Bimetal thermometers rely on the principle that different metals expand at different rates when exposed to heat. By bonding two different metals together to make a bimetallic strip, it becomes more sensitive to small temperature changes. Most of the bimetal strips use high thermal expansion alloy like steel or stainless steel coupled with low thermal expansion ally such as Invar.
Two construction methods for the bimetallic strips
- Spiral strip – the bimetallic strip is coiled into a spiral attached to a dial that indicates temperature.
- Cantilever strip – the bimetallic strips are bonded together in a cantilever. Deflection is used to indicate temperature.
Most commonly the strip are coiled into the thermometer and it will get larger or smaller depending upon the temperature change and will directly move the needle on the face of the dial to indicate temperature. Bimetal thermometers are cost effective as they do not require power – you only need to read the temperature measurement on the dial. For industrial applications, the bimetal thermometers offer large dials for easy reading and made from durable stainless steel construction. Dampened and silicone-filled bimetal thermometers provide accurate and reliable temperature read-outs in high vibration settings like manufacturing plants. Aside from the bimetal thermometers used for refrigerators, ovens and furnaces, there are penetration bimetal thermometers for food processing and production.
Ranges and accuracy of bimetal thermometers
In general the bimetal thermometer is considered to be very reliable and accurate particularly for home thermostats. A small pocket thermometer used to test the temperature in an air conditioning system or cooking has a dial of about an inch in diameter but with a temperature range from 0 to 220 degrees Fahrenheit and marked off in two degree increments. Larger units with 2, 3 and 5 “dial faces will typically be accurate to 1% of the span of the unit. Ranges as high as 1000 degrees Fahrenheit are available however those within the 500 degree Fahrenheit ranges are more common.
The critical role of bimetal thermometers in the food industry
The most common thermometer used in the food industry is the bimetallic stemmed thermometer that is capable of measuring temperatures from 0oF to 220oF (-18oC to 104oC). Among its many important features is the adjustable calibration nut for accuracy, easy-to-read numbered temperature markings, and the dimple to mark the end of the sensing area with accuracy up to +/-2oF or 1 degree Centigrade. Bimetal thermometers play a big role in food safety and food quality as they manage both time and temperature. The bimetal thermometer can be the most single important tool that can protect food.