Digital 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.
Thermometers
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?
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.
