Thermocouple output

Temperature Sensors: Thermocouples

Also in this series:
  1. Electric Sensors
  2. Temperature sensors: RTD, an accurate alternative
  3. Thermistor, a champion of sensitivity

The vast majority of temperature transducers are analog because both the temperature and the output electrical value vary with continuity. The choice of the temperature sensors must be done carefully, taking into account the environmental conditions, temperature values to be detected and the required accuracy.

In general, the requirements are:

  • high sensitivity;
  • linear output and growing;
  • operating stability over time;
  • adaptability to different application requirements

Common temperature sensors

Temperature sensors most used today are the thermocouple, the resistive temperature devices (RTDs), Thermistors and the latest Silicon Integrated sensors.

Each of these technologies can meet specific conditions of temperature and environment. The temperature range of the sensor, its robustness and sensitivity are just some of the features that are used to determine whether or not the device meets the requirements of a particular application. There is not a temperature sensor ideal for all applications. The thermocouple can boast a wide temperature range, the RTD’s strength is its linearity, while the advantage of the thermistor is its accuracy.

Table 1 summarises the main characteristics of these four temperature sensors. This table can be used for a first selection of the sensor to be used.

T-sensors comparison
TAB.1 Characteristics of most popular four temperature sensors. Credit: Microchip

The universal and inexpensive Thermocouple

A thermocouple consists of two wires of different metals welded together at one end, as shown in fig.1. The reference junction temperature – also known as cold-junction compensation point is used to mitigate the errors generated by iron-copper and copper-constantan junctions. The junction of two metals of the thermocouple is placed at the point where you want to measure the temperature. This configuration of materials produces a voltage between the two wires not soldered, which is a function of the temperature of all junctions. As a result, the thermocouple does not require excitation voltage or current.

Since there is a tension between the 2 open ends of the wires, it might seem that the thermocouple interface could be achieved simply by measuring the voltage difference between the 2 wires. This might be true if it were not for the fact that the end of the thermocouple wires are in contact with another metal, usually copper.

Thermocouple
Fig. 1. A Type J Thermocouple. Credit: Microchip

This fact creates 2 additional thermocouples and introduces a significant error in the system. The only way to compensate for this error is to measure the temperature at the reference junction (fig. 1) and subtract the error contribution of these connections through a hardware or mixed solution of hardware and software.

Error compensation. Credit: Mosaic Industries
Fig. 2. Error compensation. Credit: Mosaic Industries

In theory the thermocouple could consist of any two metals, however, in practice, some combinations have been preferred of two metals chosen according to their quality of linearity and magnitude of the voltage drop versus temperature. These standard types of thermocouple are E, J, T, K, N, S, B and R (summarized in tab. 2 and Fig. 3). Thermocouples are highly nonlinear and always require linearization algorithms. The Seebeck coefficient in tab. 2 represents the average voltage supplied by a specific thermocouple at a determined temperature.

Thermocouple output
Fig. 3. Response of different thermocouples. Credit: Microchip
Standard thermocouples
Tab. 2. Standard thermocouples. Credit: Microchip

Thermocouples are extremely non-linear compared to Thermistors, RTDS and Integrated Silicon sensors. Therefore, fairly complex algorithms must be carried out to its linearization. Typically for every type of thermocouple we need a series of coefficients to linearize the output voltage. These coefficients are then used in the equation:

    \[V_{0}=c_{0}+c_{1}t+c_{2}t^{2}+c_{3}t^{3}+...\]

Where V is the voltage supplied from the junction of the thermocouple and t is the temperature.

The alternative to using these complex calculations is to use the program memory for a look-up table. The look-up linearization table, e.g. for a thermocouple of type K, is a 11×14 matrix of decimal integers.

In addition, the thermocouple can quantify the temperature only in relation to a reference temperature. The reference temperature is defined as the temperature at the end of the thermocouple wires, furthest away from the soldered bead. This temperature is usually measured using an RTD, a thermistor or a integrated Silicon sensor.

Assembled thermocoupleExamples of commercial thermocouples

The thermal mass of the thermocouple is smaller than that of an RTD or a thermistor: hence it follows that thermocouple’s response is faster compared to these larger sensors. The wide temperature range thermocouple can measure, makes it particularly suitable for critical environments.

Conclusion

In conclusion, we can say that thermocouples are usually selected because of their wide range of measurable temperatures, for their robustness and for the price. Good linearity and accuracy are difficult to obtain and this makes them unsuitable for precision systems. If you require a high accuracy, other temperature sensors may be a better alternative.