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| Sensing temperature |
Temperature sensors are devices used to measure temperature of a medium (i.e. liquid, solid or gas). The sensor detects change in the temperature, and accordingly, change its physical or electrical property in a manner that can be measured. These sensors come in many different forms and are used for a wide variety of applications.
Process control requires precise measurement of temperature in order to control a process efficiently. To properly control monitor temperature, the proper sensing device needs to be selected.
We can generally classify temperature sensors into two primary types, which further breakdown into their own sub-categories.
- Contact Sensors: Sensors that physically come in contact with the process medium or the vessel carrying the process medium. They can be further broken into two types: Mechanical temperature sensors: Devices that use the physical expansion and contraction of materials (like non-compressible fluids or differential metals) to mechanically open or close a set of contacts. Examples are bulb and capillary thermometers and bi-metallic sensors. The second type are electrical temperature sensors: These sensors provide a measurable electrical change such as resistance, voltage, current proportional to a given change in temperature. Examples are thermocouple, RTDs, and thermistors. These devices have varying accuracies and temperature ranges.
- Non-contact Sensors: Non-contact temperature measurement is done without contacting or immersing the sensor into the process medium. These sensors used to be prohibitively expensive and used only where it was impossible (because of high temperatures) to make direct contact with the medium, such as the production of primary metals or geo-thermal research. Today however, we see the use of non-contact sensors being used in consumer medical goods and commercial HVAC application. To measure temperature, these devices use the thermal radiation and infra-red light waves reflected from the targets surface. Examples of these sensors are pyrometers, infrared sensors, and optical sensors.
