Thermocouples are probably the most widely used sensor for temperature measurement, which have been used in labs, industry and, even consumer devices
. Although widely used, thermocouples may be the least understood of temperature sensors. The main reason for that may be the existence of many different types of thermocouples, and special attention has to be paid to the installation and measurement to achieve valid results.
Main advantages of thermocouples are:
- Wide operating temperature range, from -100 ºC to higher than 2500ºC
- Simple, durable and reliable
- Relatively inexpensive
- Adaptable to many types of applications, including reactive or caustic environments
- No external power supply required
Main disadvantages of thermocouples are:
- Non-linearity
- Relatively low stability
- Low sensitivity
- Low voltage output can be affected by RFI and EMI
- Reference junction compensation required
Output voltage is on the order of microvolts per degree, and thermocouples are sometimes located far from the data acquisition equipment. Those conditions impose, most of the times, instrumentation amplifiers, high gain, filtering, and other signal conditioning techniques to maximize the signal and minimize the noise.
Furthermore, thermocouple output is non-linear, so linearization procedures must be considered for the hardware and/or software responsible for the temperature reading. It is important to note that several solutions are available to transparently accommodate these linearization procedures: at hardware level, we can find dedicated integrated circuits with a built-in linearization table, while at software level it is common availability of software library providing linearization capabilities.
Finally, thermocouple measurements require the use of a reference junction, as presented in the following section.
Operating principle of the thermocouple
In the early 1800s, Thomas Seebeck discovered that the junction between two different metals generates a voltage that is a function of the temperature. A thermocouple is a temperature measurement device made by joining materials with different Seebeck coefficients.
As referred, this function is highly non-linear over the interval range of temperatures. Depending on the type of materials used to produce the thermocouple, we can have different types of thermocouples and associated functions Voltage versus Temperature.
Each type of thermocouple is referred by a specific symbol as referred in Figure 2.2, where more common types of thermocouples are referred, and indicative temperature ranges are presented.
In the base metal thermocouple group, types T, J, E, and K are referred; they are economical, reliable, and reasonably accurate.
In the noble metal thermocouple group, those constructed with platinum and rhodium and used as industry standard, types R and S are referred; they are more accurate and stable than base metal types, but also more expensive.
Several types of thermocouples
Temperature measurement with thermocouples relies on a second thermocouple element, which is used as a reference element sensing a known temperature.
Historically, the easiest way of producing a precise reference temperature was to immerse the reference junction in an ice bath (which gave it the name of "cold junction").
Although, this set-up is not desirable for most applications, so it is currently common to use as reference junction a second junction which temperature will be read by a different temperature sensor (one that does not require a reference junction, like a thermistor or a semiconductor sensor).
In this sense, the two junctions are wired in series in the circuit and the voltage generated by the two voltages can be expressed by the following expression:
V = a (Tunknown – Tref)
where:
- a is the Seebeck Coefficient for the specific type of thermocouple;
- Tunknown is the temperature of the measurement junction, and
- Tref is the temperature of the reference junction.
After reading the voltage, it is only necessary to look up in a table for the corresponding temperature according with the specific thermocouple type.