Selection method of temperature sensor

If you want to make reliable temperature measurement, you first need to choose the correct temperature instrument, that is, the temperature sensor. Among them, thermocouples, thermistors, platinum resistance thermometers (RTDs) and temperature ICs are the most commonly used temperature sensors in testing.
The following is an introduction to the characteristics of the thermocouple and thermistor temperature instruments.
1. Thermocouple
Thermocouples are the most commonly used temperature sensors in temperature measurement. Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is strong, low in price, does not require power supply, and is also the cheapest. A thermocouple consists of two wires of dissimilar metals (metal A and metal B) connected at one end. When one end of the thermocouple is heated, there is a potential difference in the thermocouple circuit. The temperature can be calculated from the measured potential difference.
However, there is a nonlinear relationship between voltage and temperature. Since the temperature is a nonlinear relationship between voltage and temperature, it is necessary to make a second measurement for the reference temperature (Tref), and use the test equipment software or hardware to process the voltage-temperature conversion inside the instrument to Finally the thermocouple temperature (Tx) is obtained. Both Agilent34970A and 34980A data collectors have built-in measurement computing capabilities.
In short, thermocouples are the simplest and most versatile temperature sensors, but thermocouples are not suitable for high-precision measurements and applications.
2. Thermistor
Thermistors are made of semiconductor materials, and most of them have a negative temperature coefficient, that is, the resistance value decreases with the increase of temperature. Temperature changes will cause large resistance changes, so it is the most sensitive temperature sensor. However, the linearity of the thermistor is extremely poor, and has a lot to do with the production process. Manufacturers do not give standardized thermistor curves.
Thermistors are very small and respond quickly to changes in temperature. But the thermistor requires a current source, and its small size makes it extremely sensitive to self-heating errors.
The thermistor measures absolute temperature on two lines and has better accuracy, but it is more expensive than a thermocouple, and its measurable temperature range is also smaller than that of a thermocouple. A commonly used thermistor has a resistance of 5kΩ at 25°C, and a change in temperature of 1°C causes a resistance change of 200Ω. Note that the 10Ω lead resistance causes only a negligible 0.05°C error. It is ideal for current control applications requiring fast and sensitive temperature measurements. The small size is advantageous for applications with space requirements, but care must be taken to prevent self-heating errors.
Thermistors also have their own measurement tricks. The advantage of the thermistor's small size is that it stabilizes quickly without causing a thermal load. However, it is also very weak, and high current will cause self-heating. Since the thermistor is a resistive device, any current source will generate heat from power across it. Power is equal to the product of the square of the current and the resistance. So use a small current source. Permanent damage will result if the thermistor is exposed to high heat.