Thermoelectric-Effect Instruments (Thermocouples)

About thermoelectric-effect instruments or thermocouples

Thermoelectric-effect instruments rely on the physical principle that, when any two different metals are connected together, an e.m.f., which is a function of the temperature, is generated at the junction between the metals. The general form of this relationship is:

e = a₁T+a₂T²+a₃T³+ ... +a_nTⁿ
 

This is clearly non-linear, which is inconvenient for measurement applications. Fortunately, for certain pairs of materials, the terms involving squared and higher powers of T (a₂T², a₃T³, etc.) are approximately zero and the e.m.f.– temperature relationship is approximately linear according to:

e ≈ a₁T

Wires of such pairs of materials are connected together at one end, and in this form are known as thermocouples. Thermocouples are a very important class of device as they provide the most commonly used method of measuring temperatures in industry.

Thermocouples are manufactured from various combinations of the base metals copper and iron, the base-metal alloys of alumel (Ni/Mn/Al/Si), chromel (Ni/Cr), constantan (Cu/Ni), nicrosil (Ni/Cr/Si) and nisil (Ni/Si/Mn), the noble metals platinum and tungsten, and the noble-metal alloys of platinum/rhodium and tungsten/rhenium. Only certain combinations of these are used as thermocouples and each standard combination is known by an internationally recognized type letter; for instance, type K is chromel-alumel.

Certain of these show reasonable linearity over short temperature ranges and their characteristic can therefore be approximated by a series of straight-line relationships for use in intelligent instruments containing thermocouples. In general, however, the temperature indicated by a given e. m. f. output measurement has to be calculated from tables. The set of these tables corresponding to all the standard types of thermocouple available are known as thermocouple tables.

A typical thermocouple, made from one chromel wire and one constantan wire. For analysis purposes, it is useful to represent the thermocouple by its equivalent electrical circuit. The e.m.f. generated at the junction is represented by a voltage source, E₁, and it is customary also to show the temperature of the junction on the diagram. This is known as the hot junction and is represented by T_h. The junction between the open ends of the thermocouple and the voltage measuring instrument is known as the reference junction, and thermocouple tables are calibrated assuming that this is maintained at a temperature of 0°C. This is achieved by immersing the reference junction in an ice bath.

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