PT100 wiring: use different wires joined together and cable length considerations

Question

I am trying to convert a fridge into an incubator for turtle eggs. I would like to make as little modifications to the fridge as possible (i.e: avoid drilling holes through the styrofoam) so that is why I would like to reuse the original fridge wirings.

I have been able to identify the connections between the rear (exterior) of the fridge (where all the electronic boards are placed) and the various connectors inside the fridge. The idea is to take advantage of the existing temperature sensor wirings and replace the original electronic boards with a PID-RS-S-48 controller in combination with a PT-100.

Considering I will need to control the temperature in the 20-30 °C range, is there any drawback in possibly using different wires (different section, material etc.) in regard to accuracy? In this case I would need to use 2/3 different wires connected together (the PT100, the internal fridge wirings and the wirings from the back of the fridge to the PID).

Moreover, should I be worried of the offset caused by the wiring itself (roughly 2/3 m total wire length)?

Please, let me know if anything is not clear and I will try to be more specific. Any comment is greatly appreciated.

Answer 1

Pt100 sensors are platinum (Pt) resistors with a resistance of 100 Ω at 0°C rising to (from memory) 138 Ω at 100°C. That means that every 1 Ω of wiring resistance will add a 3°C error to your readout.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A 3-wire sensor.

The problem can be solved using 3- or 4-wire sensing circuits. A known current (say 1 mA) is run between on of the A wires and B. The other A wire is used to measure the voltage drop between the two A terminals and by subtracting twice this value from the voltage measured across the current terminals the voltage across the sensing element can be calculated.

What's important in this setup is that the same wire length and type are used in the A and B current feed.

For your application you may find that your cable resistance is so low that the 2-wire measurement will have an acceptable error.

Answer 2

Since you are using a commercial controller, it is likely a 3-wire type. So you need only be concerned with resistance differences in the wires (actually two of the three wires, but let's ignore that).

Since your wire length is so short, if the wires are of reasonably heavy gauge (eg. AWG18 or heavier), simply ensuring that they are all the same gauge and similar length should be more than sufficient.