Thermocouples are among the most popular temperature measurement instruments in industrial applications because of their versatility and ease of use. However, measurement errors can occur. This article discusses the six most common causes of thermocouple errors.

Thermocouples are robust temperature measurement devices that are accurate enough for many industrial and scientific applications. Relatively inexpensive compared to other temperature measurement technologies, thermocouples are valued for their ability to measure a wide temperature range: from –200° to +1250°C (–328° to +2282°F).

Thermocouples measure temperature differentials, not absolute temperatures. Two wires, each made from a different metal, are joined at the tip. This is the measuring junction. At the other end, the wires are connected to a body of a known temperature, called the reference junction. A thermocouple works by taking the difference in voltage between the two junctions, explained by the Seebeck effect. The measured voltage is converted into a temperature unit, with the temperature reading displayed on a device or transmitted to a remote location.

Although thermocouples are reliable, temperature measurement errors can occur for various reasons. The following are the six most common causes of thermocouple measuring errors, followed by ways to rectify them:

1. Selecting the Wrong Type of Thermocouple on the Transmitter

You can run into problems if you choose the wrong type of thermocouple when inputting the settings into the transmitter during installation. This is a common error, as there are numerous types of thermocouples – types K, J, N, E, T, R, S, and B – each with a different range, accuracy, and electrical output.

Solution: Almost all thermocouples are color coded by type, so you usually just need to confirm the color of the thermocouple jacket and match the settings on the transmitter.

For a quick reference on thermocouple color codes, including international standards, download our chart.

2. Problems Related to the Thermocouple Extension Wire

If you accidentally reverse the polarity of the thermocouple lead wires, the measured temperature will be incorrect by the difference in temperature of the two ends of the leads. The problem is understandable because red is the usual color for positive charges, whereas the red wire in thermocouple cables typically contains the negative signal. This coloration is ANSI standard for thermocouples, but it is not what most people expect.

Solution: Doublecheck the connection and, if necessary, swap the thermocouple lead wires.

3. Inherent Variations in Alloys

No two batches of wires are exactly alike. As the alloy percentages vary a tiny bit during each manufacturing process, some error in thermocouple accuracy is unavoidable. Standard thermocouples get within approximately 1% of the actual temperature at the measuring junction, which is accurate enough for most applications.

Solution: Order thermocouples with special-limit wires, which can improve accuracy twofold. These wires are manufactured at the highest tolerances to ensure the fewest possible impurities and the greatest consistency in alloy ratio.

4. Temperature Variations Around the Reference Junction Connection

Because a thermocouple measures temperature differentials, any temperature fluctuations around the reference junction (cold junction), which has the known temperature, result in an erroneous temperature reading.

Solution: Make sure no fans or other sources of cooling or heating are located near the reference junction. Simple insulation can also protect the junctions from extreme temperatures.

5. Thermocouple Grounded at More Than One Location

A thermocouple should be grounded at only one location. If it is grounded at more than one location, a “ground loop” can be created with current flowing through the thermocouple from one ground to the other. This is likely to generate electromagnetic fields, which can lead to radio-frequency-interference-related problems that can impact measurement accuracy.

Solution: Ground either the transmitter (connection head) or the controller/recorder, but not both. Selecting transmitters that have internal isolation between the input, output, and ground usually provides enough isolation to eliminate a ground loop. Loop isolators are also available and can be put in the loop wiring circuit to prevent this from happening.

6. Thermocouple Age

While thermocouples are reliable temperature measurement devices, they do drift with time. Maximum exposure temperature, cyclic measurements, and frequency of the cycles affect the metallurgy with a resultant drift, usually downward. Unfortunately, this drift cannot be predicted, but 10-20°F errors are common.

Solution: The only solution is to periodically replace the thermocouple based on the user’s experience.

You can learn more about industrial temperature measurement using versatile and low-maintenance thermocouples from this WIKA reference page.

    7 Responses to
    1. Jonathan Apperley

      More often than not, people who are replacing thermocouples or working with them that end up encountering errors have mostly encountered the error due to using the wrong type of thermocouple for their application

      • Jim Vincent

        I tend to want to agree with that but it worked for a few days after I replaced it with a universal.

    2. Jim Vincent

      The pilot light went out but I did notice that it was a yellow or orange flame which I read up on it and it said that that could be from a lot of dust and debris well I’ve been doing some sanding in my garage on some wood and that was plausible. Therefore I bought another thermocouple and I changed it out and it now it doesn’t come back on but I can get the pilot light to light but it doesn’t stay on when I let go of the button

    3. Jim Vincent

      I’m wondering if the universal thermocouples aren’t all that reliable from the factory and maybe that’s it but I also remember hearing a question that never got answered correctly or properly not from this site but something about a thermocouple not being able to touch metal which makes no sense because the clip that it goes in is metal or stainless steel anyway

      • Adam DeLancey

        Hi Jim. The thermocouple can be in contact with metal without any issue. I’m assuming you are dealing with a residential hot water heater or furnace. You can check that you are generating a mV signal with a handheld meter and your thermocouple by heating the “hot” end of the thermocouple and measuring the mV output at the cold end. If it is generating a mV signal when heated the thermocouple is most likely OK and you should check other issues with your system.

    4. Eng. Imran

      I face problem with thermocouple connected on SIMENS PLC module EM231. If thermocouple ST metal head touched with earth body.. value changing itself like 400…500…550….and so on… if we not touch it works OK shows only room temperature like 38.. 40 c. What is reason ?

      • Adam DeLancey

        This sounds as though there could be multiple things to trouble shoot in order to diagnose the problem. Below are a few things to starting checking:

        1. Isolate the thermocouple from the PLC module.
        a. Check thermocouple with a thermocouple reader and measure temperature
        b. Alternatively use a multimeter and measure mV, but you will need to compensate for ambient temperature where you are taking readings from
        c. Record results and check if there is still a problem
        2. If the sensor is functioning correctly when not hooked up to PLC, attach to PLC and check readings again.
        a. Check if sensor is grounded or ungrounded
        b. If grounded there could possibly be noise from instrument interfering with readings
        i. Adding a ground wire may be a possibility
        c. If sensor is not reading correctly when attached to PLC there may be a short or secondary junction in the wiring from the sensor to the PLC

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