Campbell CR800 Series, Operator'S Manual

Page: 267 / 598

Section 7. Installation
267
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
= 25 mV (maximum voltage in the
±
25 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. 268)
into the CR800. It is already programmed with the
excitation voltage from step 3.
b. Place the PRT in an ice bath (0 °C).
c. Measure the PRT. If you are doing a dry run, assume the result of
BrHalf3W() = X
0
. = 0.01000
d. Calculate RS
0
RS
0
= X
0
• Rf = 100000 mΩ
Wow! We are lucky to have a perfect PRT! In the real world, PRT
resistance at 0 °C will probably land on either side of 100 Ω.
5. Measure the sensor:
If you are doing a dry run, assume the temperature is 10 °C.
a. Enter CRBasic EXAMPLE: PT100 BrHalf3W() Three-Wire Half-Bridge
Measurement
(p. 268)
into the CR800. It is already programmed with the
excitation voltage from step 3 and RS
0
from step 4.
b. Place PT100 in medium to measure.
c. Measure with
BrHalf3W()
. If you are doing a dry run, assume the result
of BrHalf3W() = X
0
= 0.01039.