Bimetal thermometers use two strips of different metals to convert temperature into mechanical displacement. The different metals, often steel and copper or steel and brass, are joined together throughout their length and will expand at different rates as they are heated. He difference in expansion will cause the strip to bend one way if heated or the other if cooled. This movement correlates to the actual temperature and moves an indicator along the scale. The metal strips can be lengthened for sensitivity and are often coiled for compactness.

Bimetal thermometers are inexpensive, simple, and durable. As purely mechanical devices, they display only the current temperature on their dial. They can be accurate to about 1% full span. They are often used in food and beverage preparation, process temperature measurement, wastewater treatment, boiler systems, and other places.

Thermometers are devices that measure temperature or a temperature gradient using a variety of different principles. Temperature is simply the numerical measurement of hot and cold—which has great importance in a wide variety of applications. Temperature affects our comfort, cooks our food and is critical to making many products upon which we rely. Given the extreme importance of temperature across so many aspects of modern life, thermometers are a familiar tool to us.

Temperature is the most commonly measured parameter in commercial and industrial settings. Industries as diverse as food processing, pharmaceuticals, cold storage, paper manufacturing, and others absolutely rely on process temperatures being within a certain range. Though there are many temperature sensing options, thermometers provide an inexpensive, versatile and reliable choice.

Characteristic of Thermometers

Though some thermometers rely on sophisticated technology such as infrared sensing to make non-contact temperature measurements, thermometers for contact measurement—those we are describing here—are much simpler, and rely on just two important elements: a temperature sensor and a scale. Temperature sensors range from the bulb on a mercury thermometer to RTD or thermocouples for more advanced thermometers. Likewise, the scale can be simply a series of accurately placed markings printed on the side of a mercury thermometer or a digital readout capable of advanced calculations or data logging.

Scale

The scale of a thermometer is important. More than just a series of digits placed along the edge of a thermometer, scale is an internationally agreed upon value that corresponds to specific temperatures. Though an individual thermometer can measure the temperature, there is no way to compare that to the readings of another thermometer unless they conform to an agreed upon scale.

Temperature scales are based upon fixed points such as the freezing and boiling points of water. The most recent attempt to fix the values of temperature scale is the International Temperature Scale of 1990 (ITS-90). It extends from 0.65 K (−272.5 °C; −458.5 °F) to approximately 1,358 K (1,085 °C; 1,985 °F).

Temperature scales differ in two ways: the point chosen as zero degrees, and the magnitudes of incremental units or degrees on the scale. Common temperature scales include the Celsius scale (°C) which chooses the freezing point of water as 0°C and defines the magnitude of degrees such that 100°C is the boiling point of water. In the United States the Fahrenheit scale is commonly used. Another common temperature scale is Kelvin which is usually used in scientific applications.

Calibration

Like most measuring devices, thermometers need to be calibrated periodically to maintain accuracy. Calibration often consists of calibrating them with other, calibrated thermometers or by checking them against known fixed points on the temperature scale such as the freezing or boiling points of water. Since the sensors typically used in thermometers cannot be adjusted, any adjustments following a calibration need to be made to the scale, either through a manual or electronic adjustment depending upon the type of thermometer.

Things to consider when selecting a thermometer:

What level accuracy is needed? Over what temperature range?

Which scale is preferred?

Will it measure at the spot or remotely?

In what type of environment will the thermometer operate?

Are multiple channels needed?

Will the thermometer be used for food service? Will it require hygienic approvals?