Section 7. Installation
PT100 BrHalf3W() Three-Wire Half-Bridge Measurement
'This program example demonstrates the measurement of a 100-ohm PRT (PT100) in a three-wire
'half bridge with voltage excitation. See adjacent procedure and schematic.
'Declare constants and variables:
Const Rf = 10000000 '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 RS/RS0
Public DegC 'Calculated temperature
BeginProg
Scan(1,Sec,0,0)
....'Measure X:
'BrHalf3W(Dest,Reps,Range,SEChan,ExChan,MeasPEx,ExmV,RevEx,SettlingTime,
' Integ,Mult,Offset)
BrHalf3W(X,1,mV25,1,Vx1,1,2033,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
• The three-wire half-bridge compensates for lead-wire resistance by
assuming that the resistance of wire a is the same as the resistance of
wire b (see FIGURE: PT100 BrHalf3W() Three-Wire Half-Bridge
Schematic
(p. 266)). The maximum difference expected in wire resistance
is 2%, but is more likely to be on the order of 1%.
• The average resistance of 22 AWG wire is 16.5 Ω per 1000 feet, which
would give 500 ft lead wires (for example) a nominal resistance of 8.3 Ω.
Two percent of 8.3 Ω is 0.17 Ω. Assuming that the greater resistance is
in wire b, the resistance measured for the PRT in the ice bath (RS0) is
100.17 Ω, and the resistance at 40 °C (RS) is 115.71 Ω.
• At 40 °C, because of the error from wire b, the measured ratio RS/RS0 is
1.1551 while the ratio without the error would be 115.54/100 = 1.1554.
As a result, the temperature computed by PRTCalc() from the ratio with
the error is about 0.43 °C higher than the temperature measured without
the error from wire b. This source of error does not exist in a four-wire
half-bridge configuration.
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