Tuesday, March 27, 2018

Bimetal Thermometers for Industrial Process Measurement

stainless steel bimetal thermometer
Bimetal thermometers have a place in modern process
measurement systems.
Image courtesy Wika
Temperature measurement is everywhere, with broad ranges of accuracy, range and other operational requirements to bring the measurement data into a process management or control system. The process could be as simple as measuring a cooking temperature, or a part of a complex refining operation. Temperature provides an indication of heat energy level that is used in many ways throughout process control.

Though there are many instruments and technologies available to measure temperature, one that everyone is familiar with is the dial thermometer. A familiar numeric scale and a pointer indicate the temperature at the sensing location. Even within the product range of dial thermometers, there are several differing methods utilized to produce a temperature reading. One of these is the bimetal thermometer.

A bimetallic thermometer is named for the mechanism that responds to process temperature and provides the force to position the indicator needle over the scale on the dial face. A bimetal is formed from two dissimilar metals bonded together. The metals expand and contract at different rates in response to a change in their temperature. A bimetal thermometer relies on the predictable deformation of a bimetal spring or strip in response to a temperature change. The mechanical deformation is transformed into rotational movement of the indicating needle on the instrument face where the corresponding temperature can be read by a technician or operator. This design principle has been in use throughout laboratories, kitchens, and industry for many years and has proven to be predictably accurate, stable, and rugged.

The major advantages of the bimetallic thermometer are its relative cost, ease of use, and ability to function without any external power source. This class of instruments provides operability up to +1000°F.

When applying dial faced thermometers, there are several main considerations.

  • Scale - The display behind the indicating pointer. The scale divisions impact the instrument's accuracy at indicating process temperature.  
  • Range - The physical suitability of the instrument to be exposed to the temperatures which may be present in the process. May be the same as scale.
  • Dial Size - Larger diameter dial faces make reading the instrument indications easier.
  • Connection - There are numerous options for the way in which the probe or stem, which is inserted into the process, attaches to the dial portion or head of the instrument. Common arrangements are back, side, or bottom connected. If the head cannot be rotated or angled, the connection attributes may be the sole determinant of how the dial face is oriented.
  • Stem Length - The stem extends from the head into the process. Coordinating the stem length with the insertion depth into the process and the placement of the instrument is important to achieving a useful and ergonomic installation.
  • Materials of Construction - Make sure the selected instrument is rugged enough to withstand expected environmental conditions at the installation site.
These are only the primary considerations. Share your operational requirements with a product specialist. Leverage your own knowledge and experience with their product application expertise to develop the optimal solution.