Thermocouples (thermoelectric sensors) are temperature sensors that consist of two wires made of different metals joined together at one end to form a measuring weld. The free ends of the thermocouple measure the voltage that arises when the weld is heated or cooled. It is proportional to the temperature difference between the measuring weld and the free ends - the terminals.
Thermocouples are widely used in many industrial applications because they are robust, relatively inexpensive, have sufficient accuracy for most applications and do not require an external power supply. Thermocouple sensors are also valued for their wide measurement range - depending on the materials used, a thermocouple can measure temperatures from -200°C to +1250°C. However, for a variety of reasons, thermocouple measuring errors can occur.
The simplest way to check that a thermocouple is working correctly is to simulate a measurement using a millivolt generator that plugs directly into the indicating device. Set the values given in the voltage series and compare the readings with the assigned temperatures. This is the quickest way to determine whether the set linearisation or measurement range has been selected correctly.
For a quick test, isolate a piece of compensation wire, twist the wires at one end and plug in the indicating device. The twisted wires form a thermocouple with which, by heating the joint, you can check the basic operation of the device.
If it is not working properly, check thermocouple failure reasons:
T hermocouple problem: We often encounter that the wrong type of thermocouple has been selected on the transmitter. This type of error happens because there are many different types of these sensors, e.g. type K, type J or type S thermocouples for high temperatures.
You need to be careful as each has a different range, accuracy and electrical output. Choosing the wrong type of thermocouple when making settings to the transmitter during installation can cause erroneous measurements.
Solution: different types of thermocouple sensors are colour coded accordingly. Always check the colour code on the thermocouple jacket before setting up the transmitter.
Thermokoppel probleem : als een thermokoppel op meer dan één plaats wordt geaard, kan er een negatief verschijnsel optreden dat 'aardlus' genoemd wordt. Hierbij stroomt er stroom door het thermokoppel van de ene aarde naar de andere, wat resulteert in elektromagnetische velden. Deze velden interfereren met radiofrequenties, wat weer van invloed is op de nauwkeurigheid van de temperatuurmeting.
Oplossing: aard ofwel de zender (aansluitkop) of de regelaar/recorder, maar niet allebei tegelijk.
Thermocouple problem: The most common error when connecting a thermocouple is that the polarity of the lead wires is reversed or altered. In this case, the measured temperature will be incorrect due to the temperature difference between the two ends of the leads.
Such mistakes happen because intuitively expect the red wire to be positive. However, red wire in thermocouple cables typically are negative.
Solution: Double-check the thermocouple connection and, if necessary, swap the thermocouple lead wire
Problem: Keep in mind that thermocouple measures temperature differentials in actual temperature, not the absolute temperature. Therefore, nearby devices such as fans can cause temperature fluctuations and, therefore, erroneous readings.
Solution: Do not keep fans or other sources of cooling or heating near the reference junction (cold junction). Also make sure the thermocouple connector is insulated, as this helps protect it from extreme temperatures.
Thermocouple problem: The percentage of metal alloys from which thermocouple wires are made can vary slightly from batch to batch. Some errors in the accuracy of thermocouple sensors, due to the manufacturing process, are inevitable. Typical accuracy is ±1-2°C, which is sufficient for most industrial applications.
Solution: If your application requires exceptionally high accuracy, order thermocouples with wires of special limit . They are manufactured to the highest tolerances to keep impurities as low as possible and consistency in alloy proportions as high as possible.
Also take care to buy thermocouples from trusted suppliers. At JUMO, we take care to control the quality of our thermocouple sensors in order to reduce the occurrence of wire alloy problems as much as possible.
Thermocouple problem: Like any device or machine, a thermocouple used for several to several years wears out. These devices are exposed to extreme temperatures, pressures and other forces that inevitably cause them to age.
Solution: Replace the worn-out thermocouple with a new one. Regular maintenance helps to slow down the ageing process of the measuring equipment