Thermocouple and Temperature Measurement 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.
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
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)
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.
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.
Over the years a selection of the most popular combinations of wires / legs has been established based on a range of criteria including cost, availability, convenience, melting point, chemical properties, stability, and output requirement.
The thermocouple ‘type’ is usually chosen via the performance requirement in terms of temperature range and responsiveness / sensitivity.
Our standard thermocouple types are listed below, however other types are available for low sensitivities and feature lower resolutions, primarily types R, S and B.
|Code||Notes||Temperature Range (°C)|
|K||Most suited to an oxidising atmosphere, it has a wide temperature range and is the most commonly used.||0 to +1100|
|T||Excellent for low temperature and cryogenic applications. Good for when moisture may be present||-185 to +300|
|J||Commonly used in the plastics moulding industry. Used in reducing atmosphere as an unprotected thermocouple sensor. NB. Iron oxidises at low (rusts) and at high temperature||+20 to +700|
|N||Very stable output at high temperature it can be used up to 1300°C. Good oxidation resistance. Type N stands up to temperature cycling extremely well.||0 to +1250|
|E||Has the highest thermal EMF output change per °C. Suitable for use in a vacuum or mildly oxidising atmosphere as an unprotected thermocouple sensor.||0 to +800|
|R||Used for very high-temperature applications. Used in the UK in preferences to Type S for historical reasons. Has high resistance to oxidation and corrosion. Easily contaminated, it normally requires protection||0 to +1600|
|S||Type S has similar characteristics to Type R as shown directly above.||0 to +1500|
|B||Type B although similar characteristics to Type S and R is not as popular. Generally used in the glass industry.||+100 to +1700|
|G||Formerly known as Code W. Tungsten Rhenium alloy combinations offer reasonably high and relatively linear EMF outputs for high-temperature measurement up to 2600°C and good chemical stability at high temperatures in hydrogen, inert gas and vacuum atmospheres. They are not practicable for use below 400°C. Not recommended for use in oxidising conditions||+20 to +2320|
|C||Formerly known as Code W5.||+50 to +1820|
|D||Formerly known as Code W3.||+50 to +2100|