The thermoelectric EMF generated in thermocouples is proportional to the temperature difference between hot and cold junctions.
As long as the intermediate junctions are at the reference or same known temperature, the copper connecting wires have no effect on the hot junction temperature measurement.
To measure the temperature at the hot junction accurately, the error introduced by the copper connecting wires of voltage measuring instruments needs to be compensated.
Thermocouples are commonly used temperature sensors at industrial plants
Thermocouples are commonly used temperature sensors at industrial plants. These sensors employ the principle of the Seebeck effect to measure the temperature difference between hot and cold junctions. This temperature difference generates a thermoelectric voltage. Usually, multimeters or voltmeters are used to measure the generated voltage. The connecting wires from the voltmeter to the cold junction create errors in the temperature measurement. For precise measurement of temperatures at the hot junction, thermocouple cold junction compensation methods are required.
The Operation Principles of Thermocouples
Thermocouples are thermal sensors made of two dissimilar metals connected at the hot junction, where the temperature (Thot) is to be measured. The metal wires are kept open at the cold junction, which is considered the reference junction in the thermocouple arrangement, with a reference or known temperature, Tcold .
Due to the temperature gradient between the hot and cold junction, thermoelectric EMF is generated across the cold junction. The thermoelectric EMF generated is proportional to the temperature difference between the hot and cold junctions. The fundamental knowledge of cold junction temperature governs the thermocouple application.
The thermoelectric EMF can be given by the following equation, where S is the Seebeck coefficient in µV/℃, which is dependent on the thermocouple type:
If the cold junction temperature is known, the hot junction temperature can be obtained from the above equation as follows:
The Importance of Thermocouple Cold Junction Compensation
Usually, voltage measuring meters are used to read the thermoelectric voltage generated in a thermocouple. The voltmeters or multimeters are connected to the cold junction using copper wires. These copper wires form intermediate junctions in the thermocouple. As long as the intermediate junctions are at the reference or same known temperature, the copper connecting wires have no effect on the hot junction temperature measurement. Whenever the intermediate junctions are at different temperatures, errors are introduced in the hot junction temperature measurement.
The thermocouple metals bonded at the intermediate junctions to the copper wires form additional thermocouples in the actual arrangement. Thermoelectric EMF from the additional thermocouples introduces errors in the hot junction temperature measurement. To measure the temperature at the hot junction accurately, the error introduced by the copper connecting wires needs to be compensated.
Thermocouple Cold Junction Compensation Methods
There are several thermocouple cold junction compensation methods used in the industry, including the following methods.
Cold Junction at 0 ℃
In this thermocouple cold junction compensation, the thermocouple cold junction is placed at 0℃. Usually, an ice bath is arranged to achieve 0℃. The copper connecting wires are bonded to the thermocouple metals in this ice bath. The absence of a temperature difference at the intermediate junctions generates no thermoelectric EMF, thus no error in the hot junction temperature measurement. However, thermocouple cold junction compensation using an ice bath requires proper electrical isolation to prevent leakage currents. This compensation is typically very accurate.
Cold Junction at a Fixed, Known Reference Temperature
In this compensation method, the thermocouple cold junction is kept at a fixed, known reference temperature instead of in an ice bath. This thermocouple compensation method makes use of temperature-controlled circuits to maintain the cold junction temperature constant. The error due to the known cold junction temperature in the generated thermoelectric voltage is corrected to obtain the actual hot junction temperature.
Sometimes, it is impractical to keep the cold junctions in an ice bath or at a known temperature. In such cases, temperature probes can be used to measure the cold junction temperature. With the knowledge of thermocouple type and cold junction temperature obtained using temperature probes, the errors in the temperature measurement can be compensated through mathematical calculations.
The rugged, simple circuit and no power requirements of thermocouples make them the most widely used and popular thermal sensor in the industry. If you are looking for precise and accurate temperature measurements, then thermocouple cold junction compensation is one thing to consider. Cadence’s software, including the Celsius thermal solver featured in the video below, can assist you in developing thermal sensor circuits.