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How to Select a Thermocouple?

What is thermocouple?

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit (the thermoelectric effect or Seebeck effect).

Need for reference junction( cold junction)

In thermocouples, voltage is developed due to flow of electric current. This current flow depends upon the difference in temperatures at the two ends of conducting wire. That is thermocouples always measure difference in temperatures and not absolute temperature.

To measure the temperature of one junction, the other junction is kept at some reference temperature. As this is done by using ice bath, it is normally called at cold junction.

Using ice bath for constant temperature is useful for laboratory calibration, but is not convenient for most measurement and control applications. Instead of ice bath, an effect of  cold junction is   added using a thermally sensitive device such as a thermistor or diode. This is also called as isothermal block. Special care is taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction is applied. This is known as cold junction compensation.

Software compensation is the most versatile technique used for measuring thermocouples. Many thermocouples can be connected on the same block. The technique is independent of the types of thermocouples. All of the conversions are performed by the computer. The disadvantage is that the computer requires additional time to calculate the reference junction temperature. For maximum speed we can use hardware compensation.

Hardware compensation can be viewed as inserting a battery that cancels the offset voltage produced by the reference junction. These commercially available circuits provide an electronic ice point reference. Their main advantage is speed while the disadvantage is that it is suited to compensate only a particular type of thermocouple.

Thermocouple properties

The selection criteria for thermocouple materials:

1.    Temperature Range

2.    Melting point

3.    Reaction to various atmospheric conditions

4.    Thermoelectric output in combination

5.    Electrical conductance

6.    Stability

7.    Interchangeability

8.    Repeatability

9.    accuracy

10. resolution

11. Cost

12. Availability

13. Chemical properties

14. Abrasion and vibration resistance

15. Installation requirements

16. Magnetic properties

17. Ease of handling and fabrication

Things to remember:

Wire Size of Thermocouple: Selecting the wire size used in the thermocouple sensor depends upon the application. Generally, when longer life is required for the higher temperatures, the larger size wires should be chosen. When sensitivity is the prime concern, the smaller sizes should be used.

Length of Thermocouple Probe: Since the effect of conduction of heat from the hot end of the thermocouple must be minimized, the thermocouple probe must have sufficient length. Unless there is sufficient immersion, readings will be low. It is suggested the thermocouple be immersed for a minimum distance equivalent to four times the outside diameter of a protection tube or well.

Location of Thermocouple: Thermocouples should always be in a position to have a definite temperature relationship to the work load. Usually, the thermocouple should be located between the work load and the heat source and be located approximately 1/3 the distance from the work load to the heat source.

Types of thermocouples based on metal combination:

Different types are best suited for different applications. 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.

Need for Thermocouple thermowell:

Thermowells are used in industrial temperature measurement to provide isolation between a temperature sensor (often a thermocouple) and the environment whose temperature is to be measured.

They are intrusive fittings and are subjected to static and dynamic fluid forces. These forces govern their design. Vortex shedding is the dominant concern as it is capable of forcing the thermowell into flow-induced resonance and consequent fatigue failure. The latter is particularly significant at high fluid velocities.

Thermowells are used to facilitate the repair of temperature sensing devices without interrupting the process being monitored.

Thermowells are available in three main barrel or shank design types. The barrel, or shank, is the container style apparatus, which is inserted into the process flow. Since thermowells are inserted directly into the process flow, the goal is to allow for measurement while causing as little restriction of flow as possible.

When selecting between the available types of thermowells, points considered are:

Stem length (length from bore to the end of the well) and bore diameter of the thermowell.

The temperature and viscosity of the media into which the thermowell will be seated.

Lagging extensions through which the sensor will have to pass.

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