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+
... +anTn
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 Th.
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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