Thermocouples and thermistors are commonly used temperature measuring instruments in industry and laboratories. The core difference in their working principles lies in the physical effects of temperature measurement. The specific differences are as follows:

The essential difference in working principle is thermal resistance: based on the characteristic of the resistance value of conductors/semiconductors changing with temperature (i.e. thermal resistance effect). Taking the commonly used platinum thermistor as an example, the resistance value of its platinum wire increases regularly with temperature, and the corresponding relationship between resistance and temperature conforms to the calibration formula of the International Temperature Scale (ITS-90). By measuring the resistance value, the corresponding temperature can be converted.

Thermocouple: Thermoelectric effect based on the Seebeck effect when two different metal conductors form a closed circuit. When there is a temperature difference between two metal electrodes of different materials, a thermoelectric potential related to the temperature difference will be generated in the circuit. The magnitude of the thermoelectric potential is proportional to the temperature difference between the two ends. By detecting the thermoelectric potential, the temperature of the measured end (hot end) can be calculated.

Core composition and measurement logic differences in thermal resistance: The core component is pure metal wire (such as platinum, copper) or semiconductor material, and stable excitation current needs to be provided externally during measurement. The resistance change is measured through circuits such as Wheatstone bridge, which belongs to active measurement (requires power supply), and measures the change of its own resistance with temperature.

Thermocouple: The core is a closed circuit composed of two different metal conductors (such as platinum rhodium platinum and nickel chromium nickel silicon), which does not require external power supply and directly generates thermoelectric potential based on temperature difference. It belongs to passive measurement and measures the potential signal corresponding to the temperature difference at both ends.

The difference in the relationship between temperature and output signal in thermal resistance: The resistance value has an approximately linear positive correlation with temperature (platinum thermal resistance has linearity in the medium and low temperature range), and the output is a resistance signal that needs to be converted into a standard electrical signal (such as 4-20mA) through a transmitter.

Thermocouple: The relationship between thermoelectric potential and temperature is nonlinear, and the thermoelectric potential temperature curve of different types of thermocouples is different (corresponding to the scale conversion). The direct output is a millivolt level potential signal, and some scenarios require the use of a cold junction compensation device to ensure measurement accuracy.