How To Check A Faulty Temperature Sensor?

MI thermocouple with a head

Sometimes sensors can break or degrade in harsh environments. You might be suffering from intermittent thermocouple failure or instruments might be showing “Open Circuit”. Therefore, to help you to check a sensor, we created a guide to check issues with thermocouples and resistance thermometers to diagnose various failure modes to help you rectify the situation.

Troubleshooting a faulty temperature sensor steps:

  • Check -ve and +ve leads are correct
  • Check you are using the correct type of cable
  • Check for local heat sources affecting your readings
  • Check setup of your temperature controller/readout
  • Resolve out of range errors
  • Resolve Sensor Break/Open circuit errors
  • Physically inspect sensor for damage
  • Check temperature transmitter
  • Test faulty thermocouple with a multimeter
  • Test faulty resistance thermometer with a multimeter
  • Contact the experts 01246 261999 /

How to test a faulty thermocouple or resistance thermometer?

The first indication that a thermocouple or pt100 sensor is not working correctly will be from the instrument it is connected to. The readings might appear lower or higher than expected or the instrument will be showing an error code.

Instruments connected to a thermocouple or pt100 will often
be a temperature controller or a readout.

Different errors on a controller will mean different things depending on the brand and model. Check your temperature controller manual to make sure it isn’t an error with the controller itself.

Check if the -ve and +ve leads swapped

If your temperature sensor is a resistance thermometer swapping the -ve and +ve leads will not influence the reading.

Thermocouple readings do change if leads are not correctly connected.
If -ve and +ve leads are swapped between a thermocouple and an instrument then the reading will change relative to the ambient temperature e.g. a reading of 100°C with an ambient temperature of 25°C will read as 75°C, if the leads are not correctly connected. Ensure that the -ve thermocouple lead and +ve thermocouple lead are wired correctly to your instrument.

Check for any thermocouple cable wiring issues

Incorrectly wired compensating cable

Incorrect wiring of thermocouple cables can be further complicated because thermocouples often use compensating cable. It is possible that the compensating cable is incorrectly wired to the thermocouple. It may appear correctly wired at the instrument but not correct between the thermocouple and compensating cable. To avoid this, make sure -ve and +ve legs match correctly along the full length of the thermocouple, to the compensating cable, to the instrument.

Using the wrong compensating type

Each compensating cable is also tied to a specific thermocouple type. To ensure accuracy you must only use compensating cable of the same type e.g. a type K thermocouple could only be extended with type k compensating cable. Sometimes other types of cable such as general instrument cable, or copper cable is used to extend thermocouples and this also creates errors.

Using compensating cable in high-temperature environments

The final quirk with compensating cable is that it should not be used in high temperature environments. Thermocouple compensating cable is rated for a lower temperature than thermocouple cable. Try to use compensating cable in ambient temperatures to reduce errors.

Check for local heat source issues

If your thermocouple is reading high, it could be because your sensor is located too close to a heat source. If your sensor is near a local heat source it could increase the temperature being measured as opposed to the position you wish to measure temperature.

Check temperature transmitter settings

If your sensor is using a temperature transmitter it is important to check that the output of the transmitter and the input of the instrument is the same. Most commonly a signal range of 4…20 mA is used.

Check temperature controller settings

Check input settings on temperature controller

For example, if you have your temperature controller setup for Type S input but attach a Type B thermocouple sensor it would display an incorrect temperature and lead you to believe your sensor was faulty. You will need to make sure the input selected on the controller matches the type of thermocouple or resistance thermometer connected to it.

There are also a couple of controller errors that point to a fault in the temperature sensor.

Instrument shows: Input signal higher or lower than limit (Out of range error)

The controller could be displaying several things indicating that it believes there is an out of range error.

Each brand and model of controller is different so look in your temperature controller manual to find out how an out of range error will show on your specific model.

If you have an out of range error the input the temperature sensor is providing is out of the expected range according to the configured input on the controller.

For example, if you had your controller setup for Type S thermocouple input it would expect a small range of mV. If you then connected a Type E thermocouple, a thermocouple with a much higher mV range, at 300°C it would show an out-of-range error.

Instrument shows: Sensor Break Error (Open Circuit)

If the controller shows an open circuit error the connection to the sensor is not complete. This could mean either one of the sensor terminals is not connected to the controller or there is a break somewhere in the sensor.

Each brand and model of controller is different so look in your temperature controller manual to find out how a sensor break error will show on your specific model.

If you have checked that the sensor is properly connected to the controller and you still get an open circuit error you will need to disconnect the sensor and remove it from your process to further test the broken thermocouple or broken thermometer for faults.

Conduct a visual check of the temperature sensor

After removing the sensors from your process, visually check for damage along the length of the sheath and at the measuring point. On a thermocouple you will have a measuring junction at the tip, on a resistance thermometer you will have an element at the tip e.g. a pt100 element.

If there are any breaks along the sheath or the tip of the sensor is broken it will probably be the cause of the fault. If visually nothing appears to be wrong, you can verify any internal breaks with a multimeter.

Troubleshooting temperature sensors with a multimeter

In-head thermocouple multimeter check

One check you can carry out with a thermocouple is if you short the +ve and -ve wires in a terminal connection head, if the instrument and any compensating cables are setup correctly the instrument will give an ambient reading.

Testing a faulty thermocouple with a multimeter

Get your faulty thermocouple and a multimeter.

Change your multimeter to Continuity beeper/diode symbol/flow of current. Some multimeters will beep if there is a break in the circuit, some multimeters will beep when the circuit is complete. Know how your multimeter functions.

Diode Symbol: Diode symbol

Two multimeters showing the diode/continuity beeper setting selected:

Multimetter diode flow of current

Attach each leg of the thermocouple to the multimeter. It doesn’t matter which way the positive and negative leg are.

After connecting the thermocouple to the multimeter a working thermocouple will produce a beep (or no beep if your multimeter works the opposite way). The screen will display resistance in Ohms, which could be zero. If there is a break it will display OL or similar.

If you are experiencing intermittent thermocouple failure you can adjust the position of the sensor and its leads to see if you can trigger the fault.

Testing a faulty resistance thermometer with a multimeter

In this section, we will explain how to test RTD with a multimeter. So, after removing the faulty resistance thermometer, get your multimeter and change it to the Ohms setting (Ω).

Two multimeters showing the ohms (Ω) setting selected:

Two multimeter ohms

Next connect the resistance thermometer to your multimeter. It doesn’t matter which way around the positive and negative legs are. Some resistance thermometers have 3 or 4 wires. Make sure you connect the wires of a similar colour together. E.g White and white connect to one terminal and red and red connect to the other terminal.

Four wire resistance thermometer connected to multimeter clips. Showing white-white and red-red leads connected together:

4 wire multimeter

If your resistance thermometer is connected to the multimeter and it is reading OL there is a possible break in the sensor. You can move any flexible cable or tails around to try and trigger any intermittent faults. In a working resistance thermometer, the multimeter would read an ohms value, an example table is below:

Ohms reading at Room Temperature (20-25°C) Resistance Thermometer Type
107.793-109.735Ω Pt100
538.967-548.673 Ω Pt500
1077.935-1097.347Ω Pt1000
~0Ω Broken sensor or not a resistance thermometer

Tolerance to these values would depend on the accuracy of your resistance thermometer and the actual temperature in your room.

Sample resistance thermometer reading 109.7 ohms which is around 25°C:

Resistance thermometer ohms

Contact Peak Sensors

The above guide is aimed at helping you identify problems with your resistance thermometers and thermocouples. Peak Sensors are experts in temperature sensors and a shortcut to solving your RTD or thermocouple problems is to contact us. We can help solve your temperature sensor problems and either repair or manufacture replacement sensors.

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