It is important to know thermocouple malfunctioning in order to ensure the safety of the process. The quality of the product and the machine’s safety depends on the proper functioning of the thermocouple.
Let us take an example, a thermocouple used for the furnace to control the fuel to it. What happens if the thermocouple does not read correctly or goes bad. The furnace temperature may increase if we feed more fuel when the thermocouple does not work properly. If the furnace is of electric type, then the thermocouple must cut off the power supply at the set temperature.
The malfunctioning of the thermocouple can cause catastrophic damage to the process and equipment. Therefore, it is a must to do preventive maintenance and testing for a longer life of the thermocouple.
How Does a Thermocouple Work?
A thermocouple is a temperature measuring sensor. It consists of two dissimilar metals joined together at one end. The point where two metals join forms a junction called a hot junction. When the hot junction receives heat, the voltage produces at the open ends( Cold junction) of the thermocouple. This phenomenon is called the Seebeck effect.
The voltage produced at the open ends of the thermocouple is proportional to the difference between the hot and cold junction.
How to find if a thermocouple is working
Step 1: Inspect the thermocouple
Visually inspect the thermocouple for defects. The followings are the symptoms of a defective thermocouple.
Failed Thermocouple Symptoms
Contamination on the tube
The sign of contamination on the tube or thermocouple jacket is the symptom of the defective thermocouple. The sign of discoloration, cracks, or pinholes is the symptom that creates doubt about defective thermocouples.
Missing Insulation of Thermocouple Wiring
The tear of insulation on the thermocouple wire is the symptom of bad insulation of wires. The wire’s insulation resistance must be high for the reliable functioning of the thermocouple. Therefore, completely inspect the thermocouple wiring for wear and tear.
Faulty Connector
The connector connects the thermocouple leads to the temperature transmitter or thermocouple compensation cables. The connection looseness in the connector can cause the thermocouple malfunctioning. The damage to the connector also affects the reliable functioning of the thermocouple.
Step 2: Open circuit test of the thermocouple
Follow the procedure to do the open circuit test on the thermocouple.
- Remove the thermocouple from its mounting by unscrewing the connection nut and then bracket nuts.
- Now take a multi-meter and set it to ohm. Connect the meter leads to the thermocouple leads. The meter must read zero value. The zero resistance shows that both metals of the thermocouple at the hot junction are joined together. If the resistance value is infinite, then it shows that the thermocouple is open. If it shows a high resistance value, then the junction has developed contact resistance due to corrosion.
- Now to test the actual working of the thermocouple put the tip of the thermocouple into flame and leave it for some time( at least 45 seconds) for its junction heating.
- Next, attach the multimeter leads to the thermocouple. Connect one lead on the side of the thermocouple, and attach the other lead at the end of the thermocouple.
- The multimeter should read 25-30 mVDC. It shows that the thermocouple is functional.
- If the voltage reading is less than 25mVDC, replace the thermocouple.