There are several temperature element types widely used to measure the temperature of a process substance. These types are categorized into mechanical temperature elements and electronic temperature elements. Mechanical temperature element uses the principle of thermal expansion, i.e. the element expands if the temperature rise and vice versa.
Mechanical Based Temperature Element
Solid element Bimetallic is a temperature element instrument formed from two types of metal that have different thermal expansion properties. These two metals are bonded together to become one strip. The metal strips will bend if the temperature changes. At the free end of the bimetal, the pointer is attached to indicate the result of temperature measurement.
Gas Temperature Element
The gas-filled Temperature Element is based on the principle of the ideal gas law and consists of bulb/stem, capillary, and Bourdon tube. The sensing element is a rigid bulb or stem containing a gas. If the temperature rises, the volume remains constant since the bulb is rigid, while the gas pressure increases proportionally. The Pressure Element like the bourdon tube measures the change in gas pressure.
The bulb and the bourdon tube are connected by a capillary tube, allowing the temperature indicator to be located not directly attached to the measuring process. This advantage enables the temperature gauge indicator to be mounted on a convenient location, whereas the tapping point of process fluid to be measured is not permanently inaccessible.
However, the capillary might have heat loss; therefore, the addition of compensation is required to eliminate the error.
Liquid Temperature Element
Liquid temperature element utilizes mercury constrained within a bulb/stem. Mercury is no longer preferred in most process applications, although it provides fast response and good accuracy. Mercury is mostly used in glass stem thermometers for non-process industries such as body temperature measurement.
Electronic Based Temperature Element
Resistance Temperature Detector (RTD) and Thermocouple are the two most commonly used electronic temperature elements. To indicate the temperature measurement, these elements need to be connected to the control system by wired directly or using a temperature transmitter.
Resistance Temperature Detector
Resistance Temperature Detector (RTD) works based on the principle that the resistance of a metal varies with its temperature. RTD is the most commonly used type of in-process measurement due to its excellent stability, accuracy, repeatability.
Thermocouple
A thermocouple consists of two dissimilar metals of which the junction generates a voltage proportional to the junction temperature. The applications of Thermocouple requires typically a wide range (very high to low temperature). Other advantages of Thermocouple over RTD are it has a more rugged design and also provides a faster measurement response.