A dial thermometer, also known as a differential absorption lidar thermometer, is a mechanical device that uses a metal pointer on a circular scale to indicate temperature measurements. The device has two primary components; the metal pointer, or needle, and a temperature sensor. These sensors may be bi-metal or bimetallic, liquid or gas filled, or vapor-tension-based.
A bimetal dial thermometer, also known as a bimetallic strip thermometer, uses coil spring technology. The sensing device is made up of two different metals welded or fastened together, like steel and copper or steel and brass. One metal has low heat sensitivity, while the other metal has high heat sensitivity. The bimetal sensor is attached to the metal pointer or needle on the face of the thermometer. As the temperature rises, the metals respond differently, causing the metal strip to curl and move the pointer on the temperature scale.
Bimetal technology has been used in thermometers for 200 years. It is currently popular in a variety of household applications including thermostats, wall thermometers, grills, circuit breakers for electrical heating devices, and other household applications. This kind is best used for warmer temperatures, and some bimetal thermometers can measure up to 500 degrees Fahrenheit (260 degrees Celsius). Accuracy diminishes with a bimetal dial thermometer at low temperatures because the sensitivity of the two metals to low temperatures is very similar.
Liquid or liquid/gas based thermometers also operate on the principle of expansion and contraction. As the liquid expands or contracts in response to the temperature changes, the resulting pressure moves the needle on the scale. Mercury, alcohol and ether are among the liquid choices available. A liquid- or gas-based dial thermometer is more accurate at lower temperatures than bimetal, and is commonly found in HVAC systems, refrigeration plants, refrigerated trucks and photo processing equipment.
A less expensive choice of dial thermometer is the vapor-tension-based device. This type of thermometer is filled with a permanent gas. A variety of liquids are used to create the gas, and the choice is based upon the temperature range desired for the thermometer. These are often used in refrigeration, drying ovens, chemical processing, solar heating and HVAC applications.
The case for a dial thermometer is usually made of metal or stainless steel, with a glass or plastic crystal covering the circular face. Temperature may be measured in Fahrenheit, Celsius or both. Some devices, such as meat thermometers, have a sensing probe.
There are thousands of different thermometers available. One of the primary considerations should be the application requirements. For industrial applications, shock resistance and vibration tolerance could be important. The temperature range and degree of accuracy required are also pertinent considerations.