Temperature sensor working principle

Principle of operation of temperature sensors:

A temperature sensor is a device that detects temperature and converts it into a measurable signal. The principle of operation is based on a variety of physical effects, including thermoelectric, resistive, thermistor, thermocouple, thermal expansion, semiconductor, and infrared absorption effects. The following is a detailed explanation of these principles:

1. Thermoelectric effect

The thermoelectric effect is the basis of thermocouple temperature sensors. Thermocouple by two different materials of the conductor composed of a closed loop, when the two ends of the temperature gradient exists, there will be a current through the loop, at this time between the two ends of the existence of electric potential - thermal electromotive force. This phenomenon of an electric potential due to a difference in temperature is known as the Seebeck effect. By measuring this electromotive force, a measurement of temperature can be achieved. Thermocouples have the advantages of wide measuring range and high accuracy, and are widely used in various temperature measurement situations.

   - A thermocouple is a sensor formed by welding two different metal conductors together.

   - When the solder joints of the sensor are at different temperatures, a temperature difference is formed, which generates a small electric potential (voltage signal).

   - The magnitude of this electromotive force is related to the temperature difference, and the temperature can be measured by a calibrated voltage-temperature curve.

2. Resistive and thermistor effects

Resistance effect and thermistor effect are the working principles of thermistor temperature sensors. Thermistor is an element whose resistance value changes with temperature. According to the law of resistance value change with temperature, thermistor can be divided into two types: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). The resistance value of positive temperature coefficient materials increases with increasing temperature, and the resistance value of negative temperature coefficient materials decreases with increasing temperature. Measurement of temperature can be achieved by measuring the resistance value of the thermistor. Thermistor sensors can achieve high accuracy over a limited temperature range (e.g. -90°C to 130°C).

2.1 Thermistor Effect:

   - Thermistors are resistors whose resistance changes with temperature. They are usually divided into two types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC).

   - The resistance value of NTC thermistors decreases with increasing temperature, while PTCs increase with increasing temperature.

   - By measuring the change in resistance value, the change in temperature can be deduced. There is usually a specific non-linear relationship between the resistance value and the temperature that requires calibration and specific circuitry to convert to a temperature value.

2.2 Resistance Effect:

   - An RTD is a sensor that uses a pure metal (usually platinum) wire or film as the sensing element.

   - When the temperature changes, the resistance value of the metal resistor changes, and this change is linearly related to the temperature.

   - By measuring the change in resistance value, the value of temperature can be calculated.

3. Infrared absorption effect

Infrared temperature sensors are based on the principle that the internal thermal movement of an object radiates electromagnetic waves (which contain infrared rays with a wavelength of 0.75 to 100 μm) in all directions. These sensors do not require direct contact with the object to be measured, and indirectly measure the temperature by measuring the infrared energy radiated by the object. Therefore, it is particularly suitable for measuring the surface temperature of moving objects, small targets, and objects with small heat capacity or rapid temperature changes (transients).

   - Infrared sensors measure the surface temperature of an object by receiving the infrared radiation it emits.

   - The thermal radiation of an object is proportional to its surface temperature, and the sensor can determine the temperature of the object by measuring the intensity of the radiation received.

4. Thermal expansion effect: 

Thermal expansion sensor is the use of certain materials with temperature changes and volume expansion characteristics to measure temperature. When the temperature changes, the length of the material in the sensor will change, and the temperature information can be obtained by measuring the change in length.

5. Semiconductor effect: 

Semiconductor materials have a higher rate of change of resistance with temperature than metals, so they can be made into temperature sensors. When the temperature rises, the resistance of the semiconductor will decrease rapidly, and the temperature can be measured accurately by using this characteristic.

In practical applications, the appropriate temperature sensor should be selected according to the specific measurement range, accuracy requirements, environmental conditions and other factors.

1. Thermocouples: Thermocouples usually have a high measuring range and decent accuracy, but their linearity and stability may be affected by temperature and material type. At extreme temperatures, the performance of thermocouples may degrade, thus affecting accuracy.

2. Coefficient of Thermal Expansion Sensors (Expansion Meters): Expansion meters are less accurate and are typically used for relatively crude temperature measurements. They are suitable for measuring temperature changes rather than precise temperature values.

3. Resistance Temperature Detectors (RTDs): RTDs offer good accuracy and stability, especially at constant temperatures. They have a wide measuring range, but have a slow response time and are not suitable for rapidly changing temperature monitoring.

4. Thermistors (semiconductor temperature sensors): Thermistors are highly sensitive and can respond very quickly to temperature changes. However, their long-term stability may not be as good as RTDs, and performance degradation may occur at extreme temperatures.

To summarise

Temperature sensors work on a variety of principles, based on different physical effects to achieve temperature measurement. Thermocouple sensors utilise the thermoelectric effect; thermistor sensors utilise the resistive and thermistor effects; and infrared sensors utilise the infrared absorption effect. These sensors have their own advantages and disadvantages, and are suitable for different measurement occasions and needs. For example, in the requirements of higher accuracy occasions, commonly used thermocouples or RTDs; in the requirements of fast response time occasions, commonly used thermoelectric effect or semiconductor effect sensors; in the requirements of low-cost occasions, commonly used sensors of thermal expansion effect.

Overall, no single temperature sensor is the most accurate under all conditions. The choice of sensor depends on the specific application requirements, including the desired measurement range, accuracy, response time, cost, and environmental conditions. When selecting a temperature sensor, there are usually trade-offs to be made based on these factors.