Thermocouples use two wires that are different in composition, but joined to form one junction. The two junctions are at different temperatures, which then generates the Peltier emf in the circuit.
A piece of copper wire will have a temperature gradient along it if it is heated at one end; this is because the electrons in the wire will move to the cooler end of the wire. This process converts heat into energy. This principle was discovered by Volta and Seebeck.
A millivolt signal is generated when the two junctions are at different temperatures. This is called the Seebeck effect and is specified in the International Electrotechnical Commission’s standards IEC 1977.
Thermocouples only have value if they have cold junction compensation, which uses a reference temperature from an ice or water bath. The two ends of the thermocouple are held at the same temperature, while the hot junction is compared to the cold junction (as seen in the diagram). The thicker the thermocouple wire is, the higher temperatures it can measure, but with a slower response time.
If the two junctions of a thermocouple have the same temperature, they will generate an equal and opposite EMF, causing no current to flow through the circuit. If the junctions have different temperatures, the EMF will not be zero, causing current to flow through the circuit much like heat flows through a copper wire. The flow of EMF through the circuit is dependent on the metals used as well as the temperature difference of the two junctions, which can be measured with a meter.
The EMF in the thermocouple circuit is very small, in millivolts. A highly sensitive instrument is needed to determine the generated EMF. A measuring or reading instrument is needed to amplify the millivolt signal, interpret the voltage as a temperature reading, and display the reading. Galvanometers and voltage balancing potentiometers are normally used. Potentiometers are used most often.
A potentiometer is an electrical component that measures the potential difference by comparing it to a reference voltage. It is also known as a pot or potmeter. It can provide high precision measurements. Galvanometers are used to measure very small electric currents. They are used to measure null deflection or zero current.
A thermocouple must be placed against a known temperature in order to make an absolute measurement. The hot junction is the measuring assembly, while the cold junction is the reference where a cold junction compensation chip is located. The cold junction temperature may vary but provides a reference. The cold junction can be fixed by immersing it in water or ice to maintain a constant temperature.
What does a thermocouple do? #
A thermocouple is a device that measures temperature. It consists of two different metal wires that are joined together at one end. The other end of the thermocouple is connected to a controller or indicator that can read the thermocouple’s input. Thermocouples can be used to measure a wide range of temperatures, depending on the type of thermocouple you use.
