RTDs (Resistance Temperature Detectors) are temperature detectors which change their resistance when there is a change in temperature. RTDs are used for continuous temperature measurement, which shows positive temperature coefficient. When temperature increases the resistance of RTD increases.
Why is wire compensation needed in RTD?
As RTD is a resistor, the ohm meter or the resistance measuring devices are connected parallel to the lead wires of the RTD. The lead wire resistance adds to the RTD resistance as the wires got resistance and this combined resistance will be shown as the RTD resistance incorrectly. So that lead compensation techniques are used in RTD connections.
It is compulsory to use compensation technique in the industrial sector, because of the wide range of temperature measurement is needed. The nonlinearity of RTD becomes a significant problem as range got wider, so we must find a way to use low-resistance RTDs and deal with the (lesser) problem of wire resistance,
The above shown is a two-wire RTD connection which shows the wire resistance as R wire. The resistance of the lead wire will be added to the total resistance R total.
There are three connection circuits for RTD wire circuit:
We have already discussed the two-wire circuits above. Four-wire and Three-wire circuit are the compensation circuits.
Four-wire RTD circuit:
Here in the four wire RTD circuit, the current and voltage circuits are separated.
Current is supplied to through a separate circuit from the current source. This current carrying excitation wire will drop some voltage. Which voltage drop will not read in the voltmeter.
The two sense wire connects between the voltmeter and the RTD. Of course, it posses resistance and drops voltage in the sensing wire. But only a little current is drawn through the sensing wire which makes the voltage drop negligible. Thus, the resistance of the current-carrying wires are of no effect because the voltmeter never senses their voltage drops, and the resistance of the voltmeter’s sensing wires are of no effect because they carry practically zero current. Disadvantages for four-wire RTD circuit:
Need extra number of wires
Three-wire RTD circuit:
The voltmeter A measures the voltage dropped across the RTD plus the voltage dropped across the bottom current-carrying wire. Voltmeter “B” measures just the voltage dropped across the top current-carrying wire.
Thr both current carrying wire should have the same resistance so that subtracting the voltage across B from the voltage across voltmeter B give the total voltage drop across the RTD.
V RTD = V meter(A) − V meter(B)
If the resistance of the two current carrying wires is precisely same then the calculated voltage will be same as the actual RTD voltage.
Disadvantages of Three-wire RTD:
Susceptibility to self-heating error
Susceptibility to signal noise
Need for lead resistance compensation
Need for power supply.