One of the properties of great use to scientists and engineers is temperature, and measuring temperature is important for a variety of processes such as smelting steel, monitoring jet engine performance, measuring how close an experiment gets to absolute zero, or determining the effects of global warming. There are several reliable sensor categories that can be used to accurately measure temperature, and choosing one depends on the operating environment, range of expected temperature, and accuracy required by the application.
Thermometer Temperature Sensors
Historically, temperature has been measured by correlating the thermal expansion of a material to a change in temperature. The mercury thermometer immediately springs to mind as a classic example of this type of sensor, but prior to the development of the mercury thermometer in the early 1700s, other materials such as alcohol and air/water interfaces were used to create temperature measurement devices.
Thermocouple Temperature Sensors
One of the more modern temperature sensors is the thermocouple. A thermocouple consists of two dissimilar metals that are joined together. When a current is run through the junction of the two metals, a voltage is generated that is related to the change in temperature from a standard. Early thermocouples required the use of a cold junction, where part of the thermocouple was kept at a reference temperature, because a thermocouple output is related to the temperature difference between two points.
An ice bath was often used because it was straightforward to maintain a temperature of 32F this way. Today, artificial cold junctions, also known as cold junction compensation, can be employed using a thermistor or diode in the circuit. Thermocouples are manufactured in many industry standard specifications. For example, one of the most common thermocouple types is Type K, which has a chromel–alumel junction.
Resistance Temperature Sensors
Another type of temperature sensor that uses an electrical current is the resistance sensor. There are two types of resistance sensors: the thermistor, which is usually a ceramic or polymer material; and the resistance temperature detector (RTD), which is usually a metal.
In both cases, the resistance of the sensor changes based on the change in temperature. RTDs can be used over a wider temperature range, while thermistors can provide higher accuracy within a smaller temperature range. Some types of thermistors are designed to have a large change in resistance at a certain temperature, and can be effectively used as switches.
Other types of temperature sensors include:
- Liquid crystal sensors
- Infrared sensors
- Fiber-Bragg gratings
- Silicon bandgap temperature sensors
A wide range of temperature sensors are available to fit nearly any application, in any environment, at any temperature.
Susan Kristoff - Susan Kristoff is mechanical engineer by trade, but has a diverse set of professional and personal interests. The glue that binds all of ...
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