlowSequence

'Slow sequence Scan to perform temperature

Scan

(1,Sec,0,0)

' compensation on DAQ

Calibrate

'Corrects ADC offset and gain

BiasComp

'Corrects ADC bias current

Next

Scan

EndProg

'Program ends here

4.4.3 Lead Compensation using Quarter Bridge Strain with 2 Wire

Element

If the leads become so long that lead resistance compensation
calculations are required then a 2-wire gage should not be
utilized. It would be difficult to mathematically compensate, and
even though a Shunt Calibration is possible, false readings due to
lead wire temperature changes would in most cases inject more
error than the sensitivity change due to the leads resistances.
Instead a 3-wire gage should be utilized.

NOTE

This section covers some of the errors introduced into the measurement using
this type of circuit with long leads. This is not to show how to perform lead
compensation using a 2 wire gage, but to discourage its practice.

A 2-wire quarter bridge strain circuit is shown in figure 4.4-5

R

2

=1 K

Ω

R

1

=1 K

Ω

Excite V

R

D

+

-

R

L

GAUGE = R

4

R

L

FIGURE 4.4-5. Two wire quarter bridge strain circuit

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