What is a Thermocouple?
What is a thermocouple and how does a thermocouple work?
What is a Thermocouple is a common question. A thermocouple is a relatively simple device used to measure temperature. Thermocouple sensors are made from two different metal wires joined (welded) together to form a measuring junction (also known as a hot junction). This junction and the wires are usually enclosed in a metal sheath to make a thermocouple probe which can then be inserted into the medium where the temperature is to be measured. The opposite end of the two wires are also joined together at a point known as the cold junction.
As the temperature at the hot junction changes, it creates a thermal gradient between the hot and cold junctions which generates an electron flow and a resultant electromotive force (EMF), measured in Millivolts. The voltage measured at the ends of the conductors is a function of the temperature difference along the conductor length. This effect is known as The Seebeck Effect.
When connected to an appropriate thermocouple display, the cold junction compensation is electronically referenced to 0ºC and a formula is applied to the millivolt signal, which converts the incoming signal to °C or °F, using the instrumentation. An in depth, technical explanation of How a Thermocouple Works can be found here.
Choosing the right thermocouple probe for all your applications can be difficult, but our experienced engineers are here to help!
Please call us on 877 244 1777 for help or advice on any aspect of temperature measurement.
We are a large manufacturer of temperature sensors which means we have an enormous range of components in stock to make virtually any sensor you specify. We can ship custom built thermocouples typically within 5 days or sooner. Below is a selection of the different types of sensor we manufacture covering a wide temperature range with fast response times. Sensors are supplied with a choice of terminations such as thermocouple wire, connectors or terminal heads.Mineral Insulated
Our most popular thermocouple style. Rugged sensors, ideal for most applications. Wide choice of terminations e.g. pot seals, Wire, connectors, heads etc.General Purpose
A wide range of thermocouples to suit many applications. Hand held, surface, bayonet, bolt, low cost simple styles, patch sensors etc.Miniature
Ideal for precision temperature measurements where minimal displacement and a fast response is required.Heavy Duty
For industrial applications such as furnaces, kilns, ovens, boilers, flues etc. Many types of sheath material available.
What is Thermocouple Wire?
Once you have decided on the construction of the thermocouple probe, we need to consider how to connect the sensor to instrumentation. Copper wires are unsuitable for connecting thermocouples and so we have to use a dedicated thermocouple cable.
Theremocouple wire is available as 'thermocouple grade' which is wire that is used to make the sensing junction or probe part of the thermocouple. Extension wire is only ever used to extend the thermocouple signal back to instrumentation. Extension grades will generally have a lower ambient temperature rating than the thermocouple. This means that whilst it can pass the signal from the high temperature probe, the extension cable itself should not be exposed to high ambient temperatures.
Extension cables offer a cost saving for extending thermocouples and are designated by the letter 'X', for example 'KX' or 'JX'. Thermocouple grade wires are designated by double letters e.g. KK, JJ, NN etc.
How do I choose a thermocouple type?
What are thermocouple types?
As mentioned above a thermocouple is made from two dissimilar metal wires joined together to form a measuring junction. An EMF is generated according to the temperature. A few conductor combinations have been standardised under IEC 60584-1 as being the most suitable for temperature measurements in terms of stability, accuracy and repeatability and these combinations have been designated with a letter such as type K, type T, type J etc., with each type having a particular combination (e.g. Nickel Chromium vs Nickel Aluminium for Type K, Fe vs Constantin for Type J etc.
The thermocouple type will depend on the accuracy required, the operating temperature range and other factors such as the environmental conditions. A detailed explanation of each thermocouple type can be found here.
What is a type K thermocouple?
Type K thermocouples, also known as Chromel Alumel, are the most popular type of thermocouple because they are a good all round performer. They have a wide temperature range up to 2000ºF, are reasonably accurate and provide good stability in a wide range of applications. The positive leg is Nickel Chromium and the negative leg is Nickel Aluminium.
Type K thermocouples are color coded according to ANSI MC96.1-1982 which for sensors with wires attached is yellow positive leg and a red negative with an overall sheath color of brown for thermocouple grade wires or yellow for extension grade. If a thermocouple connector is used then this will be coloured yellow.
What are type J thermocouples?
Type J thermocouples are widely used in the plastics machinery industry and comprise conductors of Iron vs Constantan and is one of the few thermocouples that can be used in reducing atmospheres. They have a temperature range generally from -300ºF up to 1300ºF.
Type J thermocouples are color coded according to ANSI MC96.1-1982 which for sensors with wires attached is white positive leg and a red negative with an overall sheath color of brown for thermocouple grade wires or black for extension grade. If a thermocouple connector is used then this will be coloured black.
How do I know which thermocouple junction to use?
Sheathed thermocouple probes / sensors are available with a choice of three different types of 'hot' junction: insulated, grounded or exposed.
|Insulated Junction Thermocouple
The hot junction is insulated from sheath which gives a floating output with a typical insulation resistance in excess of 100 megohms.
|Grounded Junction Thermocouple
The hot junction is welded to sheath tip giving an earthed output and faster response to temperature changes.
|Exposed Junction Thermocouple
An exposed junction gives the fastest response time and is commonly used for the measurement of air temperature in ducts. Usually Restricted to a maximum operating temperature of 600°C for mineral insulated thermocouples.