If you want to make reliable temperature measurement, you first need to select the correct temperature instrument, that is, temperature sensor. Thermocouple, thermistor, platinum resistance (RTD) and temperature IC are the most commonly used temperature sensors in testing.

The following is an introduction to the characteristics of thermocouple and thermistor temperature instruments.

1. Thermocouple

Thermocouple is the most commonly used temperature sensor in temperature measurement. Its main advantage is that it has a wide temperature range and can adapt to various atmospheric environments. It is solid, low price, no power supply and the cheapest. The thermocouple consists of two different metal wires (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 measured potential difference can be used to calculate the temperature.

However, there is a nonlinear relationship between voltage and temperature. Due to the nonlinear relationship between voltage and temperature, it is necessary to make a second measurement for the reference temperature (Tref), and use the software or hardware of the test equipment to process the voltage temperature transformation in the instrument, so as to finally obtain the thermocouple temperature (TX). Agilent 34970A and 34980A data collectors have built-in measurement and operation capabilities.

In short, thermocouple is the simplest and most common temperature sensor, but thermocouple is not suitable for high-precision measurement and application.

2. Thermistor

Thermistors are made of semiconductor materials, mostly with negative temperature coefficient, that is, the resistance decreases with the increase of temperature. Temperature change will cause large resistance change, so it is the most sensitive temperature sensor. However, the linearity of thermistor is very poor, and it is closely related to the production process. The manufacturer cannot give a standardized thermistor curve.

Thermistors are very small in size and respond quickly to temperature changes. However, thermistors need to use current source, and its small size also makes it extremely sensitive to self heating error.

The thermistor measures the absolute temperature on the two lines, which has good accuracy, but it is more expensive than the thermocouple, and the measurable temperature range is also smaller than the thermocouple. The resistance value of a common thermistor at 25 ℃ is 5K Ω, and the resistance change of 200 Ω is caused by the temperature change of 1 ℃. Note that the lead resistance of 10 Ω only causes a negligible error of 0.05 ℃. It is well suited for current control applications that require fast and sensitive temperature measurements. Small size is beneficial for applications with space requirements, but attention must be paid to prevent self heating error.

Thermistors also have their own measurement skills. The thermistor has the advantage of small volume. It can be stable quickly and will not cause thermal load. However, it is also very weak. High current will cause self heating. Since thermistor is a resistive device, any current source will generate heat due to power. Power is equal to the product of the square of the current and the resistance. Therefore, use a small current source. If the thermistor is exposed to high heat, it will cause * * sexual damage.

Through the introduction of two kinds of temperature instruments, I hope it will be helpful to your work and study.

Selection attention

1. Whether the temperature of the measured object needs recording, alarm and automatic control, and whether it needs remote measurement and transmission;

2. Size and accuracy requirements of temperature measurement range;

3. Whether the size of temperature measuring element is appropriate;

4. When the temperature of the measured object changes with time, whether the lag of the temperature measuring element can meet the temperature measurement requirements;

5. Whether the environmental conditions of the measured object damage the temperature measuring element;

6. If the price is guaranteed, is it convenient to use.