How Do Two Color Pyrometers Work? The two color thermometer is one of the most popular instruments that Ircon sells. It has many applications in many markets and solves many difficult applications.

Two Color Pyrometers As a short review, let’s remember, all infrared thermometers measure the temperature of an object without physical contact. The ability to accomplish this is based on the fact that every object emits radiant energy and the intensity of this radiation is a function of its temperature. All infrared thermometers use an instrument called a blackbody as its source of calibration. This blackbody is a perfect radiator. In other words, this device radiates the maximum amount of infrared energy that any object can emit at any temperature or wavelength. Every other hot object emits less energy than a blackbody does at the same temperature, and the object is said to have an emissivity characteristic. The emissivity is simply the ratio of how the hot object emits energy compared to the perfect blackbody. Thus, if we say a piece of oxidized steel has an emissivity of 0.8, we mean it is emitting 80% as much as a blackbody at the same temperature. Since the infrared thermometer reads correctly when it is looking at a blackbody, we somehow have to tell it that for the steel there is a 20% loss in signal. The infrared thermometer has an emissivity control which, when set at 0.8, correctly amplifies the detector signal, so that the instrument reads the true temperature.   Infrared thermometers also have a lens system. This lens system collects the infrared energy from a specific target area. When the infrared thermometer looks at this specific target, it measures the intensity of the radiant energy over the entire target area. The energy travels from this specific target back to the lens in the form of a “Cone-of-Vision”. Most infrared thermometers require that the cone-of-vision be completely filled with the hot object and no obstruction can interfere with the cone. The thermometer acts just like a camera. If you obstruct the lens with your finger, camera strap or maybe a dirty lens, you will get a blurred or obstructed image. The light did not get to the film properly; therefore, a bad picture results. The same problems can occur with an infrared thermometer. There are many applications where there are problems present which will cause the infrared thermometer to read incorrectly; these include: l. Small objects (too small to fill the cone-of-vision). 2. Dust, smoke or steam which obscures the line of sight. 3. Windows in the process that get dirty and are difficult to keep clean. 4. Emissivity of the product changes (due to change in alloy or surface condition). A two-color or ratio pyrometer can operate properly even with these problems, and indicate the correct temperature.

Two Color Pyrometers Two Color Pyrometer Brightness Pyrometer This slide on the left shows what a normal or brightness pyrometer has to see, specifically a clear unobstructed view is necessary. The slide on the right shows that even with obstructions a two color instrument could read the right temperature Brightness Pyrometer

Two Color Pyrometers What is different about a two-color pyrometer versus a brightness pyrometer? A two-color pyrometer consists of two brightness pyrometers in the same package. It uses two detectors, operating at two separate wavelengths, but both detectors see the same hot target. The slide on the right shows a graphical picture of the detector outputs of this instrument.   In this example, the two-color thermometer is looking at a blackbody with a temperature of 1500°C and an emissivity of 1.0. Detector #1 looking at this target will give an output of 500 units. Detector #2 looking at the same target, but at a different wavelength, will output a signal of 1000 units. Since this is a ratio thermometer, we divide 1000 by 500 and get a ratio of 2. The instrument is calibrated to read 1500°C when it sees a ratio of 2.

Two Color Pyrometers Now what happens if somehow the signal from the hot target is reduced or prevented from getting to the detector? This could be caused by a dirty window, object too small to fill the cone-of-vision, or maybe there is smoke in the line of sight. In this graph, we have added a second curve which depicts an example where we have lost 90% of the signal, but the target temperature is still 1500°C. This is the same as having the apparent emissivity drop from 1.0 to 0.1.   Detector #1 looking at the lower curve (E=0.1) will output a signal of 50 units. Detector #2 will output 100 units. Both signals have been reduced by 90% as compared to the upper curve (E=1.0). Note that 100 divided by 50 is again a ratio of 2, or the instrument will read 1500°C even though we have lost 90% of the signal.

Two Color Pyrometers Reduction Ratio Standard is 90% Specials to 99% Limited by Low Emissivity Object size Obstructions Basically, a two-color thermometer works properly as long as whatever affects one wavelength, must affect the other wavelength the same amount. Technically, we say the obstruction has to be a graybody. If this obstruction is a true graybody, then the only reason the detected ratio changes is due to a change in temperature. Every two-color thermometer has a limit as to how much signal can be reduced. This is called the reduction ratio. The reduction in signal can vary from 0% to as high as 90% of the signal and still read an accurate temperature. Keep in mind that the loss in signal can come from three sources. Note; specials have been made to work with a reduction of as much as 99% of the signal.   l. Low emissivity of target. 2. Object too small to fill cone-of-vision. 3. Obstruction caused by smoke, steam, dust or a solid obstruction as well as a dirty window. The total loss in signal is an accumulation of all three of the above reductions. Suppose we have an instrument with a designed reduction of 90%. If you have an object with an emissivity of 0.25, you already have a 75% reduction of the signal which means you can only lose about 15% more signal because of small or obstructed targets. When you reach the limit of the reduction ratio, the instrument can sense this and is designed to indicate “Invalid”. This condition simply says the signal is so low that a repeatable result is not possible. Rather than indicate erroneous readings, the instrument is forced to a below zero scale output and provides a relay closure to alarm for its inability to indicate the temperature.

Two Color Pyrometers E – Slope Adjustable 0.85 to 1.15 1.0 is a graybody Required for non-graybody targets Required for non-graybody obstructions Incorrect windows Unfortunately, the two-color has characteristics which are unique. Not every application is solvable using a two-color or ratio pyrometer without making some adjustment. There are applications where the object emissivity is different for the two wavelengths. A good example of this is measuring molten metals. Usually each wavelength will have a different emissivity. Thus, when the two-color thermometer looks at the molten metal, the ratio or slope will be incorrect and an error will occur in the reading.   How can this be corrected? All two-color thermometers have an adjustment called E-Slope. When viewing the molten metal, the E-Slope adjustment is turned until the instrument reads the correct metal temperature. The correct temperature may be obtained by using a disposable thermocouple. This E-Slope adjustment simply corrects the ratio by a constant which corrects the instrument indication for the unequal spectral emissivities of the target. Once the E-Slope is adjusted, then the problems of smoke, steam, dust, small targets, etc., are handled by the instrument. Similar errors will occur if an improper window material is used. For example, “Pyrex” windows are slightly colored and transmit differently in the two spectral regions. Again, the E-Slope control can be used to correct this problem. (It is suggested that quartz be used for the window material.)

Two Color Pyrometers Reflection Errors Conclusion: A two-color pyrometer will solve many difficult applications that cannot be done with a brightness pyrometer. However, they are not the solution to all problems. There are applications where they will not measure temperature properly. As with any infrared thermometer, the application has to be considered carefully before picking the correct instrument. One application that cannot be solved with a two color is looking inside of a furnace which has reflected energy. The two color will not ignore the reflected energy. In fact it will have a bigger error then a single wavelength instrument. Reflection Errors

Two Color Pyrometers Cone of Vision Target Area Cone of Vision Small Targets As previously stated, the hot object does not have to fill the entire cone-of-vision of the thermometer. Applications such as hot wires, hot rods and molten glass streams are usually very narrow and do not fill the field of view as seen in the telescope reticle. Cone of Vision

Background (Variable Temperature) The problem that has to be considered is what fills the remainder of the reticle. If the remainder is filled with another hot object, you will simply average the two temperatures of the objects and again get an incorrect reading. If the background around the wire or rod is cool, the background contribution to the two target signals and the resulting ratio will be negligible and the indicated temperature will be correct. This figure shows the different conditions.   l. Reticle filled with target. (Perfect, always strive for this) 2. Reticle 80% covered with hot target. (Still excellent) 3. Reticle 50% covered. (Not too bad) 4. Reticle 25% covered with hot target. (Careful — background temperature getting to be more important). 5. Oops! (We are not magicians) None of these are “problems” — if the background is cool!!! Our Target is 1000°C: 1 – 5 are our cone of vision

Two Color Background Errors Here is a family of curves which indicate the amount of error in temperature for a two-color thermometer when:   • The background temperature varies from room temperature to 1200°C. • The reticle varies from completely filled with the target to only 25% filled. • The target temperature is 1000°C. Point A: The target fills 80% of the cone-of-vision, 20% is filled with the background. The error at 840°C is -10°C. Point B: Target fills 50% of the cone-of-vision and 50% is filled with background. The error is -32°C at 840°C. Point C:The target fills 25% of the cone-of-vision, the background fills 75%. The error at 825°C is -64°C. Comments: • For background temperatures below 400°C, there is no error in the indications even if only 25% of the cone-of-vision is filled with the hot target. • As the target temperature and background temperature approach each other, an averaging effect will occur. It becomes progressively worse as the background fills more and more of the cone-of-vision. • If the background temperature exceeds 1000°C, then the instrument will indicate too high for any percentage of the cone-of-vision filled with background signal. Many two-color thermometers are used in steel mills. A popular misconception in the industry is that “a two-color can see through scale”. Sorry, not true. Cold scale which fills the entire reticle will simply cause the instrument to read low; if the scale is cold, the steel is hot and the scale fills part of the reticle, then the two-color will read the right temperature. If the scale fills part of the reticle and is just a little cooler (100°-200°C) than the steel, then the instrument will average the temperatures and indicate a lower than correct temperature. If the steel is moving and there are occasional spots of scale, then consider using the “Peak-Picker” circuit which will pick (and indicate) the hot steel temperature and ignore the occasional cold scale. Peak-Picker is discussed later in a seminar.

(Wavelengths out of focus) Two Color Pyrometers No Achromatic Lens (Wavelengths out of focus) Physical Characteristics A two-color pyrometer is a complicated sensing head. There are many special features required to insure that the instrument reads the proper temperature.   Lens Infrared Thermometers have a lens to collect the infrared energy. Some units have lenses which are focusable and allow the instrument to be focused on small targets. The two-color instrument requires an achromat lens. This means it has a lens which “color” corrects the incoming infrared energy. We know that if two different wavelengths of energy go through a lens, they will each bend differently. The top picture shows this effect, the long wavelength does not bend as much as the short wavelength. The bottom picture illustrates how an achromatic lens which corrects this problem and shows the two wavelengths will be focused at the same point. All Ircon two color thermometers use the achromat lens Achromatic Lens (Wavelengths in focus)

Two Color Pyrometers No Achromatic lens? The Pyrometer “sees” two images Why is this so important? Here we see a picture of a bottle. The infrared thermometer without an achromatic lens will see two images. One is larger than the other, with the largest image being created by the long wavelength. In the figure, we show two positions of the reticles. In the left-hand position, we would see more short wavelengths than long wavelengths, and in the right-hand reticle, we see only the long wavelength. If the bottle were moving and this effect was allowed, the instrument would give very erratic readings as the target enters and leaves the field of view.

Detector Methods - Ircon There are several methods used to obtain two signals to create the two-color process. Here we see the IRCON method. The two detectors are sandwiched on top of each other and placed in a metal transistor can. The can is in an aluminum block, heated and held at a constant temperature to insure excellent temperature compensation. To transmit the infrared energy from the beam splitting mirror to the detector, a short length of non-coherent fiber optics is used. This serves as an optical mixer. A small target focused on the top of the fiber is scattered over the detectors at the opposite ends of the cable. This is necessary to eliminate the problem of non-uniform sensitivity across the sensing area of the detectors. If a small spot of energy strikes a detector in various locations, minor differences in outputs may occur. The mixing is necessary because some applications have targets which do not completely fill the detector.   Because the instrument has no chopper and operates in a DC mode, the output responds as fast as 10 milliseconds to a 95% of full scale temperature. The detectors are silicon which are very stable over long periods of time (years). Thus, the instrument is basically drift-free. Speed of response: Approx. 10 milliseconds

Two Detector Design Optical Mixer Ends Achromatic Lens Heater Chopper Achromatic Lens Optical Mixer End This slide shows a second method in which both spectral regions are simultaneously chopped and measured continuously. Again a fiber optic is used as an optical mixer. The upper circular picture shows a small target which is seen by 6 individual fibers. Because the fiber optic is non-coherent the 6 fibers will scatter all over the target (lower circular picture). This scattering permits the detection of the incoming signal and the non-uniformity of detector sensitivity is eliminated.   For this sensing head, two separate InGaS detectors are used. A special mirror splits the two wavelengths and two signals are created. Both detectors are heated to the same temperature to prevent any temperature compensation problems. This system is used to provide a low temperature two color instrument which can measure down to 250°C (500°F) Specifications of Ircon Two-Color High Temperature Response Time: 10 ms to 95% of Scale Min. Target Size: 4 mm (0.16”) Reduction Ratio: 90:1 Standard (100:1 is special) Accuracy: ± 0.5% of Full Scale Reading Temperature Range: 600°C to 6500°C (in 3 Models) Low Temperature Response Time: 25 ms to 95% of Scale Min. Target Size: 2.3 mm (0.1”) Accuracy: ± 0.6% of Reading Temperature Range: 250°C to 1000°C (in 3 Models) Detector #1 Detector #2 Heater

Detector Methods – Filter Wheel This slide shows a method which utilizes a single detector and a filter wheel. The filter wheel contains a series of two different spectral filters which are placed alternately around the outside of wheel. As each filter passes by the detector, it sees the two wavelengths alternately. This generates a train of discrete pulses with the short and long wavelength pulses alternating in time. These pulses are then demodulated to a DC signal, and then the ratio is calculated.   The basic problem with this design is response time, it has to be slow (3 seconds). To provide an accurate temperature measurement, the pyrometer must accurately measure the ratio of the two spectral signals. An accurate ratio requires the number of short and long wavelength pulses processed to be identical, or alternately a large quantity of them are processed so that the loss of cycle will not affect the reading. This requires waiting for the accumulation of a large train of pulses of both wavelengths to approximate this condition. Thus, the target must be steady during the long accumulation period (possibly 3 seconds), i.e., the response time is slow. When the target moves too fast, you have the application where the infrared energy is not in sight long enough for a proper balanced infrared output. This causes the instrument to read very high or very low. In an attempt to avoid this disturbing behavior, the instrument response is intentionally slowed down. Speed of Response: Approx. 3 Seconds

Applications for Two Color Pyrometers Steel •Slabs exiting Reheat Furnace •Coke from Furnace •Hot Strip Mill •Bar Mill •Rod Mill Heating •Induction Heaters •Vacuum Furnaces Metals •Molten Metals •Wire Drawing •Wire Annealing Glass •Gob Temperature •Melt Temperature •Glass Streams Applications: Two-color thermometers have been successfully used in the following applications:   Steel • Slabs exiting Reheat Furnace • Coke from Furnace • Hot Strip Mill • Bar Mill • Rod Mill Glass • Gob Temperature • Melt Temperature • Glass Streams Heating • Induction Heaters • Vacuum Furnaces Metals • Molten Metals • Wire Drawing • Wire Annealing All these applications have common problems of dust, smoke or steam interferences, small targets, or obstructions between the target and sensing head.

Thank you for attending! Conclusions? Thank you for attending! Conclusion: A two-color pyrometer will solve many difficult applications that cannot be done with a brightness pyrometer. However, they are not the solution to all problems. There are applications where they will not measure temperature properly. As with any infrared thermometer, the application has to be considered carefully before picking the correct instrument.