Reprinted from R&D Magazine, September, 1995
Multi-wavelength Expert System Has Many Applications
RALPH A. FELICE, President
FAR Associates, Macedonia, Ohio 44056
Editor’s Note: This novel pyrometer addresses the range from 500 to beyond 3500°C. It is claimed to require no prior knowledge of the target, including the emissivity, to yield extremely accurate temperature determinations. It provides a unique on-line tolerance, a gauge of the accuracy of each measurement. This feature allows the users to ascertain that the measurement has been successful or alerts them to interfering conditions.
Radiation pyrometry has many advantages over other temperature measurement techniques: it’s fast, it doesn’t require contact, it doesn’t affect the temperature that’s being measured, and it needs no consumables. For temperatures above about 1800°C/3300°F there isn’t really a practical alternative. The pyrometer discussed in this article is particularly of value in difficult applications which have traditionally defied solution. These include two broad classes: (1) where the target’s emissivity is unknown or changing, and (2) where the environment interferes. A third, smaller class may be added: applications where the temperature is so high that no source of known temperature exists to calibrate the instrument. In many of these applications the reproducibility of some pyrometers has allowed their “temperature” outputs to be correlated with product quality through trial and error. As long as nothing changes, the correlations hold. However, change is inevitable: process scale-up, product and process development, commissioning of new capital equipment, automation, and operator turnover all undermine the reliability of the carefully-developed correlations. Reproducibility is not enough when the target and environment change. Accuracy is needed for the knowledge base to carry over into the new situation. Without it, one may be in the position of the engineering manager who said “every time we change the product 10% we have to do 70% of the engineering over.”
A new type of pyrometer, developed and tested in a broad spectrum of industrial applications over the last 10 years, is capable of solving these problems. In the course of its testing it has demonstrated these attributes:
- Independence from knowledge of target emissivity
- An on-linegauge of accuracy (a tolerance describing how well the indicated temperature is known)
- Extreme accuracy and reproducibility accuracy of 0.1% reading is readily achievable, reproducibility of 0.01% is routine
- Unlimited high temperature response
- Ability to cope with absorbing or emitting atmospheres
- Noise immunity
- Adaptability to many situations
- Broad temperature response in one package
CAUSES OF DIFFICULTY
Making a successful pyrometric measurement requires allowance for emissivity and environmental interference. Emissivity, the ratio of the emitted radiation of a real radiator to that of an ideal one, can depend on composition, surface finish, mechanical and thermal history, and the wavelength where the measurement is made. It can also depend on the temperature; then the problem becomes a circular one, impossible to resolve. Environmental interference often is in the form of absorption or emission of thermal radiation by the material between the instrument and the target. This includes both the atmosphere and any optical elements in the path. If the detector is sensitive to the wavelengths where this occurs the result will be affected.
The overwhelming majority of pyrometers available are of two types: brightness or ratio. Brightness devices rely on capturing a known fraction of the energy emitted by the target; the user must know the emissivity to get the correct temperature value. For the reasons shown above this can be impossible.
Ratio pyrometry attempts to circumvent the emissivity issue mathematically. Intensities are measured at two different wavelengths and divided. The resulting representative equation is solved for temperature and the hope is that the division has canceled out the emissivity. This method works if the emissivity is the same at both wavelengths, but this is only certain in an ideal or semi-ideal (greybody) radiator. Concern over emissivity cancellation affects the design of ratio pyrometers: the closer together the wavelengths are chosen, the more likely the emissivities are to cancel; the farther apart, the larger the magnitude of the resultant signal, and the greater the precision.
These problems of relative and absolute emissivity as well as the separate issue of environmental interference are addressed by a new pyrometer. It can detect and avoid the influencesof emissions and absorptions in the thermal spectrum and determine the value of the emissivity. It does both of these with no prior information about the target or its environment.
THE NEW MULTI-WAVELENGTH PYROMETER
The idea for this device grew from the frustration developed when several existing pyrometers were used to measure a process temperature. Each gave a different answer. We speculated that if a huge number of instruments could be used and a large fraction of those agreed among themselves, then the result should be trustworthy. It turned out to be not quite that simple. After 10 years, the resulting “tolerant” pyrometer has solved the problems of difficult applications with a variety of targets and environments. These include ceramics, glass, composite materials, metals, combustion, chemical vapor deposition, etc.