Campbell CR3000 Micrologger, Operator'S Manual

Page: 298 / 644

Section 7. Installation
298
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 Resistance() Full-Bridge Calibration
(p.
299)
into the CR3000. It is already programmed with the excitation current
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
Resistance()
= X
0
. =
–9785.
d. Calculate RS
0
:
RS
0
= ((
–R2 • R4) – (X
0
• (R1 + R2 + R4))) / (X
0
–
R1) = 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 Resistance() Full-Bridge Measurement
(p. 299)
into the CR3000. It is already programmed with the excitation current
and RS
0
from step 4.
b. Place PT100 in medium to measure.
c. Measure with Resistance(). If you are doing a dry run, assume the result
of Resistance()
= X
10
=
–7874.
d. Calculate RS
10
:
RS
10
= ((
–R2 • R4) – (X
10
• (R1 + R2 + R4))) / (X
10
–
R1) = 103900 mΩ
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. 605).
4
Bridge resistance: RB
min
= X
–40
/ ((RS
–40
/ (RS
–40
+ R4)) – (R2 / (R1 + R2)))