R
Smax
= Maximum sensor resistance based on the maximum expected temperature to be
measured
Using the typical 35 mA maximum excitation current, the maximum excitation voltage for the
sensor when R
Smax
is 100 Ω is:
V
x
= 0.35 mA (10 kΩ + 100 Ω) = 3.54 V
When R
Smax
is 1.25 kΩ, this changes to:
V
x
= 0.35 mA (10 kΩ + 1.25 kΩ) = 3.94 V
5.2 Calibrating a PRT
The greatest source of error in a PRT is likely to be that the resistance at 0 °C deviates from the
nominal value. Calibrating the PRT in an ice bath can correct this offset and any offset in the
fixed resistor in the terminal input module.
The result of the 3-wire half bridge is:
With the PRT at 0 °C, R
s
=R
0
. Thus, the above result becomes R
0
/R
f
, the reciprocal of the
multiplier required to calculate temperature, R
f
/R
0
. By making a measurement with the PRT in an
ice bath, errors in both R
s
and R
0
can be accounted for.
To perform the calibration, connect the PRT to the data logger and program the data logger to
measure the PRT with the 3-wire half bridge as shown in the example section (multiplier = 1).
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 Ω 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 wire resistance by assuming that the resistance of wire A
is the same as the resistance of wire B (
(p. 3)). 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
BrHalf3W()
, is actually R
s
plus the difference in resistance of wires A and B.
For example, assume that a 100 Ω PRT is separated from the data logger by 500 feet of 22 AWG
wires. The average resistance of 22 AWG wire is 16.5 Ω per 1000 feet, which would give each 500-
foot wire 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 (R
0
= 100 Ω) in the ice bath would be
3WHB10K 3-Wire Half Bridge Terminal Input Module
9
