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Temperature Measurement Methods & Instruments
When considering temperature, the first thing to understand is - What is Temperature?
Well, temperature is a measure of the average heat or thermal energy of the particles in a substance; the more thermal energy in the substance the higher it's temperature. This thermal energy is expressed in one of the many different units of temperature: Kelvin, Celsius, Rankine or Farenheit. See our Temperature Conversion page to find out more about the origin of each unit, how to convert between them, and a handy online temperature conversion calculator.
There are many different ways to measure temperature all of which infer temperature by sensing some change in a physical characteristic. Of the many different methods of temperature measurement available, the instrument engineer in the process industry is only likely to encounter a handful, as discussed below.
Thermocouples are probably the most used method for temperature measurement. Thermocouples are basically two wires, made of different metals and joined at one end. Changes in the temperature at the joined end induce a change in electromotive force (emf) between the other ends. See our page on types of thermocouples for a description of the more usual materials used in thermocouple construction, and the benefits of each combination. Also, have a read of our page on thermocouple failure and burnout to learn how to protect your plant against thermocouple failure.
Resistive Temperature Devices
Resistive temperature devices work on the principle that the electrical resistance of a material changes as its temperature changes. There are two main type of resistive temperature device:
- Metallic devices (commonly referred to as RTDs). RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature,
- Thermistors. Thermistors rely on resistance change in a ceramic semiconductor with the resistance dropping non linearly with temperature rise.
Bimetallic temperature measurement devices take advantage of the difference in rate of thermal expansion between different metals. Strips of two metals are bonded together. When heated, one side will expand more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer. This simplicity in design and manufacture has helped bimetalic devices to form the basis of most industrial temperature gauges and dial thermometers. Further, because these devices do not require a power supply they are portable, but they are usually not as accurate as thermocouples or RTDs.
Infrared sensors are non-contacting devices. They infer temperature by measuring the thermal radiation emitted by a material. They tend to be used when temperature measurements are extremely high, perhaps around furnaces or fired heaters.
Temperature Instrument Selection
Each type of temperature instrument has a particular set of conditions for which it is best suited with many of these conditions overlapping between device types, and this makes temperature device selection as much an art as it is a science.
See our page on Temperature Instrument Selection Guide for things to be considered when selecting a device, a comparison of the most commonly used temperature devices, and a temperature instrument selection chart.
Installation of Temperature Sensors
Temperature sensors in the process industries tend to be delicate and need to be protected from the fluid that they are mesuring. The most common method used to achieve this is to use a thermowell i.e. a pressure-tight sheath which surrounds the temperature measuring device.
Thermowells are available with a range of process connections, lengths, shapes and materials of construction. See our pages on Thermowell Design and Specification and Thermowell Wake Frequency for more details on choosing the right thermowell for your application.
The following pages give more detail on the techniques used in temperature measurement:
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For those who want to read further about the theory and practice of measuring temperature, and broaden their understanding of the differing types of temperature instrumentation, then the following books from Amazon will be of interest: