Thermocouple Principles

TRM are one of the worlds leading sensor manufacturing companies. Here we’ll talk about thermocouple first principles and how vital temperature measurement is, including the commonly asked what are thermocouple hot and cold junctions.

Firstly, the most measured physical quantity 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 is 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 has a larger range of temperature measurements. 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 the Type N thermocouple. With the main aim to provide customers with superior products delivering greater stability and precision. Additionally smart sensor to detect the 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 that convert temperature into another form of a 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.

The 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.

Some TTs operate by measuring the output of a thermocouple, which is a simple device whose output is proportional to the difference in temperature between a hot and a cold junction. The hot junction is the one measuring the process, and the cold junction is at the head itself.

Thermistors

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

Sensor – Ceramic (metal oxides)

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.

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