Temperature element Instrument

 Temperature element Instrument

There are several types of temperature elements widely used to measure the temperature of a process substance. These types are divided into mechanical temperature elements and electronic temperature elements. The mechanical thermal element uses the principle of thermal expansion, that is, the element expands when the temperature increases and vice versa. The electronic temperature element uses the principle of changing electrical properties following a change in temperature.

Mechanical temperature element

Solid element

A bimetallic is a thermal element created from two types of metals that have different thermal expansion properties. The two metals are joined together to form one strip. Metal strips bend when the temperature changes. An indicator is attached to the free end of the bimetal to indicate the result of the temperature measurement.

Gas element

A gas-filled system is a temperature element based on the principle of the ideal gas law. It consists of a bulb/stem, a capillary and a Bourdon tube. The sensing element is a solid bulb or spindle containing gas. If the temperature rises, the volume remains constant because the flask is rigid, while the pressure of the gas increases proportionally. The change in gas pressure is measured by a pressure element such as a Bourdon tube.

The flask and bourdon tube are connected by a capillary tube allowing the placement of a temperature indicator that is not directly connected to the pipe/vessel being measured. These advantages allow the thermometer indicator to be mounted in a convenient location, while the sampling point of the measured process fluid is not permanently inaccessible.

However, the capillary can lose heat, so compensation must be added to eliminate the error. www.instrumentationportal.com

Liquid element

The liquid temperature element uses mercury in the bulb/stem. Mercury is no longer preferred in most process applications, although it provides fast response and good accuracy. Mercury is mostly used in a glass stem thermometer for non-process industries such as measuring body temperature.

Electronic device for measuring temperature

The two most commonly used electronic temperature elements are the resistance temperature detector (RTD) and the thermocouple. To indicate the temperature measurement, these elements need to be connected to the control system by cable directly or by means of a temperature sensor.

Resistance temperature detector

The Resistance Temperature Detector (RTD) works on the principle that the resistance of a metal changes with its temperature. RTD is the most commonly used type in process measurement due to its good stability, accuracy, repeatability.

Thermocouple

A thermocouple consists of two different metals, the transition of which generates a voltage proportional to the transition temperature. The thermocouple is selected for operation that requires a wide range (very high - low temperature). Other advantages of a thermocouple over an RTD are that it has a more durable construction and also provides a faster response.