Campbell CR3000 Micrologger, Operator'S Manual, Page: 282 / 644

Section 7. Installation
282
Bridge Resistor Values (mΩ)
Rf
100000
Procedure
1. Build circuit
1
:
a. Use FIGURE: PT100 BrHalf3W() Three-Wire Half-Bridge Schematic
(p.
281)
as a template.
b. For Rf, choose a 1%, 10 ppm/°C, 10000000 mΩ (10 kΩ resistor).
2. Wire circuit to datalogger:
Use FIGURE: PT100 BrHalf3W() Three-Wire Half-Bridge Schematic
(p. 281)
as the wiring diagram.
3. Calculate excitation voltage:
Use the following equation to calculate the best excitation voltage (VX) for
the measurement range of –40 to 60 °C. The equation reduces the absolute
result by 1% to allow for resistor inaccuracy:
VX
max
= VS
max
/ (RS
max
/ (Rf + RS
max
)) • 0.99
where,
VS
max
= 50 mV (maximum voltage in the ±
50 input range)
Rf = 10000000 mΩ
RS
max
= 123240 mΩ (PT00 at 60 °C)
2
so,
VX
max
= 1626420334066 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 CRBasic EXAMPLE: PT100 BrHalf3W() Three-Wire Half-Bridge
Calibration
(p. 284)
into the CR3000. It is already programmed with the
excitation voltage from step 3.
b. Place the PRT in an ice bath (0 °C).