Why do Thermocouple Temperature Measurement Errors Occur?
A thermocouple is used for temperature measurement and control in various industrial units. The most common types of thermocouples include E, J, K, and T. According to the type, they can withstand certain temperature ranges. Thermocouples with a high temperature range find applications in various industrial units, scientific research labs, and so on. These thermocouples comprise a junction made of two welded wire legs. The temperature is measured at the junction. Any temperature change at the junction leads to voltage generation. This voltage reading is used to calculate the temperature. However, at times, there are some factors which may cause errors in temperature measurements. This post discusses these very factors.
What Causes Temperature Measurement Errors in a Thermocouple?
It is natural for any probe, machine, or equipment to malfunction due to some error, at some point in their operational life. For most times, these can be fixed, but at times, these repairs can be either expensive or such errors may lead to wastage of productive time. If we know the possible causes of such errors, we can be proactive and fix them before hand through periodic maintenance. So, here’s a list of causes of temperature management errors in a thermocouple.
- The biggest potential for error is improper location or lack of contact limiting thermal transfer to the sensing T/C junction. Careful positioning of the sensor in order measure the area of concern and control with ample and consistent contact with the surface of measurement will give the most accurate feedback to controller. Rule of thumb for embedded T/C’s is to immerse at least 5X the diameter of the probe with tight contact with the bore.(
- Some errors are just human errors based on a wrong selection decision, and this is one of the causes of temperature measurement errors. It is essential to check your transmitter settings when installing a thermocouple and check if it is compatible. This is because each thermocouple type comes with different electrical output capacities. Although thermocouples are color coded such errors happen. So, the easiest way to fix this is checking the color code on the thermocouple jacket and match it with the transmitter settings.
- Do note that a thermocouple measures differential temperatures such as changes in the current temperature, and not the absolute temperature. So, the peripheral parameters or devices, such as fans, affect the reading of a thermocouple, and this temperature variation leads to a wrong reading. The way out of this is not keeping any cooling or heating devices in the thermocouple’s periphery. Also, make sure the thermocouple junction is insulated to avoid influence by any external temperature changes such as cooling systems.
- If the thermocouple is being used for several years, the operational life may eventually reduce due to wear and tear, as well as constant cyclic temperature measurements. This is bound to happen for any device or machine. While periodic maintenance does play a role, worn out thermocouple may need to be replaced.
- If a thermocouple is grounded at multiple locations, it may create a loop of electromagnetic fields, thus leading to EMI or radio frequency related errors which can hamper the accuracy of temperature measurement.
- At times, there may be issues with the extension wires of the thermocouple. This can happen if the polarity of the lead wires is reversed or exchanged. So, this is also very much a human error, and hence it is essential to check the settings even after matching the color codes. The easiest way is to follow the ANSI standards for thermocouples.
If you are looking for thermocouple probes or wires and cables need to join them, ensure you match the thermocouple type and select the right material, such as Teflon, fiberglass, stainless steel, and so on, based on your application requirement. It is important to buy these thermocouples from trusted thermocouple suppliers like Therm-x, California. The company manufactures and supplies high-quality thermal products such as temperature sensors, controllers, and offers end-to-end solutions on temperature-related process control.