Campbell CR800 Series - PT100 Brfull() Four-Wire Full-Bridge Calibration

Section 7. Installation
271
where,
VS
max
= 25 mV (maximum voltage in the ±25 input range)
R1 = 5000000 mΩ (5 kΩ)
R2 = 100000 mΩ (100 Ω)
R4 = 5000000 mΩ (5 kΩ)
RS
max
= 123240 mΩ (PT100 at 60 °C)
2
so,
VX
max
= 44972562111243 mV
4. Calibrate the PT100:
If the PRT accuracy specification is good enough, and you trust it, assume
RS
0
= 100000 mΩ. Otherwise, do the following procedure:
a. Enter
PT100 BrFull() Four-Wire Full-Bridge Calibration
'This program example demonstrates the calibration of a 100-ohm PRT (PT100) in a four-wire
'full bridge with voltage excitation. See previous procedure and schematic.
'
'Declare constants and variables:
Const R1 = 5000000 'Value of R1 bridge resistor
Const R2 = 120000 'Value of R2 bridge resistor
Const R4 = 5000000 'Value of R4 bridge resistor
Public X1 'Raw output from the bridge
Public X2 'Calculated intermediate value
Public RS0 'Calculated PT100 resistance at 0 °C
BeginProg
Scan(1,Sec,0,0)
'Measure X1
'BrFull(Dest,Reps,Range,DiffChan,ExChan,MeasPEx,ExmV,RevEx,RevDiff,SettlingTime,
' Integ,Mult,Offset)
BrFull(X1,1,mV25,1,Vx1,1,2500,True,True,0,250,1,0)
'Calculate X2:
X2 = (X1/1000) + (R2/(R1+R2)
'Calculate RS0:
RS0 = (R4*X2) / (1-X2)
NextScan
EndProg
into the CR800. It is already programmed with the excitation voltage from
step 3.
b. Place the PRT in an ice bath (0 °C).