Section 7. Installation
269
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.
Summary of Contents for CR800 Series
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Page 446: ...Section 8 Operation 446 8 11 2 Data Display FIGURE 100 CR1000KD Displaying Data ...
Page 448: ...Section 8 Operation 448 FIGURE 102 CR1000KD Real Time Custom ...
Page 449: ...Section 8 Operation 449 8 11 2 3 Final Storage Data FIGURE 103 CR1000KD Final Storage Data ...
Page 450: ...Section 8 Operation 450 8 11 3 Run Stop Program FIGURE 104 CR1000KD Run Stop Program ...
Page 452: ...Section 8 Operation 452 FIGURE 106 CR1000KD File Edit ...
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Page 523: ...Section 11 Glossary 523 FIGURE 116 Relationships of Accuracy Precision and Resolution ...
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