Thermocouple how it works | Types , advantages , Application

 THERMOCOUPLE Principle : 

The thermocouple is a device that converts thermal energy into electrical energy. Thermocouples are very simple and durable temperature sensors. Thermocouples use a junction of dissimilar metals to generate a voltage proportional to temperature. Thermocouples are based on the Seebeck effect. In 1821, a physicist T.J. Seebeck discovered that “when two conductors of dissimilar metals are joined together to form a loop and two unequal temperatures are interposed at the junctions, then an e.mf will exist between two points A and B which is primarily a function of the junction temperature”. It is known as thermoelectric effect or Seebeck effect.

Figure 1.44 illustrates Seebeck effect where two dissimilar metals A and B are used to close the loop connecting junctions at two different temperatures T1 and T2. The e.m.f. produced is found to be almost linear in temperature and very repetitive for constant materials. The e.m.f. produced by the thermocouple loop is approximately given by

seebek effect
seebeck effect

The amount of current which is produced is depending on the temperature difference between measurement and reference junction. The characteristics are two metals and attached circuit.

Types of Thermocouple :

Thermocouples come with different pairings of materials allowing for a very wide range of applications. The different compositions are standardized into thermocouple types. The different types are given letter names which are standardized across the industry.

thermocouple types
thermocouple types

Table 1.4 shows different thermocouple junction types and the normal temperature ranges. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material and whether or not it is magnetic.

Thermocouple Construction :

Figure 1.47 shows the internal construction of a typical thermocouple. The leads of the thermocouple are encased in a rigid metal sheath. The measuring junction is normally formed at the bottom of the thermocouple housing. Magnesium oxide surrounds the thermocouple wires to prevent vibration that could damage fine wires and to enhance heat transfer between measuring junction and medium surrounding the thermocouple.

construction of thermocouple
construction of thermocouple

Figure 1.48 illustrates simple thermocouple circuit. Heating the measuring  junction of the thermocouple produces a voltage which is greater than voltage across the reference junction. The difference between two voltages is proportional to the difference in temperature and it can be measured on the voltmeter (in mill volts). For ease of operator use, some voltmeters are set up to read out directly in temperature through use of electronic circuitry

simple thermocouple circuit
simple thermocouple circuit

A series of thermocouples connected together in series produces a higher voltage called thermopile. In thermopiles, all hot junctions are exposed to high temperature and all cold junctions are exposed to low temperature. The voltages of individual thermocouples add up in allowing for a larger voltage and increased power output. Thus, it increases the sensitivity of the instrumentation. Readings can approach an accuracy of 0.5%.

Applications of thermocouples

  • Two common applications of thermocouples are measuring room temperature and monitoring the presence of a pilot light in gas-fed heating appliances such as ovens and water heaters. The other applications are listed below.
  • Type S, R and K thermocouples are used extensively in steel and iron industries to monitor temperatures and chemistry throughout the steel making process.
  • Thermopiles are used for measuring the intensity of incident radiation typically, visible or infrared light which heats the hot junctions while the cold junctions are on a heat sink.
  • Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. Example: monitoring the switchgears during its heat run test.
  • Chemical production and petroleum refineries use the number of thermocouples for logging and limit testing the many temperatures associated with a process.

Advantages of thermocouples

  • It is simple in construction
  • It is inexpensive
  • It is rugged in construction
  • It has wide variety to choose for particular applications
  • It has wide temperature range
  • It has the ability to withstand mechanical and electrical stresses.

Disadvantages of thermocouples ;

  • It is highly non-linear behavior over its range of operation
  • It is capable of generating low voltage
  • It has low stability
  • Reference source is required
  • It is least sensitive.
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