Electricity and Control March 2020

MEASUREMENT + INSTRUMENTATION

Ratio pyrometers – operating theory and applications In industry there are many applications where a standard one-colour thermometer may read the temperature incorrectly and a two-colour or ratio pyrometer will be preferable to provide correct temperature measurements. R&C Instrumentation outlines the advantages of using two-colour pyrometers in particular applications.

S ome of the typical applications where a standard one-colour thermometer may read the temperature incorrectly include those where: - the object is too small to fill the cone-of-vision - dust, smoke or steam obscures the line of sight - windows in the process become dirty and difficult to keep clean - the emissivity of the product changes (due to a change in alloy or surface condition, for example). However, a two-colour or ratio pyrometer, like the Endurance pyrometer by Raytek and Ircon, operates effectively even where such problems exist and will indicate the correct temperature. Theory of operation The two-colour ratio technology provides for accurate and repeatable temperature measurements, which do not depend on absolute radiated energy values. In use, a two- colour sensor determines temperature from the ratio of the radiated energies in two separate wavelength bands (colours). The key benefit of two-colour sensors is that accurate measurements can be made under conditions where: - the field of view to the target is partially blocked or obscured - the target is smaller than the sensor’s field of view

At a glance ■ Two-colour or ratio pyrometers determine temperature from the ratio of radiated energies in two separate wavelength bands (colours). ■  The radiated energy from a target is, in most cases, equally reduced when objects or materials in the atmosphere block some portion of the field of view. It follows that the ratio of the energies is unaffected, and thus the measured temperatures remain accurate.

even if the target does not fill the resulting spot size. The convenience is that the user is not forced to install the sensor at some specific distance based on target size and the sensor’s optical resolution. Partially obscured targets The radiated energy from a target is, in most cases, equally reduced when objects or atmospheric materials block some portion of the field of view. It follows that the ratio of the energies is unaffected, and thus the measured

- the emissivity of the target object is low or changing by the same factor in both wavelength bands. Another benefit is that two-colour sensors measure closer to the highest temperature within the measured spot (spatial peak picking) rather than an average temperature. A two-colour sensor can be mounted further away,

Two-colour or ratio pyrometers can provide accurate temperature measurement in applications where one-colour thermometers may not be able to do so.

Electricity + Control

MARCH 2020

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