A simple technique for fault finding a PT100 temperature probe is to measure the resistance at known temperatures.
Zero and 100°C test points are easily created.
For 0°C fill a container such as a thermos flask with crushed ice, then top up with water. After the probe has been left to settle in the flask the resistance between the white and red wires should be approximately 100 ohms.
For 100°C fill a container such as a thermos flask with boiling water. After the probe has been left to settle in the flask the resistance between the white and red wires should be approximately 138.51 ohms.
Using a Resistance Or Decade Box
A simple technique for fault finding a PT100 temperature transmitter is to simulate the PT100 probe by using resistors or a resistance decade box.
The best values to use are those that correspond to the low, midpoint and high range of the temperature transmitter configuration.
Apply the resistance to the input of the temperature transmitter and look at the current flow through the loop with an ammeter to ensure it is correct for the given value.
Insulation test
If the insulation in the probe’s sheath is breaking down it can cause the probe to give incorrect readings. Short all the sensor wires together, then with a dedicated insulation tester check between the probe sheath and the shorted wires, look for a value of greater than 20 M ohms.
