Thermocouples: Construction, Colour Codes and Amplification

Among all the different types of temperature sensors, thermocouples have taken the spot for being the most widely used. This is mainly because of its simplicity and small size, speed with which it responds when any kind of temperature change is detected and also based on how easy it is to use. Another reason why thermocouples are so commonly used is because they provide the widest range of temperature measurements when compared with any temperature sensors available today, ranging between -200℃ to over 2000℃.

To put it in simple terms, thermocouples are basically thermoelectric sensors which have two junctions made from dissimilar metals that have been welded together; metals like constantan and copper, for instance. One of these junctions is known as the measuring (Hot) junction, and the other is known as the reference (Cold) junction, and is maintained at a constant temperature. When a voltage gets generated between the junction, the generated voltage is utilised in the measurement of the temperature detected by the sensor, provided that both the junctions are at different temperatures.

Thermocouple Construction

Understanding how a thermocouple operates is actually very simple. A “thermo-electric” effect is created when two different metals like constantan  and copper are joined together, resulting in the generation of a potential difference of a few millivolts (mV). The voltage difference that is generated between the two junctions is known as the “Seebeck effect”; the resulting temperature gradient that is produced across the conducting wires results in the generation of an emf. Due to this, the resulting output voltage is treated as a function of temperature change.

No output voltage is produced when either of the junctions are maintained at a constant temperature; which means the potential difference across the two junctions is zero. In such a case, the voltage output produced can be mathematically written as V1 = V2. Nonetheless, an output voltage will be detected when the junctions are at varying temperatures, and the resulting voltage is basically the difference in temperature between the two junctions, V1 – V2. Until such a time that the junction has not reached its peak voltage level, the difference in voltage tends to rise when the temperature continues to rise.

It is possible to make thermocouples using a multitude of materials thereby allowing the possibility to measure extreme temperatures between -200℃ to +2000℃. As there is a wide choice of temperature range and materials when it comes to thermocouples, picking the ideal thermocouple sensor that would serve a particular application, thermocouple colour codes and internationally recognised standards have been set.

Colour Codes of Thermocouples

Thermocouple Sensor Colour Codes Extension and Compensating Leads

Code Type

Conductors (+/-) Sensitivity British BS 1843:1952


Nickel Chromium / Constantan -200℃ to 900℃


Iron / Constantan

0 to 750℃


Nickel Chromium / Nickel Aluminium -200℃ to 1250℃


Nicrosil / Nisil

0 to 1250℃


Copper / Constantan -200℃ to 350℃


Copper / Copper Nickel Compensating for “S” and “R”

0 to 1450℃


Nickel-Chromium (Type K), Constantan-Copper (Type T), and Iron-Constantan (Type J) are the most widely used or popular thermocouple materials used when it comes to general temperature measurement. It is just a few millivolts (mV) of voltage which gets generated for a 10℃  temperature difference, since the output voltage of a thermocouple is often small, this shows the requirement for a circuitry to undertake small voltage output amplification.

Thermocouple Amplification

It is important to select the type of amplifier carefully as it should either be an Operational or discrete amplifier, this is a must because there is a need for amicable drift stability to avoid recalibration at regular intervals. As a result, amplifiers of the type instrumentation or chopper are most preferable when it comes to temperature sensing applications.

With over 30 years of expertise in temperature sensor and heater manufacturing, Heatcon Sensors provides a high level of quality, accuracy, and an unbeatable commitment to customer support.

If you are interested in knowing more about thermocouples, thermocouple probes or thermocouple wires and the other services we offer, call us at +91 9844233244.