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Are You Measuring the Correct Temperature?

Accuracy of all temperature measurements in process production is vital to ensure quality, efficiency and safety.


Noncontact temperature measurement is a key process metric in many industries, but there are many factors that influence accurate temperature measurement from a pyrometer. Here we look at the common influences on inaccurate temperature measurement.


Pyrometer Choice for Industrial Process Temperature Measurement

There are several different pyrometers on the market, but the key deciders on choosing the correct pyrometer for the most accurate temperature measurement of your process are the same.


What wavelength of detector is used for a given temperature range? The electromagnetic spectrum contains many different forms of electromagnetic emissions, including infrared, visible light, X-rays and radio waves (figure 1). The only difference between these emissions is their wavelength (related to frequency).


Radiation pyrometers typically are designed to respond to wavelengths within the infrared portion of the spectrum. In practice, temperature measurement is made using pyrometers that are operational over many different ranges of wavelength, which generally reside between 0.5 to 15 μm.


The intensity of the emitted energy of an object versus wavelength is given by Planck’s radiation law, and this shows the wavelength of the peak energy radiated will decrease as the temperature increases. Utilizing Wien’s displacement law and the Stefan-Boltzmann law to develop the Planck’s curves (figure 2) shows clearly why the peak energy level shifts to shorter wavelengths as the temperature increases.


The relative amount of energy emitted by an object when heated to different temperatures across the infrared spectrum is shown in figure 3. Again, this shows the characteristic of peak energy emitted at increasingly short wavelength for increasing temperature.


The implications of these laws mean that the most energy, which gives the most signal strength to a pyrometer detector and therefore the smallest noise-to-signal ratio, which gives the most accurate temperature readings, is with short-wavelength pyrometers.


Typically, high temperature processes in the steel and glass industries are measured using 1 μm pyrometers. With recent technical advances, short-wavelength pyrometers can even reach down to 122°F (50°C) with a 2.1 μm detector and still cover a wide temperature span up to 2012°F (1100°C). This allows them to be utilized across a range of industrial processes.