Campbell Scientific 3WHB10K 3-Wire Half-Bridge Terminal Input Module - Manual (Page 11)

3WHB10K 3-Wire Half Bridge Terminal Input Module
7
To perform the calibration, connect the PRT to the datalogger and program the
datalogger to measure the PRT with the 3-wire half bridge as shown in the
example section. For a 100 Ohm PRT use a multiplier of 100; for a 1000 Ohm
PRT use a multiplier of 10. Place the PRT in an ice bath (@ 0
°C; R
s
=R
0
).
Read the result of the bridge measurement. The reading is R
s
/R
f
, which is
equal to R
0
/R
f
since R
s
=R
0
. The correct value of the multiplier, R
f
/R
0
, is the
multiplier used divided by this reading. For example, if, with a 100 Ohm PRT,
the initial reading is 0.9890, the correct multiplier is: R
f
/R
0
= 100/0.9890 =
101.11.
5.3 Compensation for Wire Resistance
The 3-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 (Figure 3-1).
The maximum difference expected in wire resistance is 2%, but is more likely
to be on the order of 1%. The resistance of R
s
calculated with Instruction 7, is
actually R
s
plus the difference in resistance of wires A and B.
For example, assume that a 100 Ohm PRT is separated from the datalogger by
500 feet of 22 awg wires. The average resistance of 22 AWG wire is 16.5 Ohms
per 1000 feet, which would give each 500 foot lead wire a nominal resistance of
8.3 Ohms. Two percent of 8.3 Ohms is 0.17 Ohms. Assuming that the greater
resistance is in wire B, the resistance measured for the PRT (R
0
= 100 Ohms) in
the ice bath would be 100.17 Ohms, and the resistance at 40 °C would be 115.71.
The measured ratio R
s
/R
0
is 1.1551; the actual ratio is 115.54/100 = 1.1554. The
temperature computed by Instruction 16 from the measured ratio would be about
0.1
°C lower than the actual temperature of the PRT. This source of error does
not exist in a 4-wire half bridge where a differential measurement is used to
directly measure the voltage across the PRT.