Thermocouple

Thermocouple

-:KEY POINTS:-

1- A thermocouple is constructed of two dissimilar wires joined at one end and encased in a metal sheath.

  • 2- The other end of each wire is connected to a meter or measuring circuit.
  • 3- Heating the measuring junction of the thermocouple produces a voltage that is greater than the voltage across the reference junction.
  • 4- The difference between the two voltages is proportional to the difference in temperature and can be measured on a voltmeter.

Thermocouples measure temperature differentials, not absolute temperatures. Two wires, each made from a different metal, are joined at the tip. This is the measuring junction. At the other end, the wires are connected to a body of a known temperature, called the reference junction. A thermocouple works by taking the difference in voltage between the two junctions, explained by the Seebeck effect. The measured voltage is converted into a temperature unit, with the temperature reading displayed on a device or transmitted to a remote location.

A thermocouple is widely used in process industries for application of temperature measurement. Here, A thermocouple  is shown in figure. Size of thermocouple(length, sheath-dia) may change as per requirement/ pipe size.

These thermocouple can be used in process for temperature measurement such as-

  • 1-Pipeline fluid
  • 2-Heat ex-changers
  • 3-Vessel
  • Columns

A schematic diagram for thermocouple is shown in figure-

THERMOCOUPLE comes  with Two sensors:-

1:-Simplex Thermocouple– A simplex thermocouple Consist of single pair thermocouple element at its hot end.

2:-Duplex Thermocouple –  A duplex thermocouple consist of a pair  junctions  at its hot end instead of a single element in a single enclosure. The main reason such a configuration is to enable Redundancy.

 

There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility. All type thermocouple with figure shown.

Thermocouple Types with Standard Identification Color Code

Why used same type thermocouple cable??

Thermocouples often require specific cables called thermocouple extension cables or extension wires. Extension cables use the same material as the thermocouple, so they send the emission-motive force (EMF) accurately. X marks the spot, or in this case, the extension cable. So if you see a cable marked “JX”, for example, it is a cable for a type-J thermocouple.

Wiring Diagram

Thermocouple Connection with Transmitter

Thermocouple Connection with Indicator

Six Common Causes for Thermocouple Temperature Measurement Errors

1. Selecting the Wrong Type of Thermocouple on the Transmitter.

2. Problems Related to the Thermocouple Extension Wire.

3. Inherent Variations in Alloys.

4. Temperature Variations Around the Reference Junction Connection.

5. Thermocouple Grounded at More Than One Location.

6. Thermocouple Age.