What is a thermocouple?
A thermocouple is an electrical instrument used to measure temperature. It consists of two wires made of different metals which are joined together at one end, known as the measurement junction. The other end, where the wires are separate, is connected to signal conditioning circuitry traces, usually made of copper. This junction between the thermocouple metals and the copper traces is called the reference junction.
Avoiding mistakes while using a thermocouple
Thermocouples are the industry-standard method for measuring a wide range of temperatures, from boilers and ovens to aircraft engines. However, they are notoriously difficult to operate because of their complex behaviour. Here are five mistakes to avoid while using a thermocouple, in order to get an accurate reading:
- Selecting the wrong type of thermocouple: There are many types of thermocouples- types K, J, N, E, T, R, S and B- with different ranges, accuracies and electrical outputs. It is vital to choose the right type of thermocouple when setting the transmitter. Fortunately, thermocouples are usually colour-coded by type, so one would only have to note the colour of the thermocouple jacket and match the settings on the transmitter.
- Intrinsic differences in alloys: No two pairs of thermocouple wires have the same composition. Alloy composition ratios vary during manufacturing, so some inaccuracy is unavoidable. Most thermocouples are accuracy within 1% of the actual temperature, which is good enough for most cases. However, if 100% accuracy is essential, thermocouples with special-limit wires should be used. Special-limit wires have negligible impurity and their alloy ratios are consistent.
- Thermocouple grounded at more than one location: A thermocouple should be grounded only at one location. Otherwise, there is a risk of a “ground loop” being created, where current will keep flowing through the thermocouple from one ground to another. This could create electromagnetic fields that could hamper accuracy. To avoid this, either the transmitter or the controller should be grounded, but never both.
- Using old thermocouples: Over time, thermocouples lose their accuracy. Exposure to differing temperatures, cyclic measurements and frequency of the cycles affect the metallurgy of the thermocouples, which may lead to inaccuracies up to 20°F. To avoid this, thermocouples should be replaced after regular periods.
- Not using right type of Thermocouple Cable : Temperature is displayed by Indicator, Controller, Data Logger ..etc , which are located in Control rooms , far from Thermocouples. Cables are required to connect Thermocouples to the measuring Instruments. Use of Normal Copper cables causes large error. To avoid this Thermocouple Extension Cables and Thermocouple Compensating cables are preferred.
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