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The thermocouple is a very simple and cheap instrument, which can measure different degrees of temperature; thanks to its simplicity it is used in many industrial sectors.
The basic principle that uses the thermocouple for its operation was discovered by the physicist Thomas Johann Seebeck in 1822. He discovered that in a closed circuit composed of two conductors of different nature and subjected to a temperature gradient, a potential difference is established , that is proportional to the temperature differences.
In this video we will discover how a Thermocouple works, its characteristics and all its variants.
Jaes, leader in the spare parts industry for over 10 years, offers in its catalog every type of thermocouple from the leading manufacturers.
This is a Thermocouple. It is a probe made up of 2 different metal wires, welded at one end, this part, called HOT JUNCTION, will be positioned in the space to be measured. On the other end, we find the COLD FUNCTION, from here the 2 metal connections, generally made by copper, will then be connected to a measuring instrument to detect the temperature perceived by the probe.
But how does a Thermocouple work?
Imagine holding a copper bar with your hand from one end, while the opposite side is exposed to a source of heat, a flame for example. The heat will begin to spread along the entire length of the bar until it reaches our hand. This is because the heat excites the molecules and atoms present in the copper bar, which in turn will make free electrons to move easily reaching the cooler part and heating it. This happens because there is a TEMPERATURE GRADIENT, that is a difference in temperature from one point to another, in our case from the hottest to the coldest point of the copper bar. If we look closely at the images we will notice that the colder part now has more negative charged electrons, while the hot part deprived of its electrons will be positively charged, thanks to this difference we can measure the electric potential present in the copper bar, obtaining one specific voltage.
Now let’s grab our Thermocouple and remember that to measure the temperature we need a potential difference. If the Thermocouple was made up of 2 equal metal wires, the heat will arrange in the same way along the wires and there would be the same number of electrons. By measuring the voltage, the result will be 0, since there is no potential difference in a circuit consisting of 2 equal metal wires.
In fact, Thermocouples are composed of 2 wires of different metals, for example copper and iron, which conduct heat and free electrons in a different way, thus creating a potential difference.
This difference is only generated when the thermocouple circuit is closed, 2 copper wires, called compensated cables, are connacted to the respective cold junctions on one side and to a multimeter on the other one, it is possible to convert the potential difference into temperature.
For an accurate measurement, the cold junction must be in an environment with a known temperature, to compare it to that of the hot junction; ideally in the laboratory the cold junction was immersed inside a liquid solution of water and ice, therefore at a constant temperature of 0 ° C, but since this solution isn’t very practical, scientists have found a way around thanks to technology.
Inside the multimeter, a sensor is installed to detect the cold junction temperature; The cold junction is extended, thanks to the extended cables, inside the multimeter next to the temperature sensor.
The basic principle that uses the thermocouple for its operation was discovered by the physicist Thomas Johann Seebeck in 1822. He discovered that in a closed circuit composed of two conductors of different nature and subjected to a temperature gradient, a potential difference is established , that is proportional to the temperature differences.
In this video we will discover how a Thermocouple works, its characteristics and all its variants.
Jaes, leader in the spare parts industry for over 10 years, offers in its catalog every type of thermocouple from the leading manufacturers.
This is a Thermocouple. It is a probe made up of 2 different metal wires, welded at one end, this part, called HOT JUNCTION, will be positioned in the space to be measured. On the other end, we find the COLD FUNCTION, from here the 2 metal connections, generally made by copper, will then be connected to a measuring instrument to detect the temperature perceived by the probe.
But how does a Thermocouple work?
Imagine holding a copper bar with your hand from one end, while the opposite side is exposed to a source of heat, a flame for example. The heat will begin to spread along the entire length of the bar until it reaches our hand. This is because the heat excites the molecules and atoms present in the copper bar, which in turn will make free electrons to move easily reaching the cooler part and heating it. This happens because there is a TEMPERATURE GRADIENT, that is a difference in temperature from one point to another, in our case from the hottest to the coldest point of the copper bar. If we look closely at the images we will notice that the colder part now has more negative charged electrons, while the hot part deprived of its electrons will be positively charged, thanks to this difference we can measure the electric potential present in the copper bar, obtaining one specific voltage.
Now let’s grab our Thermocouple and remember that to measure the temperature we need a potential difference. If the Thermocouple was made up of 2 equal metal wires, the heat will arrange in the same way along the wires and there would be the same number of electrons. By measuring the voltage, the result will be 0, since there is no potential difference in a circuit consisting of 2 equal metal wires.
In fact, Thermocouples are composed of 2 wires of different metals, for example copper and iron, which conduct heat and free electrons in a different way, thus creating a potential difference.
This difference is only generated when the thermocouple circuit is closed, 2 copper wires, called compensated cables, are connacted to the respective cold junctions on one side and to a multimeter on the other one, it is possible to convert the potential difference into temperature.
For an accurate measurement, the cold junction must be in an environment with a known temperature, to compare it to that of the hot junction; ideally in the laboratory the cold junction was immersed inside a liquid solution of water and ice, therefore at a constant temperature of 0 ° C, but since this solution isn’t very practical, scientists have found a way around thanks to technology.
Inside the multimeter, a sensor is installed to detect the cold junction temperature; The cold junction is extended, thanks to the extended cables, inside the multimeter next to the temperature sensor.
The purpose of the sensor is to detect the cold junction temperature and to compensate the automatic cold junction temperature. To put it in other words, it processes a conversion to ensure that the cold junction is always at 0 ° C (or 32 degrees Fahrenheit), as in the laboratory.
The equalization takes place thanks to a specific algorithm designed for this situation; the processor of the instrument measures the electrical voltage of the joints and it adds it up to the temperature of the cold junction, in this way we obtain a number expressed in millivolts (mV) which is later converted by the device itself into degrees Celsius, thus giving us the actual temperature of the hot junction.
Now that we’ve seen how a thermocouple works, let’s talk about some of its variations. Thermocouples are mostly classified according to the minimum and maximum temperature they can record, consequently they are made up of several pairs of different metals.
The model marked with the letter K is the most common, cheap and available in many formats; composed of Chromel and Alumel, detects a measurement range from -200 ° C to 1260 ° C.
Type J, on the other hand, is composed of Iron and Constantan, perceives temperatures ranging from –40 ° C to 750 ° C, and are less common than type K because they are more limited. They are used in older devices that do not support the K model.
The type T, composed of Copper and Constantan, is very similar to the J model. It perceives temperatures between –200 ° C and 400 ° C, and is used mainly in laboratories.
Model E, composed of Chromel and Constantan, is suitable for measuring low temperatures since it is very sensitive.
The type N, composed of Nicrosil and Nisil, measures the interval between 650 ° C and 1250 ° C. Their stability and resistance to hot oxidation allow them to obtain excellent results for high temperatures, in fact they are the most suitable and cheaper solution than platinum-based thermocouples.
Types B, R and S are those composed of noble metals, that is platinum in different percentages. They are the most stable thermocouples, but their low sensitivity limits their use to measure high temperatures, above 300 ° C.
Basically, the Thermocouples are chosen basing upon the temperature value to be measured.
Previously we talked about thermocouple extension cables, , which extend the cold junction up to the multimeter for correct measurement. These cables are connected to the cold junction via a connector; there are connectors for each type of thermocouple and they are built with the same material to avoid unwanted interference, especially when the connector can’t be kept at a constant temperature.
Our journey in the Thermocouple world, an essential tool in many industrial fields, ends here.
The equalization takes place thanks to a specific algorithm designed for this situation; the processor of the instrument measures the electrical voltage of the joints and it adds it up to the temperature of the cold junction, in this way we obtain a number expressed in millivolts (mV) which is later converted by the device itself into degrees Celsius, thus giving us the actual temperature of the hot junction.
Now that we’ve seen how a thermocouple works, let’s talk about some of its variations. Thermocouples are mostly classified according to the minimum and maximum temperature they can record, consequently they are made up of several pairs of different metals.
The model marked with the letter K is the most common, cheap and available in many formats; composed of Chromel and Alumel, detects a measurement range from -200 ° C to 1260 ° C.
Type J, on the other hand, is composed of Iron and Constantan, perceives temperatures ranging from –40 ° C to 750 ° C, and are less common than type K because they are more limited. They are used in older devices that do not support the K model.
The type T, composed of Copper and Constantan, is very similar to the J model. It perceives temperatures between –200 ° C and 400 ° C, and is used mainly in laboratories.
Model E, composed of Chromel and Constantan, is suitable for measuring low temperatures since it is very sensitive.
The type N, composed of Nicrosil and Nisil, measures the interval between 650 ° C and 1250 ° C. Their stability and resistance to hot oxidation allow them to obtain excellent results for high temperatures, in fact they are the most suitable and cheaper solution than platinum-based thermocouples.
Types B, R and S are those composed of noble metals, that is platinum in different percentages. They are the most stable thermocouples, but their low sensitivity limits their use to measure high temperatures, above 300 ° C.
Basically, the Thermocouples are chosen basing upon the temperature value to be measured.
Previously we talked about thermocouple extension cables, , which extend the cold junction up to the multimeter for correct measurement. These cables are connected to the cold junction via a connector; there are connectors for each type of thermocouple and they are built with the same material to avoid unwanted interference, especially when the connector can’t be kept at a constant temperature.
Our journey in the Thermocouple world, an essential tool in many industrial fields, ends here.