Temperature sensor is a device to gather temperature information of surrounding environments and convert the signal to readable information. The most common example of a temperature sensor is a mercury thermometer which was invented in 1714 by physicist Daniel Gabriel Fahrenheit in Amsterdam. The mercury in thermometer changes slightly in volume due to the high coefficient of expansion along with the change in temperature. It is cheap and easy to read, so thermometer is still popular on the market.
There are two basic physical types for temperature sensor: contact temperature sensor and non-contact temperature sensor. Contact temperature sensor requires a physical contact with the target being measured and can be used to measure the temperature of solids, liquids and gases.
Contact temperature sensors include thermocouple, resistance temperature detectors (RTDs) and thermistor.
Thermocouple is a sensor for temperature measurement and consists of two different metals forming joints. Thermocouple is a voltage signal based sensor. Voltage signals can be developed between the two junctions when temperature changes. Thermocouple is easy to use and it produces a predictable voltage for a given temperature. The advantages of thermocouple include the capability to detect high temperature (> 2000°C), small sizes, robustness and cost-effectiveness. But thermocouple is non-linear and relatively less stable compared to other types of temperature sensors.
RTD is a different type of temperature sensor that operates on the principle that the electrical resistance of a metal varies as its temperature changes. RTDs are positive temperature coefficient sensors so their resistance increases with temperature. The most often used metals are platinum, nickel and copper. The resistance of metal follows accurately with temperature change and RTD can provide accurate indication of temperature. RTDs have higher repeatability, stability and linearity compared to thermocouple. RTDs also have limitations in industrial applications and they are rarely used in environments above 660 °C due to contamination issues.
Thermistor is an another type of temperature sensitive resistor for temperature measurement. Thermistors have two different types, positive temperature coefficient (PTC) thermistor and negative temperature coefficient (NTC) thermistor. As the name suggests, PTC thermistors are resistors with positive temperature coefficient. So when temperature increases, the resistance of thermistor also increases. NTC thermistors are exactly the opposite. Thermistors are usually made of ceramics or polymers, different from the metal materials used in RTDs. So thermistors are another type of low cost temperature sensors and they cost much less than RTDs. Thermistors are also most sensitive to temperature compared to thermocouple and RTDs but thermistors can only be operated in small temperature range and have lower stability. The applications of thermistors include current limiting devices, heating elements, food handling and processing and current protectors etc.
Non-contact temperature sensor measures heat radiation from an object and does not require a physical contact. These sensors are used away from measured object and concentrate thermal radiation onto a thermopile. The object’s temperature can be determined by calculating the radiation energy. Non-contact sensors are used in cases where the objects move, contact with objects causes contamination or potential danger may occur if in contact. The thermopile after collection of radiation generates a voltage signal that can be used to determine the temperature. Usually non-contact temperature sensors respond faster than contact temperature sensors. And non-contact temperature sensors are very useful alternative options under circumstances where contact temperature sensors cannot be used.
The temperature sensors discussed above are the most basic and widely used ones. They have been used in broad applications, such as HVAC control, food processing, medical applications and biology research etc.