Section 7. Installation
287
PT100 BrFull() Four-Wire Full-Bridge Calibration
'This program example demonstrates the calibration of a 100-ohm PRT (PT100) in a four-wire
'full bridge with voltage excitation. See previous procedure and schematic.
'
'Declare constants and variables:
Const
R1 = 5000000
'Value of R1 bridge resistor
Const
R2 = 120000
'Value of R2 bridge resistor
Const
R4 = 5000000
'Value of R4 bridge resistor
Public
X1
'Raw output from the bridge
Public
X2
'Calculated intermediate value
Public
RS0
'Calculated PT100 resistance at 0 °C
BeginProg
Scan
(1,Sec,0,0)
'Measure X1
'BrFull(Dest,Reps,Range,DiffChan,ExChan,MeasPEx,ExmV,RevEx,RevDiff,SettlingTime,
'
Integ
,Mult,Offset)
BrFull
(X1,1,mV50,1,Vx1,1,5000,True,True,0,250,1,0)
'Calculate X2:
X2 = (X1/1000) + (R2/(R1+R2)
'Calculate RS0:
RS0 = (R4*X2) / (1-X2)
NextScan
EndProg
into the CR3000. 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
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. 288)
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.
Summary of Contents for CR3000 Micrologger
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Page 485: ...Section 8 Operation 485 8 11 2 Data Display FIGURE 110 Keyboard and Display Displaying Data ...
Page 487: ...Section 8 Operation 487 FIGURE 112 CR1000KD Real Time Custom ...
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