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The Problem of Measuring the Temperature of Low Emissivity Materials Near Ambient Temperature.

                            

                                             By Ralph Rudolph

 

In the steel industry, it is often desirable to measure the temperature of unoxidized steel strip (which may be coated with other metals) at temperatures near ambient (less than 300 Fahrenheit). Temperatures this low require the use of longer infrared wavelengths to obtain enough infrared energy to make a measurement. Typically, I recommend using a 3.43 micron device as commercial radiation thermometers are commonly available using this wavelength, and emissivity is higher than  at longer wavelengths such as general purpose 8-14 micron units.

 

There are several problems in making such a measurement. First, one must look at the product mix being produced. In some cases, the product may have a range of differing emissivities depending on surface roughness or differing applied coatings. At low emissivities, even a small error in emissivity setting may cause a significant error in measured temperature.

 

Second, and this is a problem that many vendors don’t seem to know about, at long wavelengths, all the surroundings also emit significant infrared radiation which can reflect off the strip into the radiation thermometer, causing it to read too high of a temperature. The problem gets worse at longer wavelengths.

 

There are a variety of approaches which can be used to overcome these problems, some fairly inexpensive, some not. The complexity of the approach used depends on the accuracy desired and the location where the measurement is to be made. If the measurement is critical to achieving desired properties, the cost considerations should include that the cost of even one or two off-spec coils can run to many tens of thousands of dollars.

 

Here are a few methods that can be used, with their pros and cons:

 

  1. The Wedge (or Roll Nip) Method: This method is coming into common use (and misuse as it is incorrectly applied in many cases). At a point where a strip makes a wrap around a roll, if the radiation thermometer (RT) is aimed (usually from along side of the strip) into the conical cavity formed between the roll and strip, the cavity is assumed to form a blackbody with an emissivity of 1.0. Usually the sighting angle is about 6-8 degrees from the plane of the strip (very shallow) to sight into the cavity as deeply as possible, and the RT is aimed to sight slightly on the strip rather than the roll. (Laser sighting helps this adjustment). There are several requirements for this method to work well, and some peculiarities which should be understood.

 

It should be understood that blackbody conditions exist if and only if the roll and the strip are at the same temperature. This can occur over a time period if there is sufficient heat transfer between the strip and roll. For such heat transfer to be effective, the roll wrap should be maximized and the roll itself should have a low thermal mass, allowing it to be easily and quickly heated by the strip. How quickly the roll and strip equilibrate to the same temperature depends on line speed, strip thickness, sufficient line tension and details of the roll structure and wrap. I have done side by side comparisons of wedge systems and other systems, and these have easily shown that when the strip changes temperature, for instance with a thickness change, that there can be a considerable time lag in reestablishing blackbody conditions. During this time lag, the wedge method temperature reading can be in considerable error. Obviously if the incoming strip becomes hotter, the temperature measurement will be low, and if the incoming strip is cooler, the temperature reading will be high. It must be understood that this type of error is inherent in the wedge method. It is not accurate under these conditions.

 

Even with a well designed wedge system, it should be understood that the roll seldom actually reaches the same temperature as the strip (because of air cooling) and an emissivity somewhat less than 1.0 must be used to account for this. One method that can be used to improve the wedge method is to use a second RT that measures the roll temperature itself and the reading from this second RT can be used to correct the wedge RT reading. It is a bit more expensive, but it can considerably increase the accuracy of the wedge temperature measurement. If a wedge system is employed, I’d certainly recommend this variant.

 

The main advantage of a wedge system is that it is cheap to implement

 

  1. Reflective methods: If a hemispherical enclosure is built using high quality first surface mirrors, and an RT is sighted into this enclosure (usually through a fiber optic probe), and the open side of the enclosure is placed very close to the strip, most of the radiation emitted by the strip will be multiply reflected within the enclosure, making the interior approach a blackbody. This type of system, with a well designed enclosure, can work well in the laboratory.  From a practical standpoint, this system is difficult to implement in a mill because of trying to keep the mirror clean and because of the very close spacing to the strip causing the possibility of damage to the mirror system. It is a high maintenance system and requires an in-depth supply of extra mirrored enclosures. If a continuous reading is not necessary, it is advisable to move the unit into place only periodically on a reciprocating arm to minimize possibility of damage from the strip. (The reading when in place can be used to “correct” a second more conventional unit if desired).

 

  1. Constant background method: Imagine that a highly conductive flat black plate with a rough (machined rough) lower surface is placed over the strip and that an RT sights the strip through a small hole in the plate. The plate is designed to be a uniform preset temperature such as 100 F.  I select 100 F to avoid  water condensation on the plate. (One way to achieve a uniform temperature is to make the plate hollow and recirculate a high thermal conductivity fluid through it). If the strip emissivity is measured (using a laser device) and is reasonably known, the fraction of radiation received from the black plate versus from the strip can be calculated to determine the strip temperature. This system can be costly as the device to measure emissivity is not cheap, and the system requires embedded computers and extra heaters and instrumentation.

 

  1. Driven Source Closed Loop System: As in 3 above, a large flat black plate is placed above the strip and an RT is sighted through a small hole in the plate. The RT has its emissivity set at 1.0 and the output from the RT is used as a setpoint temperature for the plate heater system. (A minimum temperature of 100 F or so is required). With careful adjustment, the system will stabilize at a point where the plate temperature tracks the strip temperature. The main problem with this system is that it is useful only on higher emissivity product (over about 0.25) and has a fairly slow response time, but it is less expensive than #3 as an emissivity measuring device is not needed.

 

  1. Strip foldback System: If a location can be found (often at certain sets of bridle rolls) or designed so that the strip changes direction and forms a “cavity” bounded by the strip on two sides, a right angle fiber optic probe can be inserted into the middle of this cavity which will have an emissivity somewhat less than 1.0.  This is somewhat similar in a way to a wedge system but has higher losses depending on strip-to-strip spacing. The main problem is fiber optic probe vulnerability with strip breaks, and determining exactly what emissivity to use.

 

  1. Flat mirror system:  A large, flat first surface mirror is suspended several inches above the strip.  The RT is sighted into the gap between the mirror and strip, typically at about a 45 degree angle. With enough bounces between the strip and mirror, emissivity of this “cavity” may approach 1.0. As with the reflective cavity (#2) the main problem is keeping the mirror clean and determining exactly what emissivity (somewhat less than 1.0) to use.

 

  1. Subtractive Method: This works only if emissivity is stable. Two RTs are used, one sighting the strip and one sighting a cooled dummy target of similar material side by side. The signal from the cool dummy target is assumed to be primarily reflective and is subtracted from the signal from the main RT to get a strip-only signal.