MeasurementBlog

Even More Limitations of Optical Pyrometers (Chapter 3)

This problem is one that affects a smaller number of users, but to them it is significant because it limits their ability to use the “power” of Optical Pyrometry to gain precise AND repeatable temperature measurements of their products. The limitation is , of course, the subject of this brief note, speed of response.

How quickly does an optical pyrometer respond?

Sounds simple, but it is not and does not have a simple answer. Like a lot of other parameters affecting Optical Pyrometers, the answer depends on the conditions prevailing at the time of measurement AND the properties of the actual Optical being used.

Suffice it to say, the response time for following a step change in apparent temperature is of the order of 1 to 10 seconds, possibly longer, for an Optical Pyrometer in the hands of a skilled user, regardless of how you define the response time (e.g. exponential response time, 90% response, or 95%, 98% or 99% response)!

That sort of time domain is a very long one for someone interested in monitoring temperatures that change in fractions of a second, let alone trying to control the temperature of a product in a process where the product temperatures are varying with random time swings in tens of milliseconds to tenths of seconds.

Fast temperature changes will add averaging errors of random amounts to any reported temperature values made with a “slugged” or much slower-responding sensor, and can greatly increase the overall measurement uncertainty of any reported, nominal average temperature.

Those sorts of errors occur in manufacturing and research cases where temperatures are driven by the object’s heating or cooling rate, rapid variations in the spectral emissivity of the object and the occurrence of unpredictable obstructions that randomly attenuate the passage of thermal radiation into the instrument.

Such events occur regularly in high temperature processes such as refining and casting of ceramics, liquid metals like copper, brass, gold, iron, nickel, silver and steel.

Note that glass is not covered here, simple because the melting and refining of liquid glass usually takes place in relatively slow processes in furnaces which are at temperatures close to that of the molten glass. (Some glass forming processes do run much faster, but they are also special cases for optical temperature measurement, most best treated one at a time)

Glass temperature measurement in melting and refining sections of industrial furnaces is one of the many furnace application areas for Optical Pyrometers with a long history of successful use. Even in those cases, closer examination of conditions can show other limitations of Optical Pyrometers. I’ll get to them a little later.

High speed temperature events can and do occur in the many processes used to shape and form products made from such materials where temperatures high enough to warrant use of Optical Pyrometry. These include, among others, heat treating, reheating, hot rolling, forging, extruding and welding.

One final comment on speed of response has to do with the few cases of “Automatic Optical Pyrometers”.

There have been a few on the market over the past thirty or so years. Pyrometer Instrument Company used to make one and some may still dwell in storerooms that somehow these days seem to connect to Ebay! The basic design was one by E.A. Nutter and is described in design detail in a paper he gave at the Fifth International Temperature Symposium in Washington DC in 1971!.

Spectrodyne in Pennsylvania makes one even today (not to Gene Nutter’s design, but similar to the one developed at L&N in their waning days of existence).

The problem is that even these automatic devices either have a time response that is no faster than a manual instrument or the specification in not provided.

On the other hand, Optical Pyrometer users should not feel put upon by reading these pages. I strive very hard to be as objective as possible based on my knowledge. The fact is Opticals have been bypassed by technology!

Superior and inferior alternatives to Optical Pyrometers exist on the market today, but since there are no standards for any of these noncontact temperature measuring devices, it can be a swampy morass to explore. I am aware of at least two companies that makes an equivalent wavelength response model with through the lens sighting, built-in spectral emissivity control and shorter response time than traditional units. They all use absolute, calibrated photo detectors and not the comparison and feedback method of Opticals.

The Model M90V from Mikron Infrared and Chino Instruments Model IR-AHU both are modern, automatic radiation thermometers that operate at the 0.65 micrometer waveband region. The Chino unit will measure as low as 900 Deg C, while the Mikron goes about 100 degrees C lower to 800.

To add even more fuel to the competitive spirit, Land Instruments offers a very unique handheld model, that they call the Meltmaster model. It operates at even a shorter effective wavelength of 0.55 micrometer. Since one of the biggest claims to accuracy in the Optical Pyrometer’s history is a very short wavelength, one would think that a 0.55 micron instrument would be more accurate that a 0.65 micron one.

Unfortunately, that is an over simplified assumption. One thing for sure is the fact that all these devices have a 0.5 second response time, not a heck of a lot faster than the manual Optical, but none the less, faster and automatic.

I’ll get into over simplifications next time and hope to make the issue a little clearer.

Ray Peacock

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