As you may already know, a thermocouple is formed by joining two different metal alloys at a point called a junction. This junction is called the measuring or hot junction. The thermocouple leads are usually attached to a temperature indicator or controller. This connection point is called the reference or cold junction.
When the measuring junction is heated, a small DC voltage is generated in the thermocouple wires. The temperature controller measures the small voltage signal and converts it to a temperature reading. However, the voltage generated in the thermocouple is so small that
it is measured in milivolts. This small thermocouple signal presents one major application difficulty: Susceptibility to electrical noise prevalent in most industrial environment.
Noise Reduction Techniques
- Twist and shield (grounded foil sheath) the extension or lead wires from the thermocouple to the measurement system.
- Ground the measurement junction at the point of measurement. The grounding is typically to the inside of the stainless steel sheath that covers the actual thermocouple. The advantage of grounding the measurement junction is that the electrical noise is distributed equally on each wire of the thermocouple.
- Use a transmitter with very good common mode voltage rejection, and locate as close to the thermocouple as possible.
The above actions will substantially reduce the susceptibility of thermocouples to electrical noise in ‘’noisy’’ industrial environment.