OPTION : Constant Current Sources
43
3.2.2 Five-Wire Resistance Measurements
This method is used when there is a requirement for a wider range than is
possible using the four-wire technique when measuring temperature with an
RTD (Pt-100).
By using the "differentiality" of the analogue channels a base level resistance
(like 100
Ω
of the Pt-100 at 0°C) can be subtracted.
This is how to do it shown in schematic form:
0.500mA
Constant Current
0.500mA
Constant Current
I
I
R1
(Pt-100)
R2
(100)
-
+
U=R1*I-R2*I
GND
2I
AAC-2
The plus input of the channel measures the voltage developed across the
temperature sensor (R1).
The minus input measures the voltage developed by a similar current source
across a reference resistor (R2).
The lead return resistance is with this method elegantly compensated for as it is
included in both the minus and plus signal and thus subtracted from the sensor
measurement.
How to calculate temperature from resultant voltages using different values for
R2 is common high school mathematics and is left to the operator. We
recommend the use of Pt-100 tables however.
Apply the best fit straight line (equation) transform to the temperature range that
is to be recorded. This will give best and most accurate readings. A very good
accuracy can be achieved if the constant current sources and the reference
resistor are calibrated.
NOTE also that the two current sources needed for each sensor must be ”in
step”. Use for example the pairs 1 and 9, or 2 and 10, or 3 and 11, and so on.
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