Campbell CR800 Series - Page 272

Section 7. Installation
272
c. Measure the PRT. If you are doing a dry run, assume the result of
BrFull() = X
0
= 0.
d. Calculate RS
0
X2
0
= (X
0
/ 1000) + (R2 / (R1 + R2) = 0.01961
RS
0
= (R4 • X2
0
) / (1 – X2
0
) = 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 BrFull() Four-Wire Full-Bridge
Measurement
(p. 273) 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 BrFull(). If you are doing a dry run, assume the result of
Resistance() = X
10
= 0.7491.
d. Calculate RS
10
:
X2
10
= (X
10
/ 1000) + (R2 / (R1 + R2) = 0.02036
RS
10
= (R4 • X2
10
) / (1 – X2
10
) = 103900
6. Calculate RS
10
/RS
0
, K, and temperature:
a. RS
10
/RS
0
= 1.039
b. K = (RS
10
/RS
0
)-1 = 0.039
c. T = g * K^4 + h * K^3 + i * K^2 + j * K = 9.99 °C
d. T = (SQRT(d * (RS
10
/RS
0
) + e) - a) / f = 9.99 °C
1
A Campbell Scientific terminal-input module (TIM) can be used to complete the resistive bridge
circuit. Refer to the appendix Passive-Signal Conditioners — List
(p. 563).
4
Get this value from a PRT-resistance-to-temperature table