Temperature Sensing Elements

A Brief Introduction to Temperature Sensing Elements

The temperature of a process is an important measure to know as it indicates whether or not the process is in control.

The selection of the temperature sensing method depends on

• the required temperature range
• the measuring accuracy required
• the speed of response needed from the control system
• the process chemicals and conditions

Changing temperatures produce proportionate changes in the properties of materials. These changed properties can be measured and used to indicate the temperature.  The thermometer used at home to measure room temperature can be a filled system type, where trapped liquid or gas expands or contracts, or a bi-metallic strip device, where two dissimilar metal are coiled together and the length of each metal changes at a different rate and moves a pointer. These same devices, though more refined, are also used in industry. Their temperature range is generally -50oC to 600oC.

When high temperatures exist and accurate measurements are needed thermocouples and Resistance Thermal Devices (RTD) are two common industrial sensing methods used.

Thermocouples consist of two wires of dissimilar refined metals in contact at both the sensing and the measuring ends. A difference in temperature between the ends produces a proportional electric current that can be measured. Thermocouples respond quickly to temperature changes with the speed of response being affected by the wire thickness. They are used at temperatures from –200oC to 2000oC depending on the metals selected for the wires.

In a Resistance Thermal Device (RTD) temperature changes produce a proportionate electrical resistance in a length of refined metal wire that can be measured. They are used at temperatures from –250oC to 600oC. The sensor is a fine wire wound around an insulated core and the lot is encapsulated in epoxy resin or glass and protected by an outside sheath of stainless steel.

Encapsulating the wire significantly increases the response time (lag time) and makes RTD’s less favourable for control purposes where process temperature changes quickly.

RTD’s can be damaged by being subjected to excessive temperatures, by being chemically attacked by the process chemicals and from physical damage.

Mike Sondalini – Maintenance Engineer

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If you found this interesting, you may like the ebook Process Control Essentials.