MeasurementBlog

You would think that a highly accurate temperature measurement device like the Optical Pyrometer would excel at measurements looking into high temperature furnaces, wouldn’t you? Lots of people believed that, too not so very many years ago, like in the 1970s.

Then the really serious workers, those involved in trying to optimize the trade-off between lifetime of the special alloy metal tubing used in petrochemical thermal reformer furnaces and production throughput began to look a little closer at the measurement errors involved.

The workhorse instruments, Optical Pyrometer (”Opticals”), suffered from a difficulty that all non-contact temperature sensors share: it is invariably quite difficult the measure the temperature of the object by any other means.

The fact is, people were getting by with the assumption that “Optical” were infallible.

The first person, that I am aware of, to put some good measurements and theory together on the subject was Dr. Tohoru Iuchi, then the temperature measurement specialist for the Nippon Steel Company in Japan. His topic was a slightly lower temperature but equally knotty measurement problem: moving steel strip in a reducing atmosphere furnace.

(The last time I met Dr. Iuchi was nearly two years ago when he gave a paper at the 2006 ThermoSense Conference in Orlando Florida. He gave an excellent paper on Radiation Thermometry of Semitransparent Silicon Wafers. He was then a staff member of the Sensor Photonics Research center at Toyo University in Saitama, Japan.)

In fact, most of the measurement advances made under this particular set of measuring conditions were made by those addressing the measurement problems in both reducing atmosphere annealing furnaces and ordinary steel reheating furnaces in the late 1970s and 1980s.

Many of the solutions for steel furnace measurements are described along with appropriate references and equations in the book, “Applications of Radiation Thermometry”, Eds. J.C. Richmond & D.P. DeWitt, that may be purchased even today from the Digital Bookstore at the ASTM International website (Click on the link below).

It includes a Chapter by Dr. Iuchi describing many of the devices developed by his group at the Nippon Steel Company’s Research Center.

“Applications of Radiation Thermometry” Eds. J.C. Richmond & D.P. DeWitt, ASTM STP 895 (1985)

The principle measurement problem correctly identified by Dr. Iuchi, was the effect of reflected thermal radiation coming from from sources hotter than the object being measured. Both the emitted radiation and the reflected radiation are received by the Pyrometer.

The poor Pyrometer can’t tell which is which… so, it adds them up.

What’s worse, the highly nonlinear properties of thermal radiation mean that just a little from the higher source is very much more than that emitted by the lower temperature object.

Even without an emissivity correction, in most cases, an object in a hotter environment will appear to be at a higher temperature than it actually is to an Optical Pyrometer.

Under ideal furnace conditions, when the furnace walls and roof are at about the same temperature as the object, the reflected radiation from the furnace components makes up for the imperfect emission properties of the object and enables one to make a temperature measurement without need of an emissivity correction.

So, the next worse thing about the nature of reflected thermal radiation, is that unskilled operators often attempt to make an emissivity correction to the reading they get when they have a little knowledge of the measurement physics.

When reflected thermal radiation, even from an equal temperature source, is present, the emissivity correction skews the reading higher than true. When the reflection source is significantly hotter than the object, an emissivity makes an already higher than true into a much higher than true one.

In 1986 a special symposium was held in Houston Texas, sponsored by the American Institute of Chemical Engineering (AIChE), entitled “Measurement of High Temperatures in Furnaces and Processes”. It was chaired by Drs D.P DeWitt & L. F. Albright of Purdue University.

The proceedings were published in The AIChE Symposium Series, No 249, Volume 82, 1986.

In the meeting, Dr, Hoyt Hottel of MIT gave an overview of the measurement problems and T.R. R. Beynon & R. Barber of Land Infrared described the choices of optimum measurement wavebands, describing in some depth the experimental work done by Land both in Europe and the USA of Steel Mill reheating furnaces and simulators. The published Proceedings captured all the talks and a very interesting panel discussion.

The problem was recognised by many organizations in the Oil Industry and a unique instrument was developed and patented by Exxon Research to help solve it and get around the limitations of the Optical Pyrometer.

A special unit now manufactured by the Pyrometer Instrument Company under license from Exxon called The PyroLaser is used effectively to correct for emissivity and attempts to solve the measurement problem of reflected radiation in such Petrochemical furnaces.

The Quantum I Portable Laser IR Thermometer, now made by Mikron Infrared, is a similar device and was designed by the original inventor of the patented Exxon Research device.

A competing Land Instruments device, the portable Cyclops 390B Furnace Pro uses a different approach by using an optimum measuring waveband, where estimated emissivity errors can be more readily tolerated.

In addition, Land, now a division of Ametek, also offers a device known as the Furnace Thermometer System, aimed specifically at fixed, online measurements rather than portable.

The Optical Pyrometer has too short an effective wavelength (See ASTM STP 895 and the AIChE Proceedings, described above for an excellent article by E. A. Nutter that describes the term ‘Effective Wavelength’ and choice of optimum wavebands by Beynon & Barber).

The errors made by Opticals when attempting to measure tube surface temperature in thermal reformer furnaces were higher than with any other type of noncontact temperature measuring device then on the market! Now there are other ways to do the job better.

Also, at last, someone has done a very thorough job on pulling together all the loose ends involved in solving the measurement problem and written a book about it. In it both the near IR and Mid IR measurement approaches are described in detail and the problems of other verification means are described.

The person is an objective worker in the field, Dr. Peter Saunders, a well-known physicist who works at The Measurement Standards Laboratory (MSL), New Zealand’s national metrology institute. His book, Radiation Thermometry: Fundamentals and Applications in the Petrochemical Industry (SPIE Press Book) by Peter Saunders, was just released 3 August 2007 It is available from the SPIE Press, ISBN: 9780819467836, price for SPIE Members: $39.00(USD), and Non-member: $47.00 (USD). It’s the latest, but possibly not the last word on this measurement problem area. But after skimming through the chapters, I think it will be a while before anyone digs as deeply into this area of measurement.

Bottom Line:

Don’t try to use an Optical Pyrometer to measure temperatures in Thermal Reformer or Thermal Pyrolysis Furnaces, Steel Mill Reheat and Anneal furnaces and possibly other high temperature furnaces unless you have a very good idea of what to expect!

I have only got a little more to say about the shortcomings of Optical Pyrometers and it is better left for another time when I can add a little balance by telling what’s right with them, too.

Later, then.

Thanks for visiting!

Ray Peacock

TEMPERATURES.COM, INC. publishes information about measurement devices and measurement on its websites. The sites have articles, directories and news to foster competent measurements & analysis in industry & science. Sites are free. Submissions by visitors are encouraged and reviewed. Sites as of August 2007 are: lehos tecHeadlines, measureNEWS, About Temperature Sensors, TempSensor Directories, TempSensorNEWS, Measurement Databases, (MeasurementBlog.com)MeasurementMedia.com and MeasurementDevices.com

---

This entry was posted on Monday, August 13th, 2007 at 5:19 pm and is filed under Temperature.You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.