The most common temperature sensors are thermocouples, thermistors, and resistance temperature detectors (RTDs).Fiber-optic sensors, while more specialized, are growing in popularity for temperature measurements
Thermocouples, The most popular temperature sensors, are effective in applications that require a large temperature range. They are inexpensive ($1 to $50 USD) and have response time fractions of a second. Due to material properties and other factors, temperature accuracy of less than 1 °C can be hard to achieve.
RTDs are nearly as popular as thermocouples and can maintain a stable temperature reading for years. In contrast to thermocouples,RTDs have a smaller temperature range (-200 to 500 °C), require current excitation, and have a slower response time (2.5 to 10 s).RTDs are primarily used for accurate temperature measurements (±1.9 percent) in applications that are not time critical. RTDs can cost between $25 and $1,000 USD.
Thermistors have a smaller temperature range (-90 to 130 °C) than previously mentioned sensors. They have the best accuracy (±.05°C), but they are more fragile than thermocouples or RTDs. Thermistors involve excitation like the RTD; However, the thermistor requires voltage excitation rather than current excitation. A thermistor typically ranges between $2 and $10 USD in price.
Another alternative is the use of fiber optics to measure temperature. Fiber-optic temperature sensors are effective for environments that are hazardous or where there could be regular electromagnetic interference. They are nonconductive, electrically passive, immune to electromagnetic interference (EMI)-induced noise, and able to transmit data over long distances with little or no loss in signal integrity
A bimetallic strip is used to convert a temperature change into mechanical displacement. Bimetallic thermometers are not recommended for continued use above 420 degree Celsius. The thermal stability of the bimetallic thermometer is an inherent characteristic of the metals used and continued operation cannot be assured above 471°C.