Sensing Element
Temperature sensors based on the Seebeck effect require a primary measurement point where two dissimilar conductive metals are joined. This junction, known as the hot junction, is positioned in direct contact with the medium or component whose temperature is being measured. The temperature difference between this point and the reference point generates a proportional thermoelectric voltage that is measured by the instrument.
Thermal Contact
Physical installation of the sensor determines the efficiency of heat transfer and the accuracy of the transient response. The hot junction must be positioned to minimize thermal resistance to the target while avoiding electrical short circuits to ground. Utilizing thermally conductive paste, mechanical clamping, or ceramic insulation ensures proper thermal coupling and shields the sensor from ambient air currents that could lower the measured value.
Signal Generation
Thermoelectric voltage is generated along the temperature gradient of the wires, but the measured potential is determined by the temperatures at the two terminal boundaries. The hot junction acts as the high-temperature boundary in most industrial sensing applications. Because the voltage is small, electrical shielding is utilized to prevent electromagnetic noise from distorting the signal.
Calibration Method
Metrological calibration of the thermocouple assembly involves comparing its output to a reference standard in a dry block calibrator or a calibration bath. Placing the hot junction in a stable thermal medium alongside a calibrated platinum resistance thermometer allows the thermoelectric voltage to be mapped to precise temperature points. This calibration routine identifies drift in the output caused by wire contamination or oxidation, allowing correction factors to be programmed into the measurement instrument.
Routine checks at fixed points, such as the freezing point of water, verify that the thermocouple remains within its specified tolerance class over its operational lifetime.
