Campbell CR800 Series, Operator'S Manual

Page: 270 / 598

Section 7. Installation
270
7.7.16.5 Example: 100 Ω PRT in Four -Wire Full Bridge with Voltage
Excitation (PT100 / BrFull() )
FIGURE 67: PT100 BrFull() Four-Wire Full-Bridge Schematic
Procedure
1. Build circuit
1
:
a. Use
FIGURE: PT100 BrFull() Four-Wire Full-Bridge Schematic
(p. 270)
as a
template.
b. Choose a 1%, 10 ppm/°C, 5000000 Ω (5 kΩ) resistors for R1 and R4
c. Balance the bridge.
i. Find the midpoint of the temperature range. The range of –40 to 60
°C is selected for this procedure, so the midpoint is 10 °C.
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). 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. 270)
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