A thermal probe connected in LCD display is embedded with a PT100 probe
The PT100 can be accessed directly or after a length of coiled wire
Connecting the ohmmeter to the PT100 probe
109.5, the PT100 is accurate (platinum) 0.385 Ohm / ° C
Error = 1.3 Ohm or almost 4 ° C for a very precise PT100 component: mess
3-wire method: R is subtracted from 1 of the 2 wires and the double
110.5 Ohm - 1.12 Ohm (0.56 * 2) = 109.3 Ohm
Actual measurement with R = U / I. 1.096 V / 1.006 mA = 109.5 Ohm
Live: 108.5 or 1% error which is much on a PT100 ...
4-wire measurement: 2-wire for power supply, 2-wire for voltage measurement
I insert 1 Ohm in the circuit of the voltmeter to respect the equivalence of the lengths
It does not change anything! Because if the wires that power the PT100 are crossed by 1 mA
And therefore 1 mA * 1 ohm of wires = 1 mV of error! On the other hand the branch of the ...
0.1 Volt / 10 MegaOhm = 0.01 μA, ie 1 Ohm * 0.01 μA = 0.01 μVolt of error!
The error thus passes from 1% to 0.000 001%!
The high impedance of the voltmeter cancels any error
3-wire measurement
The yellow voltmeter in parallel on 1 of the 2 wires measures the error of a wire
The double is here subtracted here from 0.53 mV * 2 to the value in green 109.5
There are 108.4 mV. The advantage of this method is the saving of a thread
As in 4 wires, the resistance in series with the 3rd wire does not pose any significant error
Only important point: the wires must be identical! Inevitably the case with a cable
Here are the differences between the 2, 3 and 4 wire measurements!