Campbell Scientific 4WPB100, 4WPB1K PRT Terminal Input Modules - Manual (Page 11)

4WPB100, 4WPB1K PRT Bridge Terminal Input Modules
7
4.5 CR1000
'CR1000 Series Datalogger
Public Rs_R0, Temp_C
DataTable (Hourly,True,-1)
DataInterval
(0,60,Min,0)
Average (1,Temp_C,IEEE4,0)
EndTable
BeginProg
Scan
(1,Sec,0,0)
BrHalf4W (Rs_R0,1,mV25,mV25,1,Vx1,1,2035,True ,True ,0,250,1.0,0)
PRT (Temp_C,1,Rs_R0,1.0,0)
CallTable Hourly
NextScan
EndProg
5. PRT in 4 Wire Half Bridge
A 4 wire half bridge is the best choice for accuracy where the Platinum
Resistance Thermometer (PRT) is separated from other bridge completion
resistors by a lead length having more than a few thousandths of an Ohm
resistance. Four wires to the sensor allow one set of wires to carry the
excitation current and a separate set of sense wires that allow the voltage
across the PRT to be measured without the effect of any voltage drop in the
excitation leads.
Figure 2-1 shows the circuit used to measure the PRT. The 10 kOhm resistor
allows the use of a high excitation voltage and low voltage ranges on the
measurements. This insures that noise in the excitation does not have an effect
on signal noise and that self heating of the PRT due to excitation is kept to a
minimum. Because the fixed resistor (R
f
) and the PRT (R
s
) have
approximately the same resistance, the differential measurement of the voltage
drop across the PRT can be made on the same range as the differential
measurement of the voltage drop across R
f
.
The result of the four wire half bridge Instruction is:
V
V
2
1
the voltage drop is equal to the current (I), times the resistance thus:
V
V
I R
I R
R
R
s
f
s
f
2
1
=
⋅
⋅
=