Campbell CR800 Series, Руководство оператора

Страница: 264 / 598

Section 7. Installation
264
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. 265)
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
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. 563).
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
CRBasic Programs and Notes
PT100 BrHalf4W() Four-Wire Half-Bridge Calibration
'This program example demonstrates the calibration of a 100-ohm PRT (PT100) in a four-wire
'half bridge with voltage excitation. See adjacent procedure and schematic.
'Declare constants and variables:
Const Rf = 100000 'Value of bridge resistor
Public X 'Raw output from the bridge
Public RS0 'Calculated PT100 resistance at 0 °C
BeginProg
Scan (1,Sec,0,0)
....'Measure X:
'BrHalf4W(Dest,Reps,Range1,Range2,DiffChan,ExChan,MeasPEx,ExmV,RevEx,RevDiff,
' SettlingTime,Integ,Mult,Offset)
BrHalf4W (X,1,mV25,mV25,1,Vx1,1,2053,True,True,0,250,1,0)
'Calculate RS0:
RS0 = X * Rf
NextScan
EndProg