Thermocouples are a combination of dissimilar metals joined at one end, called hot junction, and the other end, cold junction, is left open.
The merits of thermocouples include a wide temperature range, safe operation, robust, compact, rapid response, no power requirement, and no self-heating.
The various types of thermocouples are K-type, J-type, N-type, T-type, E-type, S-type, and R -type.
Measuring temperature accurately is critical
Whether it is a chemical boiler, electric oven, or aircraft engine, the temperature is a common physical parameter that needs to be measured in all these environments. A wide range of temperature measuring sensors based on transducers, thermistors, thermostats, resistive temperature detectors (RTD), and thermocouples exist. A thermocouple is a differential temperature measurement device with a wide operating range and precise accuracy, making it a standard temperature measurement method widely used in many industries. The accuracy of temperature measurement depends on thermocouple output voltage and its associated signal conditioning circuitry. In this piece, we discuss the basics of thermocouples, their advantages and disadvantages, and the many different types available.
Schematic of a thermocouple sensor
How Is Temperature Measured from Thermocouple Output Voltage?
Thermocouples are a combination of dissimilar metals joined at one end, called hot junction, and the other end, cold junction, is left open. The schematic of a thermocouple based temperature sensor is shown above. The cold junction, otherwise termed as reference junction, is connected to signal conditioning circuitry. The temperature at a hot junction is the unknown physical quantity in the thermocouple based temperature measurement.
When the hot and cold junctions are at different temperatures, thermocouple output voltage or thermoelectric EMF is generated across the two open ends of the conductors at the cold junction. This effect is known as the Seebeck effect, which correlates the thermocouple output voltage and the temperature differences between the hot and cold junctions. Equation (1) defines the thermocouple output voltage (Vth) relation with hot junction temperature (Th) and cold junction temperature (Tc) in degree celsius. The EMF generated are in millivolts, and a signal condition circuit is necessary for accurate conversion of the voltage to a temperature reading.
where S is Seebeck coefficient in V/K.
The Seebeck coefficient value is different for thermocouples with different metal combinations, and the value and temperature share a non-linear relationship. For determining the hot junction temperature, we need to know the thermocouple output voltage, thermocouple type, and reference or cold junction temperature. The temperature at the hot junction can be given by equation (2):
where ɑ(Tc) represents the Seebeck coefficient expressed as a function of cold junction temperature (Tc) in µV/℃.
Types of Thermocouples
There are many different types of thermocouples, including:
K-type thermocouples- The k-type thermocouples are made of nickel-based alloys of chromium and aluminum. Their temperature range is -270℃ to 1372℃ and the Seebeck coefficient is 41µV/℃ at 25℃. The k-type thermocouples are used in microcontroller-based temperature sensing systems in industry sectors. They are cost-effective, accurate, and reliable over a sufficient range of operating temperatures.
J-type thermocouples- The temperature range of J-type thermocouples is -40℃ to 750℃ and the Seebeck coefficient is 52µV/℃ at 25℃. They are a combination of iron and copper-nickel (constantan) alloys and are susceptible to corrosion. The J-type thermocouple is the least costly but has a short shelf life in moist environments.
N-type thermocouples- The silicon alloys of nickel and chromium, namely nicrosil and nisil, are combined to form an N-type thermocouple. The temperature range of N-type thermocouple is -270℃ to 130℃ and the Seebeck coefficient is 27µV/℃ at 25℃. The N-type thermocouple is suitable for extreme thermal environments such as in nuclear and furnace applications.
T-type thermocouples- The temperature range of T-type thermocouples is from -200℃ to 350℃ and the Seebeck coefficient is 41µV/℃ at 25℃. This is a stable thermocouple consisting of copper-constantan alloys. These thermocouples are suitable for deep-freeze, extreme-low temperature applications such as in cryogenics and superconductor systems.
E-type thermocouples- The E-type thermocouple is composed of nickel-chromium and constantan alloys and has higher stability and accuracy compared to K-type thermocouples. The temperature range of E-type thermocouples is -270℃ to 870℃ and the Seebeck coefficient is 61µV/℃ at 25℃. These thermocouples are commonly used in an inert and oxidizing atmosphere.
S-type thermocouples- The high temperatures used in the biomedical and pharmaceutical industries are measured using S-type thermocouples containing platinum-rhodium alloys and platinum metal. The temperature range of S-type thermocouples is -50℃ to 1480℃ and the Seebeck coefficient is 6µV/℃ at 25℃.
R-type thermocouples- The R-type thermocouple is used for high-temperature measurement, even though the composition is similar to that of an S-type thermocouple. The percentage of rhodium is increased in R-type thermocouples, which provide better stability and accuracy compared to S-type thermocouples. The temperature range of S-type thermocouples is -50℃ to 1600℃ and the Seebeck coefficient of R-type thermocouple is 9µV/℃ at 25℃.
Thermocouples vs. RTD
Thermocouples and RTDs are employed for temperature measurement in industrial applications. The table below presents a comparison between thermocouple and RTDs.
A comparison between thermocouples and RTDs
Thermocouples are excellent temperature sensors that satisfy industry standards. Since there are many variations of thermocouples, you can choose the appropriate type depending on the environment, ensuring an accurate measurement. Even though the thermocouple output voltage is in the millivolts range and has the need for signal conditioning, thermocouples are cost-effective, compact, speedy, and possess a wide temperature range. If you are planning to employ a temperature measurement unit in your system, consider thermocouples.