Campbell CR3000 Micrologger, Руководство оператора

Страница: 279 / 644

Section 7. Installation
279
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 la
nd 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 BrHalf4W() Four-Wire Half-Bridge
Measurement
(p. 280)
into the CR3000. 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 BrHalf4W(). If you are doing a dry run, assume the result
of Resistance()
= X
10
= 1.039.
d. Calculate RS
10
:
RS
10
= X
10
• Rf = 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. 605).
2
The magnitude of the excitation voltage does not matter in mathematical terms because the result of
the measurement is a ratio rather than an absolute magnitude, but it does matter in terms of reducing
the effect of electromagnetic noise and of losing of resolution. A maximum excitation helps drown
out noise. A minimum input-voltage range helps preserve resolution.
3
Get this value from a PRT resistance-to-temperature table