Thermocouples are simple, robust temperature sensors, very reliable in their measurements, and low cost. Being widely used in the most varied temperature measurement processes, the thermocouple is made up of two different metals joined at one end. When there is a temperature difference between the joined end and the free ends, there is a potential difference that can be measured by a voltmeter, different types of thermocouples have different types of the curve, this is a curve that shows the potential difference in relation to temperature change
A thermocouple is the best known among the methods of measuring temperature because it is the cheapest and also covers a wider temperature range, one end of its joints is called a hot joint and the other end where we connect the compression devices or cables it’s called a cold joint. Hot, when subjected to a heat source, causes a small voltage to be generated, in the millivolts, which is proportional to its cold joint, this effect is called Seebeck.
But it is not just any metal that causes a perceptual effect, some alloys are adopted by the international system due to their reliability and precision in changing tension in relation to temperature change.
This possibility of temperature measurement was discovered accidentally using the junction of two different types of metals, generating an electrical voltage as a function of temperature by the physicist Thomas Seebeck in the year 1822.
There are several types of industrial thermocouples that allow you to measure temperatures between -270 degrees to + 2320 degrees Celsius. They are:
The cheapest thermocouples are those made of iron, copper and nickel and the most expensive are those that use platinum in their composition, the most used is type K. Although the type J thermocouple is the cheapest because it is made with iron, its oxidation process is very fast at temperatures above 542-celsius cranes, which makes it unfeasible, so it is less used.
Type K thermocouple.
The type K thermocouple is one of the most used, it has a low cost due to its popularity, several probes are available. They cover temperatures ranging from -200 ° C to 1200 ° C, with a sensitivity of approximately 41µV / ° C
Positive thermoelement (KP): Ni90% Cr10% (Chromel)
Negative thermoelement (KN): Ni95% Mn2% Si1% Al2% (Alumel)
Usage range: -270 ° C to 1200 ° C
f.em produced: -6,458 mV at 48.838 mV
Type E thermocouple
This thermocouple is suitable for low temperatures as it has a high sensitivity (68 µV / ° C).
Positive thermoelement (EP): Ni90% Cr10% (Chromel)
Negative thermoelement (EN): Cu55% Ni45% (Constantan)
Usage range: -270 ° C to 1000 ° C
produced: -9.835 mV at 76.373 mV
Type J thermocouple.
It is mainly used in obsolete equipment that is not always compatible with more ‘modern’ thermocouples. The use of type J above 760 ° C leads to an abrupt magnetic transformation that spoils the calibration. It is possible to protect the thermocouple tip to prevent corrosion using glass, ceramics or metal alloys, interfering as little as possible with the thermocouple measurement.
Positive thermoelement (JP): Fe99.5%
Negative thermoelement (JN): Cu55% Ni45% (Constantan)
Usage range: -210 ° C to 760 ° C
f.em produced: -8.096 mV to 42.919 mV
Type N thermocouple
Its high stability and resistance to oxidation at high temperatures make it more suitable for measurements at high temperatures, without resorting to thermocouples that incorporate platinum in their constitution (types B, R and S). It was designed to be a K-type “evolution”
Type B thermocouple
Type B, R and S thermocouples have similar characteristics, they are two more stable thermocouples, however, due to their reduced sensitivity, they are only used to measure temperatures above 300 ° C. Note that due to the reduced sensitivity of these thermocouples, their measurement resolution is also reduced.
Suitable for measuring temperatures up to 1800 ° C.
Against what is usual in other thermocouples, this causes the same temperature at the outlet at 0 and 42 ° C, which prevents its use below 50 ° C. In compensation, it uses common copper extension cables as long as their connection to the thermocouple is in this range (0 ° C to 50 ° C). The other thermocouples require connection cables with the same material as the thermocouple, at the risk of forming a “other thermocouple” with copper, if the connection is at a different temperature than the signal processing instrument (eg transmitter)
Positive thermoelement (BP): Pt70.4% Rh29.6% (Rhodium-Platinum)
Negative thermoelement (BN): Pt93.9% Rh6.1% (Rhodium-Platinum)
Range of use: 0 ° C to 1820 ° C
f .in produced: 0.000 mV to 13.820 mV
Type R thermocouple
With its high cost it is suitable for measuring temperatures up to 1600 ° C. Low sensitivity (10 µV / ° C).
Positive thermoelement (RP): Pt87% Rh13% (Rhodium-Platinum)
Negative thermoelement (RN): Pt100%
Usage range: -50 ° C to 1768 ° C
f.em produced: -0.226 mV at 21.101 mV
Type S thermocouple
Suitable for measuring temperatures up to 1600 ° C. Low sensitivity (10 µV / ° C), high stability and high cost.
Positive thermoelement (SP): Pt90% Rh10% (Rhodium-Platinum)
Negative thermoelement (SN): Pt100%
Usage range: -50 ° C to 1768 ° C
f.em produced: -0.236 mV at 18.693 mV
It is one of the most suitable thermocouples for measurements in the range of -270 ° C to 400 ° C.
Positive thermoelement (TP): Cu100%
Negative thermoelement (TN): Cu55% Ni45% (Constantan)
Usage range: -270 ° C to 370 ° C
f.in produced: -6.258 mV to 20.872 mV
Note that the choice A thermocouple must ensure that the measuring equipment does not limit the temperature range that can be measured.
The test carried out to calibrate a thermocouple uses room temperature together with a high-precision thermistor, a comparison table or digital calibration equipment. To test our thermocouple sensor just plug in a multimeter or voltmeter using the appropriate voltage scale and vary the temperature using a heat source, or even an ice stone to have changes in the voltmeter.
If the intention is low cost and temperature measurement with great precision, the use of the thermocouple is more indicated because it allows measuring temperatures between -270 degrees to + 2320 degrees Celsius and because it contains several models that meet your need, and occurring some damage at the junction you do not need to weld it by making a mechanical lock by twisting or tightening the two materials thus making the junction, as the models are vast the correct use is correct for the purpose to be exercised.