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
35