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.
Types of Thermometers
As noted, there are many, many type of thermometers that are used across a huge number of applications. Thermometers hang outside our windows to tell us the ambient temperature. They are built into
HVAC systems to initiate heating or cooling of buildings. They are built into cars to show when the cooling system is failing. We could go on and on.
For the purpose here, we are focusing on three types of thermometers used in commercial and industrial processes.
Bimetal thermometers
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.
Digital / thermocouple thermometers
Digital / thermocouple thermometers rely on either built-in thermocouples or input from external thermocouples to measure temperature. A thermocouple is a temperature sensing device consisting
of two dissimilar metals (conductors) joined together at one end. Unlike bimetal thermometers which use the mechanical displacement of dissimilar metals to indicate temperature, thermocouples
use a phenomenon known as the Seebeck Effect.
When any conductor is subjected to a thermal gradient it will generate a voltage, a condition known as the Seebeck Effect. Different metals generate different voltages when exposed to a thermal
gradient. The small difference in electrical voltage produced by the dissimilar metals is proportional to the temperature difference between the sensing end of the thermocouple and a reference
temperature. From this an accurate temperature can be determined.
Thermocouples are one of the most widely used temperature sensors available. They are very common in measurement and control applications in industrial and commercial settings and are also found
in thermostats and flame sensors in residential applications. The popularity of thermocouples stems, in part, from their simplicity, adaptability and cost. The main limitation with thermocouples
is accuracy, system errors of less than one degree Celsius (°C) can be difficult to achieve.
Digital / thermocouple thermometers read the electrical signal of thermocouples and display the temperature. Often they can read multiple thermocouples simultaneously. Advanced functions such as
data logging, alarms, hi/lo, etc. are sometimes included as well.
Precision thermometers
Precision thermometers are very similar to thermocouple thermometers but use RTDs or SPRTs instead of thermocouples. By using these more accurate sensors, precision thermometers are capable of
making reference-quality temperature measurements meaning they can be used as a calibration standard for other temperature sensing equipment.
RTDs, or resistance temperature detectors, are temperature sensing devices that work by correlating the resistance of a highly pure conductor to temperature. They work on the well-known principle
that the resistivity of a conductor increases as the temperature increases and decreases as the temperature decreases. In practice, a small electrical current is passed through a conductor, which
serves as the RTD element. The resistance to that electrical current is then measured and correlated to a specific temperature based upon the known resistance characteristics of the material that
makes up the RTD element.
RTDs are generally considered to be among the most accurate temperature sensors available. In addition to offering very good accuracy, they provide excellent stability and repeatability. Though
many conductors can be used in making RTDs, platinum is the preferred material. As a noble metal, platinum doesn’t react with other materials making it highly stable with a very linear and repeatable
resistance-temperature relationship over its temperature range. Platinum RTDs are often called PRTs (Platinum Resistance Thermometers). SPRTs (Standard Platinum Resistance Thermometers)
have the highest accuracy of all temperature sensors and can achieve an accuracy of up to ±0.001°C.
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?
If you have any questions regarding thermometers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
