Campbell CR6 Series, Operator'S Manual

Page: 339 / 690

Section 7. Installation
339
ii. Select a 1% resistor for R2 with a resistance that is approximately
equal to the resistance of the PRT at 10 °C. See
Procedure Information
(PT100 BrFull() Full Bridge)
(p. 349).
Since a 103.9 Ω resistor is hard to
find, use a 100 Ω resistor. It is close enough. Use 5 ppm/°C resistors.
Frequently, all the resistors in a full bridge are submerged in the medium
to be measured, so they may see large temperature changes. 5 ppm
resistors are more thermally stable than 10 ppm resistors.
2. Wire circuit to datalogger:
Use FIGURE: PT100 BrFull() Four-Wire Full Bridge Schematic
(p. 338)
as the
wiring diagram.
3. Calculate excitation voltage:
Use the following equation to calculate the best excitation voltage (VX) for
the measurement range –40 to 60 °C. The equation reduces the absolute
result by 1% to allow for resistor inaccuracy:
VX
max
= (VS
max
/ ((RS
max
/ (RS
max
+ R4)) – (R2 / R1 + R2))) • 0.99
where,
VS
max
= 200 mV (maximum voltage in the
±
200 input range)
R1 = 5000000 mΩ (5 kΩ)
R2 = 100000 mΩ (100 Ω)
R4 = 5000000 mΩ (5 kΩ)
RS
max
= 123240 mΩ (PT100 at 60 °C)
2
so,
VX
max
= 44972562111243 mV
4. Calibrate the PT100:
If the PRT accuracy specification is good enough, and you trust it, assume
RS
0
= 100000 mΩ. Otherwise, do the following procedure:
a. Enter