Thermocouples/Compensating Cables

1.Thermocouple Measurement Principle

The thermocouple principle/theory was not established by just one person in one go. ASTM STP470 says that Alessandro Volta was a pioneer for the thermometer principle, who concluded in 1800 that touching two different metals together via a solution generated electrical potentials from the result of an experiment by Galvani where touching two different metals together generated convulsions in the legs of a frog. As other researchers who contributed to establishing the thermocouple theory, Thomas Johann Seebeck (Seebeck effect found in 1821), Jean Charles Althanase Peltier (Peltier effect found in 1834) and William Thomson (Lord Kelvin, later) (Joule-Thomson effect found in 1852) have been mentioned. Among them, the following theory is the most famous.
When both ends of two homogeneous conductors A and B, whose type differs from each other, are connected electrically to make a closed circuit like Figure-1 and the temperature T1 and T2 are provided to both ends, an electric current flows in the circuit.

Thermocouple Circuit Diagram Image Figure-1 Thermocouple Circuit Diagram

This phenomenon is generally called the Seebeck effect and it is said that T.J.Seebeck found the phenomenon in combination of bismuth - copper and bismuth - antimony in 1821. Actually as a result of a study it was found later that the Seebeck effect was a combination of the Peltier effect and the Thomson effect. The detail is not mentioned here; however, since many documents have been issued separately, please refer to them.

The electric power which causes an electric current in this circuit is called the thermo-electromotive force and it is confirmed that the polarity and size of the thermo-electromotive force are determined only with the materials (A and B) of the two types of conductors and the temperature (T1 and T2) of the junctions at both ends. Therefore, the size and length of the conductors and temperature at parts other than both ends are not related to the thermo-electromotive force.

Normally the end part, which is the opposite side of the side where the temperature is measured, is kept at a constant temperature (0°C) and the thermo-electromotive force is measured so that the temperature of the measurement target is known from the relation of the thermo-electromotive force and the temperature known in advance. The junction at the side where the temperature is measured is called a measuring junction or hot junction and another junction which is to be kept at a constant temperature at the opposite side is called a reference junction or cold junction.

When a thermocouple is actually used as a temperature sensor, the measuring junction side is set at the measuring place and the opposite side is connected to a voltmeter. However, since the reference junction temperature does not become constant as it is, the part of the reference junction is put into ice water so that it becomes 0°C or a measuring apparatus with a temperature compensation circuit integrated is used. Refer to Figure-2.

Thermocouple and Reference Junction Figure-2 Thermocouple and Reference Junction

When an compensating cable is connected to a thermocouple, the connection part is called a “compensation junction. ”Since normally the compensation junction temperature is equal to the measuring instrument terminal temperature, the error of the compensating cable is not included in the characteristics of the thermocouple. Therefore, note that if the compensation junction temperature is higher than the instrument terminal temperature, the error of the compensating cable is added to the characteristics of the thermocouple.

2.Basic Thermocouple Circuit Laws

(1) Homogeneous Circuit Laws
When the A and B element wires of a thermocouple circuit consist of homogeneous materials, even if the circuit has local heating parts such as t2 and t3, the thermo-electromotive force is determined only with the temperature t1 and t0 at both junctions.
Conversely, if they have heterogeneous parts with a temperature gradient, thermo-electromotive force which differs from the case of homogeneous materials will be generated.

Homogeneous Circuit.jpg Figure-3 Homogeneous Circuit Laws

(2) Intermediate Metal Laws
When the A and B element wires of a thermocouple circuit consist of homogeneous materials and the circuit has a third metal C inserted, if the temperatures of t2 and t3 at both ends are equal each other, the thermo-electromotive force is determined only with the temperature t1 and t0.
In an actual use example, a copper alloy is used for the terminal in the connection box; however, since the distance between the connection points is short and there is no temperature difference, an error is not generated.

Intermediate Metal.jpg Figure-4 Intermediate Metal Laws

(3) Intermediate Temperature Laws
With the diagram below, if V1 and V2 are the same type of thermocouple as each other, the thermo-electromotive force is V1 when the temperatures of both junctions are t1 and t2, and the thermo-electromotive force is V2 when the temperatures are t2 and t0, and the thermo-electromotive force is VAB when the temperatures are t1 and t0.
In an actual application example, it is equivalent case where the thermocouple installed at site and compensation cables are connected from the terminal box of the field installation, or a case where the thermo-electromotive force is obtained by calculation.
Since the characteristics are equal, when compensation cables are connected, measuring is possible; however, since it is not the same characteristic completely, if the connection temperature becomes higher, the compensation cables errors are added.

Intermediate Temperature.jpg Figure-5 Intermediate Temperature Laws

[Case]
For small outer diameter sheathed thermocouples, products with approx. 50 mm below a sleeve exist. However, unexpected errors may occur when this product is used at a temperature of 100 ° C. or higher or temperature test is performed.

[Factor]
When the product is inserted into a testing tank, the sleeve part (compensation junction) is overheated with radiant heat from the testing tank liquid surface and the compensation cables errors are added.

[Measures]
For products with a short insertion length, the products are manufactured with a length which allows inspection and they are processed to the specified length after intermediate inspection. Therefore, there are some cases where reinspection can not be done, so be careful. If correct temperature measurement and reinspection are required, the sheath length must be 150 mm or longer.

Short Sheathed Thermocouple Calibration.jpg Figure-6 Testing Tank

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