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How to choose a temperature sensor

The temperature sensor is 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.

If you want to make reliable temperature measurements, you need to choose the right temperature sensor for your application. Among them, thermocouple, thermistor, platinum resistance (RTD) and temperature IC are the most commonly used temperature sensors in the test.

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

1, thermocouple

Thermocouple is the most commonly used temperature sensor in temperature measurement. Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is sturdy, low-cost, does not require power supply, and is the cheapest. A 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, voltage and temperature are non-linear. Because temperature and voltage are non-linear, 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 in the instrument. Finally, the thermocouple temperature (Tx) is obtained. The Agilent 34970A and 34980A data collectors have built-in measurement capabilities.

In short, thermocouples are the simplest and most versatile temperature sensor, but thermocouples are not suitable for high-precision measurement 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 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.

Thermistors are very small and respond quickly to temperature changes. However, the thermistor requires a current source, and its small size also makes it extremely sensitive to self-heating errors.

Thermistor measures absolute temperature on two lines and has better accuracy, but it is more expensive than a thermocouple and the measurable temperature range is smaller than that of 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.

Thermistors also have their own 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.

When selecting a temperature sensor, you should also pay attention to:

1. Whether the temperature of the measured object needs to be recorded, alarmed and automatically controlled, and whether it needs long-distance measurement and transmission;

2, the size and accuracy requirements of the temperature measurement range;

3, whether the temperature measurement element is appropriate;

4.Whether the hysteresis of the temperature measurement element can meet the temperature measurement requirements when the temperature of the measured object changes with time;

5. Whether the environmental conditions of the measured object are harmful to the temperature measuring element;

6. Whether the price is guaranteed and convenient to use.

The selection of temperature sensor is mainly based on the measurement range. When the measurement range is expected to be within the total range, a platinum resistance sensor can be selected. The narrower range typically requires that the sensor must have a fairly high basic resistance in order to obtain a sufficiently large resistance change. The sufficiently large resistance change provided by the thermistor makes these sensitive components ideal for narrow measurement ranges. Thermocouples are more suitable if the measurement range is quite large. It is best to also include the freezing point in this range, because the index table of the thermocouple is based on this temperature. Sensor linearity within a known range can also be used as an additional condition for sensor selection.