INFRARED TEMPERATURE SENSORS OPERATION AND SELECTION

INFRARED TEMPERATURE SENSORS OPERATION AND SELECTION By Vern Lappe, Vice President, Technical Services, Ircon, Inc. There are many ways to measure temperature in a process. Sensors, such as thermocouples, RTDs and infrared thermometers are the most common temperature sensors utilized today. A noncontact infrared thermometer has the advantage of being able to measure the product temperature while it is moving or if the product is in an oven, the instrument measures the product and not the environment. However, to obtain accurate temperatures with an infrared thermometer requires the consideration of the following factors: aiming and focusing, optical obstructions, interface with other instruments and maintenance. In addition, the selection of the right instrument is very critical. HOW DOES AN INFRARED THERMOMETER WORK? The oldest noncontact thermometer in the world is your eyeball. It is fairly accurate if the object is incandescent. There are people in the steel and glass industry that can look at a hot target and tell the temperature within 10-15 degrees. But if the target is below about 648ºC/1200 F, then it no longer glows and the eye cannot tell you the temperature. This is where the infrared thermometer comes into action. Infrared thermometers can measure temperatures as low as -50 C/ 58 F to as high as 3500 C /6322 F. Not one instrument can cover this entire range. It takes many different series to be able to cover this entire range of temperatures. Every object in the world emits infrared energy. As the target gets hotter, the more infrared energy is emitted and detected by the infrared thermometer. The thermometer uses two basic kinds of detectors to determine the temperature. The first and probably the most common detector is called a thermopile. This detector is actually a small chip about 6mm/.25inches square and on it are deposited about 50 tiny thermocouples. This detector looks out at the target and is actually heated by the target. Does this mean that when the detector is looking at a target at 537ºC/1000 F, the detector reaches this temperature? In actuality, the detector may only change temperature 1 or 2 degrees, but the thermocouples put out a very strong signal that can be measured and converted into the temperature of the target. This kind of detector is slow - 150 ms and requires Page 1

large spot sizes, usually at least 13-25mm/.5-1inch in size. They are very common in portable thermometers and the low priced online sensors. The alternate type of detector is called a quantum detector. Infrared energy travels in little bundles of energy called photons. When the photons strike a quantum detector, an electrical signal is created. As an example, a very common detector is the silicon detector, which is identical to the solar cell in your calculator. The photons strike the detector and a current is generated. The hotter the object, the greater the output current generated. Other materials used for quantum detectors are Indium Gallium Arsenide, Lead sulfide, Germanium, as well as many others. These detectors have several advantages; they are fast, 10 ms, they can see small targets, as small as 6mm/.25 inches, and they are rugged. They are, however, much more expensive then the thermopile detector. Fig.1. IR Sensing Head Internals Page 2

Figure 1 is a graphic display of the inner workings of an infrared thermometer. First, there is a lens system. Some lenses are focusable and some are fixed focus (the differences will be discussed later on). Let s examine an instrument that has thru-the lens focusable optics. With a focusable instrument, the lens not only collects the infrared energy, but it also collects the visible light from the hot target. Both the visible and infrared energy go through the lens and encounter a special mirror. This mirror has a special coating, which reflects the infrared energy down to the detector and allows the visible energy to pass on through the mirror to your eye. The visible energy is really focused at a point in the sensing head where a piece of glass is located. On this glass, there is an etched small black circle. This circle is called a reticle and its diameter is basically the same diameter as the detector. The sensing head eyepiece is focused on this reticle, as well, so that when you are viewing through the eyepiece and focusing the lens on the hot target, the reticle appears to be on the target. When the lens is in focus on the target, the infrared detector is receiving infrared energy from everything appearing inside of the reticle. In order for the instrument to read the correct temperature, the target must completely fill the reticle. Ideally the target should be at least 20% larger than the reticle. Failure to fill the reticle with the hot target means that objects other that the target will fill the detector and will cause an error in the reading. The infrared energy from the hot target is reflected off the mirror down to the detector. By the way, this mirror has several names that describe it, such as a folding mirror, dichroic mirror and beam splitter. As the infrared energy travels toward the detector, it encounters a spectral filter with specific coatings. These coatings are designed to allow only specific wavelengths to transmit through the filter and all other wavelengths are reflected away. This is how an infrared thermometer sees only specific wavelengths, which will become important later on. As the IR energy strikes the detector, a reaction occurs that results in an electrical signal output from the detector. The output from the detector is normally a DC signal, and is at an extremely low level. Outputs of picoamps are quite common. Since the signal has to be amplified and linearized before a temperature can be presented, we need to change this low level DC signal to a high level AC signal, amplify it, and then synchronously demodulate it back to DC. Thus, directly over the detector is a device that can be a rotating flat blade or possibly a vibrating blade. This device is called the chopper. This chopper blade is placed in the optical path of the incoming infrared energy. As the blade rotates or vibrates, it chops the incoming signal. The detector output is now an AC signal. This AC signal can now be amplified and then synchronously demodulated back to a DC signal. As you might suspect, the chopping frequency is critical. The demodulation at the correct frequency is critical so that the proper signal is being Page 3

converted to the high level DC output. There are two ways to do this. First and most difficult is to accurately control the chopping frequency of the chopper and only demodulate that frequency. The second and more reliable method is done by what is called a sync lamp and sync detector, which generates a sync signal. As shown in Figure 1, the sync lamp is placed under the chopper and on top is the sync detector, which sees the lamp at the same frequency as the infrared detector sees the incoming infrared energy. The sync detector produces a trigger or sync signal that allows the actual temperature signal to be demodulated. This system provides for stable and drift free temperature measurement. After the signals are processed, they are still non-linear with temperature. Sometimes, they are linearized in the sensing head for standalone sensors or they are sent via a cable to an indicator where they are linearized. The final temperature is then presented on digital meters, linear analog outputs or digital outputs. INSTALLING AN INFRARED THERMOMETER AIMING One of the questions most often asked during installation is: Does the thermometer have to be aimed perpendicular to the surface? The answer is No. Figure 2 shows that for measurements of smooth surfaces, such as plastic, glass and paper, the instrument can be aimed from a 45 to 90 angle. If you exceed the 45 angle, the surface becomes reflective, which in turn lowers emissivity and gives a temperature reading lower than what it really is. For rough surfaces, like steel, textiles and food, the instrument can be aimed as low as 15 from the horizontal and the temperature indication will be accurate. (Figure. 3) If is often advisable in a steel mill not to position the sensor to look straight up or down at the target because of the debris that can fall into the lens, as well as the heat and steam that can overheat the sensor. By placing the sensor off to the side and looking at an angle, the sensor will survive the environment very easily. Page 4

Fig.2. Viewing Angle for Smooth Targets Fig.3 Measuring Rough Targets Page 5

FOCUSING All infrared thermometers measure temperature within a certain area. To define the target size requires a simple calculation or reference to the manual. Figure 4 shows an infrared thermometer that has a focusable lens. To determine the target being measured, the formula is d= D/F, where d= the spot size, D= the distance from the sensor to the target and F=the focal factor of the instrument. The focal factor for any instrument is included in the manual and usually varies from a low number of 20 for low temperatures like -17-260ºC/0-500 F, to a high of 300 for high temperatures, such as 815ºC/1500 F and higher. As an example, if the focal factor is 50 and the instrument is 100 inches away then d = 100/50=2 inches. If the target is smaller than 2 inches, then the sensor has to be placed closer or an instrument used that has a higher resolution factor. The D or focal factor can be in any dimension - inches, feet, mm or cm and the answer will be calculated accordingly. Fig.4, Optical Resolution, Focusable Instruments For instruments with a fixed focus, there are charts in the manual that show the spot size vs. the distance. Figure 5 shows a typical chart. This chart shows that at 50 inches, the spot is 5.7 inches. This means that in order to measure the correct temperature, the target has to be larger than 5.7 inches. Failure to fill the spot size will allow the instrument to measure anything else that is in the spot and usually this will lead to an incorrect temperature. Page 6

Fig.5. Optical Resolution, Fixed Focus Instruments Do you have to always be in focus? No. Figure 6 shows an application where the target may move up and down, such as a paper web. At the # 2 location, the instrument is in focus. At locations #1, 3 and 4, they are out of focus. On locations #1 and 3, the target is larger than the spot being measured and the infrared thermometer averages the temperature of a larger area. The 4th position will not work because the spot size is larger than the target. For wide targets, such as strips of steel, glass and textiles, wide-angle lenses are available to provide an average temperature over the entire width of the web. However, the average temperature is usually not the desired temperature, so many installations use 3 or 4 instruments spaced across the web to provide a more accurate temperature profile. Fig.6, Out of Focus Targets Page 7

Fig.7, Obstructions OBSTRUCTIONS Figure 7 shows that the line of sight (often called the cone of vision ) between the sensor and the hot target, which needs to be clear and unobstructed. However, there are many possible obstructions that can cause problems: A. Solid obstructions, such as pipes and steel structures. The ideal solution is to remove the obstruction, but this is not always possible. The solution could be to look at the target at an angle, or maybe use a fiber optic instrument that can go around the obstruction. B. Windows. Some applications require windows to maintain a vacuum in a chamber or pressure in the oven. If a window is necessary, be sure that it is transparent for the wavelength of the instrument that is being used. In addition, the window has to be kept clean. If the window gets dirty, the instrument will measure the temperature of the dirt on the window. In addition, the window has to be large enough so that the cone of vision is not obstructed by too small of a window opening. C. Intermittent targets and obstructions, such as smoke, steam and dust cause the instrument to provide erratic temperature indications. An electronic feature, called a Page 8

peak picker solves this problem. The electrical circuit allows the indication to rise as fast as the response time, but a delayed decay rate does not allow the temperature to go down when there is interference in the line of sight. Figure 8 shows an application with bottles at different temperatures. Without peak picker, the instrument would indicate room temperature when there is no bottle present. With peak picker, the spaces are ignored and only the product temperature is indicated. Now the user has to decide which decay rate he wants to use to provide the output for controlling or indication. D. Flames. Clean gas flames are transparent to an infrared thermometer, so the infrared thermometer will see right through them and not measure the flame temperature. The same is true of inert gases, such as argon, nitrogen or hydrogen. These gases are transparent and the infrared thermometer will not see the gases, but will instead measure the temperature of the target immersed in these gases. Dirty flames like coal, oil or garbage flames are opaque and the infrared thermometer will actually measure the flame temperature rather than see through it. Fig.8 Peak Picker Page 9

INSTRUMENT INTERFACE Infrared sensors often interface with computers and other data devices. The outputs from the sensor include linear 4-20Ma, 0-10V or RS232 and RS485. All of these outputs need to be ungrounded. Computers like to work with what is known as floating inputs. If the outputs are improperly grounded, the temperature indications may be incorrect or non-existent. When working with digital outputs, be sure to match the sensor and computer baud rates. MAINTENANCE The three points of maintenance include keeping the sensor cool, keeping the lens clean and proper sensor calibration. Sensor cooling: Most sensors can operate in ambient temperatures of -17ºC to 62ºC/0 F to 145 F. If the ambient air surrounding the sensor is hotter or cooler than these temperatures, the sensor will drift or may even be destroyed. Overheating the sensor is the most common problem. To keep the sensor cool may require a watercooled jacket or attachment. (Figure 8) Do not over cool the sensor. Operate the sensor at 37-43ºC/100-110 F and this will be above the dew point temperature. If the sensor is too cool, condensation will build up inside of the sensor and destroy it. Lens cleaning: Keep the lens clean by using an air purge. (Figure 9) The goal is to maintain enough air pressure to keep the dirt and fumes away from the lens. If the lens gets dirty, the instrument will indicate too low of a temperature. To clean a dirty lens, use isopropyl alcohol to wash the lens. Dry with a soft cloth. On the instruments with focusable lens, be sure to clean both sides of the lens, as well as the window behind the lens. Fig.9 Sensor with Air Purge and Water Cooling Page 10

Calibration: It is common practice to calibrate infrared thermometers once a year. They are certainly rugged enough to go several years without calibration, but ISO 9000 usually requires annual calibration. To calibrate an infrared thermometer requires a black body source. This is a special oven that is a calibrated temperature source with a specific cavity design. However, before using a black body, this source itself should be calibrated annually by an authorized lab. CHOOSING THE RIGHT INSTRUMENT Choosing the right instrument really is picking the right wavelength instrument. Within each wavelength, there are several models to select, but if the wrong wavelength is used, the instrument may be of no value. Within the industry there are 9 basic wavelengths that are used to measure infrared energy. Table 1 lists the basic wavelengths, temperatures covered and some of the common applications. WAVELENGTH TEMPERATURE SPAN APPLICATIONS. 0.65 microns 871-3500ºC (1600-6500ºF) Molten glass and metals 0.7-1.0 microns 482-3037ºC (900-5500ºF) Steel, heating, semiconductor 1.6 microns 260-1204ºC (500-2200ºF) Non ferrous metals 2-2.6 microns 37-760ºC (100-1400ºF) Low temperature metals and small targets. 3.4 microns 10-815ºC (50-1500ºF) Thin film organic plastics 3.9 microns 815-3593ºC (1500-6500ºF) High transmission thru flames and hot gasses 4.8-5.2 microns 37-1371ºC (100-2500ºF) Glass bending, annealing, tempering 7.9 microns 10-426ºC (50-800ºF) Thin films of polyester plastics, special ranges available for high temperature ceramics 8-14 microns -50-537ºC (-58-1000ºF) General purpose low temperature applications Fig.10 Thermometer Selection Page 11

In choosing the correct instrument for your application, there are some easy rules to follow to get started. First, let s look at metals. The rule to follow is: USE THE SHORTEST WAVELENGTH INSTRUMENT THAT WILL MEASURE YOUR TEMPERATURE. Why do we say that? The shorter the wavelength, the higher the emissivity of the material, and errors due to changes in emissivity will be less at these shorter wavelengths. Figure 10 shows that if we have the choice of 4 different wavelength instruments and there was an error of 10%, the instrument at 0.9 microns will only have an error of 10 C at 1000ºC/50ºF at 1832ºF. If you used an 8-14 micron instrument, the error would be 60 C/140ºF. This is simply the laws of physics and not the defect of any one instrument. When you are working with metals, such as steel or non-ferrous metals, the choice is usually the instruments with wavelengths of 0.7-1.0, 1.6 and 2-2.6 microns. For plastic film, you have two choices - 3.4 and 7.9 microns. The 3.4 is for polycarbonate films, like PVC and polyethylene, where the film can be as thin as 1 mil (.001 thick). The 7.9 micron is for ester films, like Kapton and Mylar, and as thin as 1 mil. They are also great instruments for colored targets, like paint and laminates. At these two wavelengths, the color does not affect emissivity. The 7.9 micron wavelength is also the ideal wavelength of high temperature ceramics, such as alumina. At this wavelength, those materials are opaque and have an emissivity of 0.98. The 5 micron instrument is the most used instrument for glass applications. At this wavelength, the glass is totally opaque. This means the instrument cannot see thru glass but will actually measure the glass temperature. Again, the color of the glass has no effect on the temperature indication. The applications include annealing, tempering, laminating, bending and coating. The wavelength of 3.9 microns is a special wavelength designed to look thru hot gases and clean flames. At this wavelength, these flames and gasses have the highest transmission and therefore, the instrument actually sees thru the flames and does not measure the flame temperature. This is ideal for applications, like measuring refractory in glass furnaces and preheating ladles in steel mills. The most used wavelength is the 8-14 micron. While it is a low temperature instrument, there are thousands of these applications including food, textiles, paper, thick plastics and maintenance applications. These instruments are usually portable and can be quite inexpensive. Page 12

There is one instrument that was not mentioned so far and that is the two color or ratio pyrometer. This instrument is probably the most popular instrument in the industry, but requires a separate paper or presentation to discuss it features. We will say it has several advantages over what are known as single wavelength thermometers. First of all, it is a thermometer that has two detectors operating at two different wavelengths looking at a single hot target. This instrument has the advantage that if there are minor changes in emissivity or if smoke, steam and dust obscure the line of sight, it does not affect the two color instrument. It also has the advantage that if you only fill 10% of the reticle with the target, it will still indicate the true target temperature. These instruments are used very heavily in the steel and heating industry. CONCLUSION Infrared thermometers are very reliable instruments for measuring temperature. Care has to be taken in the installation, maintenance and selection of the right instrument. They are rugged, accurate instruments that can run for a long time. Page 13