If you want to make reliable temperature measurement, you first need to choose the right temperature instrument, that is, the temperature sensor. Among them, thermocouples, thermistors, platinum resistance resistance (RTD) and temperature ICs are the most commonly used temperature sensors in testing.
The following is an introduction to the characteristics of two temperature instruments, thermocouple and thermistor.
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 sturdy, low in price, does not require power supply, and is also the cheapest. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end, and 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 non-linear relationship between voltage and temperature. Because of the non-linear 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 transformation inside the instrument to The thermocouple temperature (Tx) is finally obtained. The Agilent 34970A and 34980A data collectors have built-in measurement computing power.
In short, thermocouples are the simplest and most versatile temperature sensors, but thermocouples are not suitable for high-precision measurements and applications.
Thermistors are made of semiconductor materials, and most of them have a negative temperature coefficient, that is, the resistance value decreases as the temperature increases. 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 temperature changes. But the thermistor requires a current source, and its small size also 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 smaller than that of a thermocouple. A common thermistor has a resistance of 5kΩ at 25 degree , and a 1 degree change in temperature results in a 200Ω resistance change. Note that the 10Ω lead resistance causes only a negligible 0.05 degree 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 small size of the thermistor is an advantage, it stabilizes quickly and does not cause a thermal load. However, it is also very weak, and high current will cause self-heating. Since a thermistor is a resistive device, any current source will cause heat on it due to power. 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.
Through the introduction of the two temperature instruments, I hope it will be helpful to everyone's work and study.
1. Whether the temperature of the measured object needs to be recorded, alarmed and automatically controlled, and whether it needs to be measured and transmitted remotely;
2. The size and accuracy of the temperature measurement range;
3. Whether the size of the temperature measuring element is appropriate;
4. In the case where the temperature of the measured object changes with time, whether the lag of the temperature measuring element can meet the temperature measuring requirements;
5. Whether the environmental conditions of the measured object damage the temperature measuring element;
6. The price is guaranteed and whether it is convenient to use.
