Temperature Sensing Devices: The Basics

Temperature sensors have a wide range of uses in many different applications. Some devices, like the equipment used to produce life-saving medicine, require highly accurate and responsive temperature sensors. On the other hand, applications like the thermometer in your car do not need to be as accurate or responsive. As such, there are many types of temperature sensors with varying accuracy and responsiveness. The four most common types of temperature sensors are negative temperature coefficient thermistors, resistance temperature detectors, thermocouples, and semiconductor-based sensors.

  1. Negative Temperature Coefficient Thermistor

The negative temperature coefficient thermistor, or NTC, is the most accurate type of temperature sensor. It is a thermally sensitive resistor that exhibits a constant, minute, periodic change in resistance related to variations in temperature. This type of sensor provides higher resistance at lower temperatures. As the temperature increases, its resistance drops slowly, relative to the device’s R-T (resistance to temperature) table. Due the large changes in resistance, even miniscule changes are reflected very accurately. The output of an NTC thermistor is non-linear because of its exponential nature, although, based on its application, it can be made linear. For standard thermistors, the effective operating range is -50 to 150°C, but this range is increased to 250°C in glass-encapsulated thermistors.

  1. Resistance Temperature Detector

The function of a resistance temperature detector, or RTD, is to use temperature to change the resistance of the RTD element. An RTD consists of a film, or in some cases, a wire wrapped around a ceramic or glass core. RTDs made from platinum are the most accurate, but those made from nickel or copper will be lower cost, though not as stable or repeatable. Platinum RTDs offer a highly accurate linear output in a wide temperature range from -200 to 600°C but are very costly.

  1. Thermocouples

A thermocouple is a temperature sensing device that consists of two wires of different metals that are electrically bonded at two points. The dissimilar metals create a varying voltage which reflects proportional changes in the temperature. Like NTCs, thermocouples are non-linear and require a conversion table when used for temperature control. This is usually done with a lookup table. The accuracy of thermocouples is low, but they operate across the widest temperature range (-200 to 1750°C).

  1. Semiconductor-based Temperature Sensors

Semiconductor-based temperature sensors are commonly part of integrated circuits. Sensors of this type feature two identical diodes with temperature-sensitive voltage vs. current characteristics that they utilize to measure changes in temperatures. These sensors provide a linear response but are the least accurate of all the basic sensor types. Additionally, they also have the slowest responsiveness and the lowest temperature range (-70 to 150°C).

Temperature sensors are important in both technical applications and everyday life. Whether you need a sensor for your home or an application in an industry such as petrochemical, automotive, aerospace and defense, consumer electronics, or other, be sure you are getting your device from a trusted and reputed source.


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