How to choose a temperature sensor?
Release time：2020-03-16 Clicks：839
A sensor that uses various physical properties of a material to change with temperature and converts temperature into electricity. The physical properties of these regular changes are mainly physical. The temperature sensor is the core part of the temperature measuring instrument. According to the measurement method, it can be divided into two types: contact and non-contact. According to the characteristics of the sensor material and electronic components, it can be divided into two types: thermal resistance and thermocouple. If you want to make reliable temperature measurements, you need to choose the right temperature sensor for your application. Thermocouples, thermistors, platinum resistance (RTD), and temperature ICs are the most commonly used temperature sensors in testing.
1 Thermocouple
Thermocouples are the most commonly used sensors in temperature measurement. Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is sturdy and low-cost, without the need for power supply, especially the cheapest. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end, as shown in Figure 1. When one end of a thermocouple is heated, there is a potential difference in the thermocouple circuit. The measured potential difference can be used to calculate the temperature.
In short, thermocouples are the simplest and most versatile temperature sensor, but thermocouples are not suitable for high-precision 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 increasing temperature. Temperature changes cause large resistance changes, so it is the most sensitive temperature sensor. However, the linearity of the thermistor is extremely poor, and has a great relationship with the production process. The manufacturer cannot give a standardized thermistor curve.
The thermistor is very small and responds quickly to temperature changes. However, 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 with better accuracy, but it is more expensive than a thermocouple and the measurable temperature range is smaller than a thermocouple. A common thermistor has a resistance value of 5kΩ at 25 ° C, and each 1 ° C temperature change causes a 200Ω resistance change. Note that the lead resistance of 10Ω only causes a negligible 0.05 ° C error. It is ideal for current control applications that require fast and sensitive temperature measurements. Small size is advantageous for applications with space requirements, but care must be taken to prevent self-heating errors.
2.1 Measurement techniques
The small size of the thermistor is an advantage, it can quickly stabilize without causing a thermal load. However, it is also very weak, and large currents can cause self-heating. Since the thermistor is a resistive device, any current source will generate 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. If the thermistor is exposed to high heat, it will cause permanent damage.
