MeasurementBlog

Technology has usurped the capabilities of Optical Pyrometers in almost every measurement situation. First, let me back up and explain about them, how they work and why I think they are obsolete.

For decades, the by-word in industrial temperature measurement in many processes was the Optical Pyrometer (aka: DFP, Disappearing Filament Pyrometer, Optical, etc.). A unit well-maintained and calibrated, in the hands of a trained and skilled user could be expected to deliver accurate, non-contact temperature measurements in many process lines under specific conditions and with appropriate corrections.

Thermal processes like glass melting, special semiconductor and ceramic heating, steel annealing & reheating, vacuum alloy melting & heating were among the applications for these measurement devices that came to depend on their regular use. One of the regular uses was to check the validity of online or fixed mount radiation pyrometers (thermometers) sighted into closed end target tubes in large furnaces, such as steel mill slab & bar reheating furnaces.

In glass & glass product manufacturing, the Optical was nearly a universal standard device for monitoring internal melter, refiner and forehearth temperatures and profiles, especially in very large, regenerative/recuperative furnaces.

Another outstanding use was the measurement of liquid iron temperatures in production of cast iron parts. In fact, this unique measurement was one of the very few for which the former Leeds & Northrup Company (L&N) manufactured units with special, emissivity-corrected temperature scales for the spectral emissivity of liquid iron at the wavelength of 0.65 micrometer, the operating wavelength of most devices; typically the scale was preset for spectral emissivity of about 0.39.

A skilled user could correct for those measurements made outside furnace environments, where nearly blackbody radiation conditions prevailed, and a different spectral emissivity correction was needed. The use of a very narrow waveband in the Optical meant that a single wavelength approximation to Plank’s Law could be applied.

[Wein's approximation to Plank's law was automatically ensured due to the temperature ranges measured in most applications, since c2/(lambda x T) >>1 for all temperatures below about 5,000 K, the temperature where the peak thermal emission would be at a wavelength of about 0.65 micrometer. That means, in all measurements below about 5,000 K, that the the instrument's response to temperature is very non-linear while its response to emissivity is linear. The net result is that an optical pyrometer can be quite accurate in reading temperature despite emissivity estimates being far less accurate.]

HOW THEY WORK

They are called disappearing filament optical pyrometers because their use in temperature measurement requires the pyrometer operator to perform a visual match between the brightness of the object of measurement and a heated lamp filament while viewing the filament, superimposed on the field of view of the object,in the optical viewing system.

The optical viewing system includes filers that restrict the waveband response to a relatively narrow wavelength range near 0.65 micrometer. A wavelength region of about 0.65 micrometer is in the red, so that the images appear red.

Some examples of the visual variations seen in a measurement are shown on the web pages at About Temperature Sensors and Spectrodyne, a manufacturer of their own design and repairer of used L&N optical pyrometers (there are no new ones since L&N stopped making them more than 10 years ago). Another optical pyrometer design and an example of the visual effects in the Pyro brand is on the Pyrometer Instrument Company website.

The operator’s goal with this measurement technique is to perform a match in the brightness of the object and the filament by varying the filament’s brightness. In many instances the match can be difficult to precisely achieve, especially near the lower end of the measurement range. When a match is made, the filament appears to vanish. Thus, operator practice and experience are very necessary.

OK, so two of the limitations inherent in optical pyrometers are emissivity sensitivity and operator training and experience required.

There are other limitations. I’ll get them to them in the next post and beyond; there’s a lot to go through and I’d like to take them one bit at a time.

Meanwhile, how quickly do you think you could make a brightness match between two objects in a field of your ’scope if you had to use a manual brightness control, like a rotary control?

Thanks for visiting. See you next post!

Ray Peacock

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