Campbell CR800 Series, Operator'S Manual, Page: 265 / 598

Section 7. Installation
265
PT100 BrHalf4W() Four-Wire Half-Bridge Measurement
'This program example demonstrates the measurement of a 100-ohm PRT in a four-wire
'half bridge using current excitation. See previous procedure and schematic.
'Declare constants and variables:
Const Rf = 100000 'Value of bridge resistor
Const RS0 = 100000 'Resistance of PT100 at 0 °C from calibration program
Public X 'Raw output from the bridge
Public RS 'Calculated PT100 resistance
Public RS_RS0 'Calculated ratio of RS/RS0
Public DegC 'Calculated temperature
BeginProg
Scan (1,Sec,0,0)
....'Measure X:
'BrHalf4W(Dest,Reps,Range1,Range2,DiffChan,ExChan,MeasPEx,ExmV,RevEx,RevDiff,
' SettlingTime,Integ,Mult,Offset)
BrHalf4W (X,1,mV25,mV25,1,Vx1,1,2053,True,True,0,250,1,0)
'Calculate RS and RS/RS0:
RS = X * Rf
RS_RS0 = RS/RS0
....'Calculate temperature from RS_RS0:
'PRTCalc(Dest,Reps,Source,PRTType,Mult,Offset)
PRTCalc (DegC,1,RS_RS0,1,1.0,0)
NextScan
EndProg
Notes
• Why use four-wire half-bridge?
Use a four-wire half-bridge when lead resistance is more than a few
thousandths of an ohm, such as occurs with long lead lengths.
• Why use 10 kΩ series resistor?
Referring to figure PT100 BrHalf4W() Four-Wire Half-Bridge Schematic
(p.
262),
the 10 kΩ series resistor allows the use of a higher -excitation voltage and
a low analog voltage input range.
• Why use high excitation and low range?
High excitation and low range minimize the effects of signal noise.
•
Why use a bridge resistor near value of PT100?
By using a bridge resistor (Rf) that is close in value to that of the PT100
(RS), the differential measurement of V2 (voltage drop across PRT) can be
made on the same range as the differential measurement of V1 (voltage drop
across Rf). Using the same range eliminates range translation errors that can