Thermocouple Principle

The most measured physical quantities is temperature hence measuring it precisely is of vital importance.

So many critical factors can be affected by the precision and frequency with which temperature is measured. A diverse range of factors are highly influenced by temperature; plant and equipment life span, fuel resources, process efficiency etc. Hence temperature measurement accurately and repeatedly is essential.

The difference between a resistance thermometer and a thermocouple is that a thermocouple is generally characterized to be rugged and versatile and a larger range of temperature measurement. While resistance thermometer permits superior measurement accuracy and stability.

Both possess the crucial characteristic that their outputs are in form of electrical signals, this allows the signals to be easily transmitted, switched, displayed and recorded with further technology.

Even though this is old technology recent advances have been made, for example the introduction of Type N thermocouple. With the main aim to provide customers with superior products delivering greater stability and precision. Additionally smart sensor to detect rate of degradation and failure

Thermocouples, Resistance Thermometers and Thermistors are in effect electrical temperature transducers and not direct-indicating thermometers such as mercury-in-glass devices.

In the majority of industrial and laboratory processes, the measurement point is usually remote from the indicating or controlling instrument. This may be due to necessity i.e. an adverse environment or convenience i.e. centralised data acquisition.

Devices are required which convert temperature into another form of signal, usually electrical and most commonly, Thermocouples, Resistance Thermometers and thermistors.

Alternative and in-direct techniques for sensing and measuring temperature include optical pryometry, other non-contract (infra-red), fibre-optic and quartz oscillation systems.

Use of thermocouples, resistance thermometers and thermistors requires some form of physical contact with the medium. Such contact can be emersion or surface depending on the sensor construction and the application.

Resistance Thermometer:

Utilise a precision resistor, the Ohms value of which increases with temperature (in the case of a positive temperature coefficient). Such variations are very stable and repeatable basis.

Sensor – Platinum wire wound or flat-film resistor

Thermistors:

An alternative group of temperature sensors which display a larger value of temperature coefficient resistance (usually negative, sometimes positive). They provide high sensitivity over a limited range.

Sensor – Ceramic (metal oxides)

Thermocouples

Essentially comprise of a thermo-element (a junction of two specified dissimilar metals) and an appropriate two wire extensions / compensating lead. A thermocouple operates on the basis of the junction located in the process producing a small voltage which increases with temperature. It does so on a reasonably stable and repeatable basis.

Sensor – Thermoelement, two dissimilar metals/alloy

When there is a temperature gradient in an electrical conductor, with an electron flow along the conductor is associated with the energy (heat) flow, hence an electromotive force (emf) is then generated. The temperature gradient itself is dependent on the size and direction of the emf. The voltage is a function of the temperature difference along the conductor length.

A thermocouple consists of two dissimilar conductors which when exposed to the same temperature gradients will produce different thermoelectric emfs which will intersect to generate a reading.

Most conducting materials can produce a thermoelectric output. However the selection becomes highly restricted to have the ideal material for temperature range, useful signal output and repeatability.  After years of research the full range of thermocouples can function in temperatures from -270°C to 2,600ºC. The full range does not naturally cover this temperature span under one thermocouple.

When it comes to your thermocouple selection one must consider physical conditions, duration of exposure, sensing application, sensor lifetime and accuracy.

Thermocouple Types (Link)

We manufacture the widest range of mineral insulated cable sheath materials available, if what you need isn’t standard we can look at virtually any metal you need via our in-house metallurgist. Our standard range covers;

Stainless Steels

  • 304
  • 304L
  • 321
  • 347
  • 316
  • 316L
  • 316Ti
  • 310
  • 310S
  • 446

Nickel Alloys

  • 600
  • 601
  • 825

Nicrobell

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