ABOUT THERMOCOUPLES
Measuring temperature

The most commonly used type of temperature measuring instruments in most production processes, both due to the very wide temperature ranges covered by different types and relatively low price, are temperature measuring instruments with thermocouples.

A thermocouple consists of two conductors, one of which is a positive and the other negative thermoelectrode of a thermocouple. Thermoelectrodes are joined at one end, forming a thermocouple measuring junction, and they are longitudinally electrically insulated from each other.

A thermocouple is a temperature measuring instrument that works according to the principle of the Seebeck effect. Seebeck’s thermoelectric effect is a physical phenomenon of generation of an electromotive force in an electric circuit of two conductors made of different metals or alloys, the junctions of which are at different temperatures. One junction is called the measuring junction of a thermocouple and represents the junction of two thermoelectrodes of a thermocouple which is exposed to temperature being measured. Second junction is a reference junction of a thermocouple and represents a junction of two thermoelectrodes that is placed at a known – reference temperature of 0°C which the measured temperature is compared to.

Complete measuring circuit consists of a thermocouple, compensating line, possibly compensating bridge, copper line with resistance for calibration and temperature indicator. The measuring circuit is shown in the following figure.
MEASURING CIRCUIT

NOTE: ALL ELEMENTS OF THE MEASURING CIRCUIT MUST BE OF THE SAME TYPE.

Thermocouples that are installed in industrial production facilities are a typical example of a demountable construction design. Measuring voltage signal of a thermocouple is fed to the converter, i.e. indicating (or recording) device for reading the temperature through compensation lines, often several tens or even hundreds of meters long. Indicating devices are usually specially adjusted mV-meters, analog or, lately, most often digital, with special processing units for ambient temperature compensation and conversion of a thermocouple voltage signal into the appropriate temperature. If, on the other hand, extension lines are too long and a measurement signal is weakened, voltage-current converters (transmitters) installed at the output of a thermocouple connection head are used so that the signal is further conducted as a current, and the indicating device in that case represents a current- temperature converter. Most often, these indicating (or recording) devices have built-in control output units for feedback control and temperature regulation.

Today, a huge number of different types of thermocouples are produced worldwide, depending on the materials used for thermoelectrodes. However, due to their characteristics of sensitivity, stability and reproducibility, only certain types of thermocouples are standardized internationally and they are shown in the following table.
Internationally standardized and in our country prescribed types of thermocouples:

Given that the colors for identifying thermocouples are not standardized at the international level, and almost every country has kept its own marking system, for easier and safe determination and recognition of the type of thermocouple, in the section of the catalog – Guidelines – an overview of International colors for identification in the technique of temperature measurement is given.

Due to their characteristics in terms of accuracy and stability even at high temperatures, thermocouples based on precious metals, i.e. S, R and B type thermocouples, occupy a special place. Widely used in industry is the much cheaper K-type thermocouple, but only in cases where requirements regarding the level of error are not so strict.

Thermocouples, depending on measurement error and measurement range, are classified into three accuracy classes in which the measurement errors must be within the limits of the permissible error, namely:

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