Principles of Noncontacting Infrared Temperature Measurement. Making Sense of Smart Infrared Temperature Sensors

Transcription

1 Principles of Noncontacting Infrared Temperature Measurement Making Sense of Smart Infrared Temperature Sensors More Than Specifications: Selecting the Right Sensor for Your Application

2 C O N T E N T S Spectrum is a publication of Raytek Corporation. This tutorial is provided to help engineers, operators, and others working with process control applications to better understand the use and benefits of infrared temperature measurement. Raytek is a leading international supplier of infrared temperature measurement instruments. Applications specialists, located around the world, are available for further training and on-site evaluation. Principles of Noncontacting Infrared Temperature Measurement 1 Basic principles of infrared thermometry Making Sense of Smart Infrared Temperature Sensors 1 7 Introducing the latest in IR technology More Than Specifications: Selecting the Right Sensor for Your Application 1 11 Choosing an infrared temperature sensor can be a straightforward procedure 1 Reprinted with permission of InTech/ISA. Raytek

3 Principles of Noncontacting Infrared Temperature Measurement Introduction Temperature measurements are fundamentally important to numerous industrial processes. However, if one is not able to contact the object, measuring temperature may appear difficult. Contact temperature sensors like thermocouples and RTD s are accurate and cost-effective, but are simply not practical for many industrial applications. Infrared temperature sensors routinely carry out measurements virtually impossible for contact sensors involving objects that Move, rotate, or vibrate Are in strong electromagnetic fields (e.g., induction heating) Undergo rapid thermal changes (faster than several hundred milliseconds) Are located in process chambers or behind windows Require surface temperature measurement Have temperatures too high for contact sensors (usually above 1400 C; 2550 F) Are physically inaccessible to contact thermometers Are damaged or contaminated if contacted Have varying surface temperature distributions Temperature would change if contacted Are made from materials with low heatcapacity or low thermal-conductivity Appear transparent; or are gaseous (combustion gases/flames) Require prompt or frequent temperature measurement. Several important factors influencing IR temperature sensor selection and performance are The object s ability to emit thermal radiation a property called emissivity The sensor s spectral sensitivity the wavelength region where the IR sensor is most sensitive The intervening atmosphere between the IR sensor and the object Structures that partially obscure the instrument s field-of-view Heat sources in proximity to the IR sensor or target object. Infrared temperature sensors provide superior response time, durability, noncontamination, and ease-of-use to meet the stricter requirements of today s demanding industrial environments. This article provides essential information about infrared thermometers, their performance characteristics, how they are tested and calibrated, and offers insight into selecting the best radiation thermometer for a given application. RELATIVE BLACKBODY RADIANT EMITTANCE Thermal radiation The physics of thermal radiation is helpful in understanding how IR thermometers work. However, one does not need a detailed knowledge of thermal radiation to properly use IR thermometers. Essential concepts of thermal radiation are summarized below. Basically, thermal radiation is the rate at which a material emits energy because of its temperature. Thermal radiation relates to the energy released from oscillations or transitions of a material s electrons, atoms, ions, or molecules sustained by the material s internal energy. All forms of matter at a temperature above absolute zero emit thermal radiation. In gases and other transparent materials (i.e., materials with negligible internal absorption), thermal energy radiates from throughout the material s volume. For materials exhibiting high internal absorption like metals, only a few atomic layers (or maybe up to a few hundred atomic layers) effectively contribute to the radiated thermal energy. For these materials, the emission of thermal radiation is primarily a surface phenomenon. Figure 1. Spectral characteristics of blackbody radiation at different temperatures C (2730 F) 1000 C (1830 F) 542 C (1000 F) 260 C (490 F) 20 C (70 F) Contact temperature sensors are not workable for applications with any of the above characteristics WAVELENGTH (microns) 3

4 Raytek In 1900, Max Planck established his quantum theory of radiation. Planck s Law mathematically describes how much radiation an object at a given temperature T emits at each wavelength l. That is, Planck s Law describes the spectral distribution of radiation emitted by a blackbody at a given temperature. Figure 1 illustrates the spectral distribution of radiation from a blackbody at several different temperatures calculated using Planck s Law. From these curves, one observes three important characteristics: (a) The emitted radiation varies continuously with wavelength. (b) At any given wavelength, the amount of radiation increases with increasing temperature. (c) The shape of the spectral distribution changes with temperature. As temperature increases, the spectral region where the radiant energy concentrates shifts toward shorter wavelengths. Planck s Law is fundamental to designing IR thermometers and understanding their operational details. However, Planck s Law applies only to a perfect radiator. The central issue in applying IR radiation thermometers to real objects is that real objects do not behave like a perfect radiator. Radiation thermometers Emissivity Radiation thermometers (also referred to as radiometers or pyrometers) are calibrated to measure the temperature of a blackbody. No real object can emit more radiation than a blackbody at the same temperature and wavelength. Emissivity e equals the ratio of radiation emitted by an object to that of a blackbody at the same temperature and wavelength. A blackbody emits radiation isotropically (i.e., the same in all directions) whereas real objects may not. Emissivity accounts for differences in the spectral distribution of radiation emitted from an object and also accounts for differences in the directional distribution of radiation emitted from an object compared to 4 Figure 2. Relative spectral distribution curves of blackbodies, graybodies, and non-graybodies. that of a blackbody. Figure 2 illustrates a comparison of emitted radiation from a blackbody and that of a real object. Real objects do not behave like perfect blackbody radiators primarily because the physical or chemical nature of their surface governs their ability to radiate thermal energy. As shown in Figure 2, a blackbody has e=1, a graybody has constant emissivity (but e<1), and a non-graybody has an emissivity that varies with wavelength (but not temperature). Radiation interacts with an object in threeways: reflection, absorption, or transmission. If R, A, and T represent a measure of an object s ability to reflect, absorb and transmit thermal energy, conservation of energy requires R+A+T=1. If an object is in thermal equilibrium with its environment, the rate at which energy radiates equals the rate of energy absorption. That is A=e and e+r+t=1. To measure an object s temperature, one selects an infrared instrument with a spectral response where the object appears opaque (T=0). Thus, e + R =1. Accordingly, highly reflective objects have low emissivity and vice versa, objects with low reflectance exhibit high emissivity. Sensor design Figure 3 illustrates the main features of an IR thermometer. A photo-sensitive detector having the desired spectral response receives radiant energy from the target object through an optical system. The optical system determines the target area or spot size viewed by the detector and effectively defines the instrument s field-of-view. The viewing angle or the ratio of target-distance to target-size (the D:S ratio) defines the field-of-view or FOV. A D:S of 60:1 corresponds to a viewing angle of about 1. If the D:S is 30:1 and the distance to the object divided by the diameter of the object is also 30, the target completely occupies the instrument s FOV. Filling the FOV is a critical consideration for one-color IR thermometers. If the object does not fill the FOV, the apparent measured temperature will represent an average of object and background temperatures. To insure proper performance, IR thermometer manufacturers typically suggest that the nominal target area exceed the FOV area by perhaps a factor of at least 1.5. Most IR temperature sensors allow the user to visually sight through the instrument much like a camera or telescope to aim and focus the sensor on the desired target region. Some IR thermometers have a built-in laser to facilitate aiming. Detectors The detector converts IR energy to a measurable voltage, current, or resistance. The detector s spectral sensitivity in combination with an optical filter placed in front of the detector (to confine the incident energy to a desired band of wavelengths) determines the sensor s spectral response.

5 Figure 3. Block diagram of an infrared temperature sensor. The sensor collects energy emitted from the object based on its optics and location. Detectors measure the energy and convert it into an electrical signal. Three general types of detectors commonly used are photon, pyroelectric, and thermovoltaic. Photon detectors release electrical energy as a voltage or current in response to incident radiation. Pyroelectric detectors respond to changes in incident radiation and require choppers to modulate the incident radiation. Thermovoltaic detectors such as thermopiles (an array of multiple thermocouples) and bolometers ( blackened thermistors) provide a voltage (emf) in response to incident IR radiation. The detector output signal requires amplification and processing to extract temperature information. Detector amplifier and electronics The main electronic component is an amplifier that increases the level of the detector signal for subsequent processing. IR temperature sensors need stable electronic circuitry to measure the small signal levels provided by the IR detector. The amplifier circuitry must resolve and amplify detector signals as low as 1 mvolt, which could correspond to object temperature variations on the order of 1 C. Once a stable signal is available for processing, there are various electronic circuits available to linearize the amplifier output signal. Some IR thermometers employ A/D conversion allowing a microprocessor to linearize and process the measured data via software. Through D/A conversion, the measured temperature can be converted to a 4-20 ma current-loop signal, a voltage corresponding to a thermocouple response, a 0-5 V analog output, or an appropriate digital signal (e.g., RS-232 or RS-485). A superior advantage of IR temperature sensors compared with contact sensors is response time. The thermal mass of a contact sensor, the process of conducting heat from the object into the sensor, and the associated thermal resistance at the point of contact dramatically limit a contact sensor s response time. By contrast, infrared sensors have no such limitations. Because of their noncontacting nature, IR temperature sensors respond almost Figure 4. High ambient temperatures in applications such as ovens or furnace walls may result in a temperature reading that is higher than the object. Instruments include a T-ambient function to compensate for background temperatures higher than the target. instantaneously to temperature changes permitting measurement of fast moving objects or objects whose temperature changes rapidly. Some Raytek infrared sensors respond to temperature changes as quickly as one millisecond. In many applications, knowing an object s temperature may not be sufficient. The peak temperature attained within a given time interval may be of greater importance than knowing the instantaneous temperature. IR thermometers usually offer signal processing choices allowing the unit to indicate peak-temperature readings within a specified time interval. 5

6 Raytek sitive to emissivity variations and other environmental influences than one-color devices because they measure temperature by ratioing two signals. Instead of measuring in one wavelength region, ratio thermometers measure in two wavelength regions typically using two detectors (with suitable filters) and determine temperature from the ratio of the two detector signals S1 and S2. Whereas single-wavelength devices calibrate their internal measurement signal level S to temperature, ratio thermometers calibrate the signal ratio, S1/S2, against temperature. The temperature measurement also depends on the ratio of the object s emissivity at the two wavelengths, e1/e2. Ratio thermometers have a similar expression relating the signal ratio and emissivity ratio to temperature: T = B / [ A + ln(e1/e2) - ln(s1/s2) ] The subscripts correspond to wavelength bandpass regions 1 and 2; A and B are constants determined at calibration. Note that the temperature T depends on signal ratio, S1/S2, and on the emissivity ratio e1/e2 (referred to as slope). Consequently, ratio thermometers are less sensitive to emissivity variations and to the absolute emissivity values. Graybodies exhibit constant e versus wavelength and thus e1/e2 = 1. With reference to the above formula, the temperature of graybodies requires little, if any, adjustment for slope since ln(1) = 0. Objects with e that varies with wavelength (but not temperature) have a constant emissivity ratio. depends on signal ratio, the corresponding temperature remains unaffected by the obstruction. cable typically has a small optical lens assembly (for focusing) mounted at its tip. Fiber optic sensors are available in both single-wavelength and ratio thermometer Similar reasoning applies to (1) an object versions. Their main advantage is that one occupying less than the full field-of-view; can route the flexible fiber optic assembly (2) atmospheric effects such as smoke, around obstructions to view targets that haze, or dust obscuring the object; or (3) otherwise are impossible to see by line-ofsight. Also, because the fiber-optic front dirt contaminating the sensor s front lens, assuming such factors influence S1 and S2 end contains no active electronics, it can equally. Some ratio thermometers maintain withstand higher ambient operating temperatures (typically, up to 200 C; 392 F), accuracy despite signal-level reduction as much as 95%. Ratio thermometers cost a and operation in high electromagnetic little more than single-wavelength units. fields. However, if the object size or other considerations (like physical or atmospheric obstructions) preclude using a single-color device, ratio thermometers may be ideal. Practical operation Real objects seldom behave like a blackbody. The measurement environment There are numerous applications where the target (1) moves or vibrates, (2) is consists of structures close to the object and the intermediate atmosphere between smaller than the FOV, (3) has its FOV partially the sensor and object. These factors are blocked, or (4) there is considerable smoke or dust present in the atmosphere. Such applications, while virtually impossible for contact sensors, are routine for twocolor ratio thermometers. not the same nor are they as well controlled as during calibration. The radiant energy received by the infrared thermometer is influenced by several factors: thermal emission from the non-blackbody object, reflection of emitted radiation (or radiation Fiber optic IR thermometers from other sources of heat nearby), and Some target objects cannot be viewed by atmospheric absorption/emission of radiation in the viewing path of the instrument. direct line-of-sight. For these applications, an IR sensor with a fiber-optic front end This represents more of a real world may solve the problem. The fiber optic Figure 5. Spectral transmission of glass mm (10 Mil) 0.8 Ratio thermometers provide a highly accurate temperature measurement given the proper value of e1/e2. Most commercially available ratio thermometers allow for adjusting the value of the emissivity ratio. TRANSMISSION mm (60 Mil) Ratio thermometers offer additional advantages besides reduced sensitivity to emissivity. If some structure in the sensor s field-of-view obscures a fraction of the target, both detector signals S1 and S2 are reduced the same fractional amount. Thus, the signal ratio, S1/S2, remains unchanged by the obstruction and, since temperature mm (240 Mil) WAVELENGTH (Microns)

7 Figure 4. Temperature errors due to emissivity uncertainty are reduced by selecting an instrument with as short a wavelength as possible. At the shorter wavelengths, differences in temperature exceed differences due to emissivity. 10% corresponds to a temperature error of 5.5% when measured in the 5 micron band but an error of only 1.2% when measured at 1 micron. Whenever possible, it is advisable to use an IR sensor exhibiting the shortest possible spectral response. Figure 4 illustrates temperature errors due to emissivity uncertainty when measuring temperature with instruments having various spectral sensitivities. Dual wavelength or two-color ratio thermometers Another type of IR thermometer is the ratio thermometer (also called dual-wavelength or two-color). These devices are less sen- Testing & calibration Sources of thermal radiation The best way to test the response of a radiation thermometer is with a blackbody source of thermal radiation (heat). Designed specifically for testing and calibrating IR thermometers, some blackbody sources resemble a small furnace with an opening to view a surface or cavity heated and controlled to a selected temperature (see Figure 4). By design, blackbody sources have e = 1. Source manufacturers calibrate blackbody source temperatures relative to known temperature standards (or NIST standards) to ensure accuracy of their temperature settings. Single-wavelength or one-color instruments The instrument manufacturer calibrates an IR thermometer by aiming it at a blackbody heat source and, by varying the source temperature, calibrates the sensor s internal measurement signal to known blackbody temperatures. The calibration relationship is stored (or coded) into the instrument s electronics. Infrared thermometer suppliers may calibrate at only two temperatures (at the extremes of a temperature measurement range) and use an equation based on thermal radiation physics to characterize the sensor s response to intermediate temperatures. IR thermometers combine the internal measurement signal with the stored calibration information to produce an output signal proportional to temperature. Typically, the output signal is a current or voltage (some instruments also include a serial digital output). Some units indicate temperature with a built-in display. It is important to emphasize that the calibration process is based on a blackbody and embodies Planck s Law (or a suitable approximation) to establish a relation between the output signal and temperature. A simplified relationship between signal S and temperature T is T = B / [A + ln(e) - ln(s)] Constants A and B are determined at calibration. Note the dependence on the emissivity e whose value must be known in order to relate the measured signal S to the object temperature T. These considerations apply to a single-wavelength instrument where the temperature T depends on the signal level S and emissivity e. Most radiation thermometers allow for adjusting e. As seen from the above formula, the ability to measure temperature T relates directly to how well one knows the emissivity. An object with e = 0.7 emits 70% of the energy of a blackbody. Unless one corrects for an object s actual emissivity, the indicated temperature reading will be lower than the object s actual temperature. Toward the shorter wavelength end of the spectrum, energy differences due to temperature exceed those due to emissivity. For example, when measuring an object at 1000 C (1830 F), an emissivity error of How to Select an IR Temperature Sensor Is an IR thermometer needed? Refer to check list in Introduction. Temperature range & desired accuracy Influences choice of IR model/type. Object size? How close can you get? Influences D:S ratio requirements. Will object fill FOV? Yes/no? If no, consider ratio thermometer. Physical obstructions? Yes/no? If yes, consider ratio thermometer. Smoke/dust/particulates? If yes, consider ratio thermometer. Measurement/control frequency? Influences response time requirements. Parts moving? How fast? Influences response time requirements. See object by line-of-sight? Yes/no? If no, consider fiber optic unit. Surface Shiny? Emissivity may be low; consider ratio thermometer. Outputs/interface requirements. Determines analog and digital output requirements. Need a PC interface? Look for RS-232/485 outputs & software. Confident IR is best choice? Yes/no? If no, arrange for a test drive demonstration. 7

8 Raytek situation wherein the IR thermometer reading corresponds to the temperature of an equivalent blackbody. In order that this equivalent temperature relate to the object s actual temperature, the user needs information pertaining to the object s measurement environment. Measuring an object s temperature requires the object appear opaque to the sensor. To achieve this, one adapts or selects the spectral sensitivity of the infrared sensor to the spectral transmittance properties of the target object. Suppose one needs to measure both the filament temperature and the glass temperature of an incandescent lamp. Glass is transparent (T = 1) at wavelengths below 1.5 to 2 µm (see Figure 5). An IR thermometer with a nominal 1µm spectral response (typical of a Silicon detector) can measure the filament temperature by viewing the filament through the glass. However, glass appears opaque (T = 0) at wavelengths above 5 µm. An IR sensor with this spectral response can measure the lamp s glass bulb temperature independent of the filament. Similar considerations apply to selecting a detector and associated optical filters when measuring objects through windows or measuring the temperature of otherwise transparent targets like plastic films, gases, or flames. Another factor to consider is atmospheric absorption. While the target object must appear opaque to the IR thermometer, the intervening atmosphere must appear transparent. The atmosphere may appear opaque in several wavelength regions primarily due to strong absorption bands of H 2 O and CO 2. If one ignores atmospheric absorption, temperature readings will be more accurate with the instrument closer to the object than when it is farther away (where it will indicate lower temperature readings). Changes in humidity or the presence of certain gases may influence temperature reading accuracy. However, infrared thermometers usually employ optical filters to exclude the adverse effects of H 2 O and 8 CO 2, as these gases exhibit transmission windows in their absorption spectra allowing infrared radiation to pass with little attenuation. Fortunately, most applications do not have issues dealing with target transparency, atmospheric absorption, reflected radiation, or complex emissivity variations. Most applications have targets that freely radiate into their surroundings with known, constant emissivity. Other applications routinely employ ratio thermometers without any knowledge of target emissivity. IR sensors typically exhibit accuracies in the range from several percent of reading down to several tenths of percent of reading. For the vast majority of applications, using infrared thermometers is simply a matter of aiming and measuring. As long as users are aware of how various factors influence measurement accuracy, one will have greater confidence in selecting and using infrared thermometers. Summary and conclusion Infrared temperature sensors are ideally suited for noncontact, noncontaminating measurements: hot, moving objects, inaccessible objects, and applications requiring critical process control. Modern IR temperature sensors include a variety of high performance models exhibiting accuracies on the order of several tenths percent of reading, response times down to 1 millisecond, and optical resolutions exceeding 300:1. Ratio thermometers provide accurate measurements in high temperature applications in which environmental conditions obscure the target or do not fill the instrument s FOV. Instruments with fiber optics allow measuring targets in high temperature ambients, areas with strong electromagnetic fields, or targets in hard-toreach locations. Infrared sensors with selected spectral response characteristics allow measurement of plastics, glass, thin films, gases, or flames. Compared with traditional contact sensors, IR sensors have longer life and require less maintenance resulting in less downtime. The economic benefits associated with improved product quality, increased productivity, and energy savings more than offset the higher initial cost of infrared temperature sensors. Industries That Use Infrared Sensors: Metals: Molten metals, forging, extruding, casting, heat treating, wire fabrication Glass: Furnaces, float glass processing, annealing, tempering, fiberglass Cement: Cement and lime kilns, refractory monitoring Semiconductor: Wafer processing, thermal processes, oxidation, annealing Ovens/Dryers: Drying, laminating, curing, coating, molding Paper: Web monitoring, printing, coating, laminating, silk-screening Plastics/Rubber: Thermoforming, calendering, molding, welding, extruding Maintenance: Motors/compressors, electrical connections, transformers, HVAC, steam, ice Food: Processing, bottling, coating, packaging, sealing, storing, preparation Automotive: Exhaust systems, cooling systems, repaint, hot-spot detection

9 Making Sense of Smart Infrared Sensors Temperature Keeping up with continuously evolving process technologies is a major challenge for process engineers. Add to that the challenge of keeping up with rapidly evolving process monitoring and control methods and the combination can be quite intimidating. However, by using the latest hardware, software, and communications equipment along with leading-edge digital circuitry, infrared temperature sensor manufacturers are giving users the tools they need to meet those challenges. One such tool is the next generation of infrared thermometer, the smart sensor. Today s new smart infrared sensors combine two rapidly evolving sciences: infrared temperature measurement and high-speed digital technology, which is usually associated with computers. They are called smart sensors because they house microprocessors programmed to act as transmitters and receivers, allowing bidirectional, serial communications between sensors on the manufacturing floor and computers in the control room. Because the circuitry is smaller, the sensors are smaller, and installation in tight or awkward areas is easier. Integrating smart sensors into new or existing process control systems provides process control engineers a new level of sophistication in temperature monitoring and control. Sensor upgrades usually required buying a new unit, calibrating it to the process, and installing it. This often left processes inactive for long periods of time. In a wire galvanizing plant, for example, sensors once were mounted over vats of molten lead, zinc, and muriatic acid and were accessible only by reaching out over the vats from a catwalk; a hot and often dangerous undertaking. To ensure worker safety, this type of process would have to be shut down for at least 24 hours to cool before changing and upgrading a sensor. Today, process engineers can configure, monitor, address, upgrade, and maintain their IR temperature sensors remotely. Smart infrared sensors, with bi-directional communications capabilities, are simple to integrate into process control systems. Once a sensor is installed in a process, sensor parameters can be easily changed to accommodate changing conditions that warrant modifications all from a personal computer in the control room. For example, if ambient conditions change (e.g., room temperature), or the process itself changes (e.g., type, thickness, or temperature), all a process engineer needs to do is customize or restore saved settings at a computer terminal. If a sensor fails due to high ambient temperature conditions, a cut cable, or failed components, a smart sensor s failsafe conditions engage automatically, sending a digital signal denoting which condition failed. The sensor activates an alarm to trigger a shut down, keeping the product or machinery from becoming damaged. If ovens or coolers fail, high and low alarms also signal problems and/or shut down the process. For smart sensors to be compatible with the thousands of different types of processes, they must be fully customizable. Because smart sensors contain EPROMs (Erasable Programmable Read Only Memory), users can reprogram them to meet their specific process requirements using field calibration, diagnostics, and/or utility software from the sensor manufacturer. Integrating smart sensors into processes While widespread implementation of smart infrared sensors is new, infrared temperature measurement has been successfully used in process monitoring and control for decades. In the past, if sensor settings needed to be changed, the process engineer would have to either shut down the process to remove the sensor or try to manually reset the sensor while in place, which could be dangerous or cause delays. Figure 1. Marathon DataTemp software configuration screen for Marathon MR1S ratio thermometer. 9

10 Raytek Extending sensor life Figure 2. Example of a process line Another benefit of owning a smart sensor is that its firmware (i.e., the software that is embedded in the sensor s chips) can be upgraded via the communications link to newer revisions as they become available without removing the sensor from the process. Firmware upgrades extend the sensor s working life and can actually make a smart sensor smarter. Figure 1 shows an example of a smart infrared temperature sensor s configuration screen. The graphical interface is intuitive and easy to use. This particular screen allows process engineers to monitor and adjust sensor settings, or reset the sensor back to the factory defaults. All the displayed information comes from the sensor by way of the serial connection. The first two columns shown on Figure 1 are for user input. The third column shows returned sensor parameters, some that can be changed through other screens or custom programming. Parameters that can be changed by user input include the following: Temperature units can be changed from Celsius to Fahrenheit, or viceversa Display and analog output mode can be changed for smart sensors that have combined 1-color and 2-color capabilities Switch panel lockout can be turned on to lock out a sensor s onboard switches, which secures the sensor from tampering or accidental changes, but still allows supervisory control over a process Baud rate settings can be changed to match settings of other equipment in the process if the default rate is too fast or too slow for the monitoring computer s processor Milliamp output temperature settings can be used as automatic process triggers or alarms 10 Signal processing defines the temperature parameters returned: peak hold returns an object s peak temperature, average returns an object s average temperatures, non will return all changes in an object s temperature Energy reduction alarm can be used to signal a dirty window Serial mode selection can be changed so the sensor transmits (1) the userdefined output string continuously (burst mode), which may contain most, but not all, of the sensor s parameters, or (2) the current value of any parameter, which can be requested by the host computer and the sensor responds once with the requested value (poll mode) Configure burst string allows a user to custom program the sensor to meet particular process requirements. Using smart sensors Smart infrared sensors can be used in any manufacturing process where temperatures are critical for quality products. For example, Figure 2 shows a generic web/converting process with an oven, a corrugating/embossing die, and a cooler. Figure 2 shows how six infrared temperature sensors monitor product temperatures before and after the various thermal processes and before and after drying. The smart sensors are configured on a highspeed multidrop network and are individually addressable from the remote supervisory computer. Measured temperatures at all sensor locations can be polled individually or sequentially, and the data can be graphed for easy monitoring or archived to document process temperature data. Set points, alarms, emissivity, and signal processing information can be downloaded to each sensor using remote addressing features. This results in tighter process control. Remote on-line addressibility In a continuous process similar to the one shown on Figure 2, smart sensors can be connected to each other or to other displays, chart recorders, and controllers on a single network. Sensors may be arranged in multidrop or point-to-point configurations, or just by themselves. In a multidrop configuration, multiple sensors (up to 15 in some cases) can be combined on a network-type cable. Each sensor can have its own address, which allows each sensor to be configured separately with different operating parameters. Because smart sensors use RS-485 or Frequency Shift Keyed (FSK) communications, they can be long distances from the control room computer up to 1,200 meters (4,000 feet) away for RS-485, and up to 3,000 meters (10,000 feet) away for FSK. Some processes use RS-232

11 communications, but cable length is limited to less than a hundred feet. In a point-to-point installation, smart sensors can be connected to chart recorders, process controllers, and displays, as well as the controlling computer. In this type of installation, digital communications can be combined with milliamp current loops for a complete all-around process communications package. Sometimes, however, specialized processes require specialized software. A wallpaper manufacturer might need a series of sensors programmed to check for breaks and tears along the entire press and coating run, but each area has different ambient and surface temperatures, and each sensor must trigger an alarm if it notices irregularities in the surface. For customized processes such as this, engineers can write their own programs using published protocol data. These custom programs can remotely reconfigure sensors on-the-fly all without shutting down the process. Field calibration and sensor upgrades Whether using multidrop, point-to-point, or single sensor networks, process engineers need the proper software tools to calibrate, configure, monitor, and upgrade those sensors. Simple, easy-to-use data acquisition, configuration, and utility programs are usually part of the smart sensor package when purchased, or custom software can be used. Field calibration software permits smart sensors to be calibrated and allows the new parameters to be downloaded directly to the sensor s circuitry. Calibration software also allows a sensor s current parameters to be saved as computer data files. These parameters can be stored in data files so a complete record of calibration and/or parameter changes is kept. One set How Infrared Temperature Sensors Work Infrared radiation is part of the electromagnetic spectrum, which includes radio waves, microwaves, visible light, and ultraviolet light, as well as gamma rays and X-rays. The infrared range falls between the visible portion of the spectrum and radio waves. Infrared wavelengths are usually expressed in microns with the infrared spectrum extending from 0.7 microns to 1000 microns. Only the 0.7 to 14 micron band is used for infrared temperature measurement. Using advanced optic systems and detectors, noncontact infrared thermometers can focus on nearly any portion or portions of the 0.7 to 14 micron band. Because every object emits an optimum amount of infrared energy at a specific point along the infrared band, each process may require uniquesensor models with specific optics and detector types. For example, a sensor with a narrow spectral range centered at 3.43 microns is optimized for measuring the surface temperature of polyethylene and related materials. A sensor set up for 5 microns is used to measure glass surfaces. A 1 micron sensor is used for metals and foils. The broader spectral ranges are used to measure lower temperature surfaces, such as paper, board, poly, and foil composites. emissivity, that indicate an object s temperature. Emissivity is a term used to quantify the energy emitting characteristics of different materials and surfaces. Infrared sensors have adjustable emissivity settings, usually from 0.1 to 1.0, which allows accurate temperature measurements of several surface types. Sources of infrared energy The emitted energy comes from an object and reaches the infrared sensor through its optical system, which focuses the energy onto one or more photosensitive detectors. The detector then converts the infrared energy to an electrical signal, which is converted into a temperature value based on the sensor s calibration equation and the target s emissivity. This temperature value can be displayed on the sensor, or, in the case of the smart sensor, can be converted to a digital output and displayed on a computer terminal. An object reflects, transmits, and emits energy, as shown in the figure. The intensity of an object s emitted infrared energy increases or decreases in proportion to its temperature. It is the emitted energy, or the target s 11

12 Raytek of calibration techniques can include onepoint offset, two-point, and three point with variable temperatures. One-point offset: If working with a single temperature in a process and the sensor reading needs to be offset to make it match a known temperature, one-point offset calibration should be used. This offset will be applied to all temperatures throughout the entire temperature range. For example, if the known temperature along a float glass line is exactly 982 C (1800 F), the smart sensor, or series of sensors, can be calibrated to that temperature. Two-point: If sensor readings need to match at two specific temperatures, the two-point calibration shown on Figure 3 should be used. This technique uses the calibration temperatures to calculate a gain and an offset, which is applied to all temperatures throughout the entire temperature range. Three-point with variable temperature: If working with a wide range of temperatures in a process and sensor readings need to match at three specific temperatures, the three-point variable temperature calibration shown on Figure 4 should be used. This technique uses the calibration temperatures to calculate two gains and two offsets. The first gain and offset is applied to all temperatures below a midpoint temperature, and the second set of gain and offset is applied to all temperatures above the midpoint temperature. Three-point calibration is not as common as one- and twopoint, but occasionally manufacturers need to perform this calibration technique to meet specific standards. Field calibration software also allows routine diagnostic tests to be run on smart sensors, including power supply voltage tests, and relay tests. The results let process engineers know if the sensors are performing at their optimum and, if not, make troubleshooting easier. The new generation of smart infrared temperature sensors allows process engineers to keep up with changes brought on by newer manufacturing techniques and increases in production. They now can configure as many sensors as necessary for their specific process control needs, and extend the life of those sensors far beyond that of earlier non-smart designs. As production increases, equipment downtime must decrease. By being able to monitor equipment and fine tune temperature variables without shutting down a process, engineers can keep the process efficient and product quality high. A smart infrared sensor s digital processing components and communications capabilities provide a level of flexibility, safety, and ease-of-use never available until now. Figure 3. Example of Two-Point Calibration Displayed Temperatures ( C) Displayed Temperatures ( C) Figure 4. Example of Three-Point Calibration Calibration after Two Point Calibration Factory Calibration before Two Point Calibration Internally Calculated Temperatures ( C) (Error correction exaggerated for clarity) mid cal point lowest cal point highest cal point Internally Calculated Temperatures ( C) (Error correction exaggerated for clarity) Calibration after Three Point-Variable Calibration Factory Calibration before Three Point-Variable Calibration 12

13 More than Specifications: Selecting The Right Infrared Temperature Sensor Your Application For Infrared temperature sensors have been successfully used for years in process industries for ongoing temperature monitoring and control. Although the technology is proven, choosing between units with different specifications is sometimes confusing, leaving the process engineer to rely on more traditional temperature measurement methods (e.g., those involving contact) or on vendor recommendations. Recent innovations in infrared temperature sensor design have provided the process engineer with enhanced functionality and more questions about how to integrate and use infrared temperature sensors. Infrared technology explained An infrared temperature sensor collects radiation from a target in the field of view defined by the instrument's optics and location. The infrared energy is isolated and measured using photosensitive detectors. The detectors convert the infrared energy to an electrical signal, which is then converted into a temperature value based on the instrument's internal algorithms and the target's emissivity (a term referring to the emitting qualities of the target's surface). Infrared or noncontact temperature sensors are very successful in measuring hot, moving, or difficult-to-reach objects, or where contact temperature sensors would damage the target. The type of infrared temperature sensor to use is based on an understanding of the process application. What is the temperature range of the target? How big is the measurement spot? How far away is that spot from the sensor? These are the first of several questions to ask to help find the right temperature sensor for your application. Environmental and operating conditions determine other sensor specifications (e.g., ambient temperature, display and output, and protective accessories). Finally, ease-of-use, maintenance, and calibration considerations may uncover hidden costs that will further influence the choice of an infrared temperature sensor. Determine temperature range Infrared instruments are available for low temperature application (from below freezing) to high temperature applications (over 2760 C; 5000 F). In general, the narrower the temperature range, the better the resolution of the output signal for monitoring and controlling process temperatures. If monitoring start-up or cool-down temperatures is critical, it is necessary to choose a temperature sensor with a wider measurement range. This is critical in heat treating applications, for example, where temperature must be held within a specific temperature range for a period of time to affect a material's metallurgical properties. 13

14 Raytek Establish target size In infrared temperature measurement, the area to be measured (i.e., the target) should fill the instrument's field of view. Suppliers of infrared temperature sensors typically recommend that the measurement target exceed the field-of-view by 50%. If the target is smaller than the field of view, background objects (e.g., furnace wall) will influence the temperature reading. Conversely, if the target is larger than the instrument's field of view, the instrument will not capture a temperature variation outside the measurement area. To collect all the emitted radiation, single wavelength infrared temperature sensors (i.e., point sensors) need a clear line of sight between the instrument and the target. Sighting optics allow the user to visually sight through the instrument on the target. Some instruments have a built-in laser that pinpoints the target, which is especially helpful in dark areas. Two-color or ratio instruments, where temperature is determined from the ratio of the radiated energies in two separate wavelength bands, are a good choice when targets are very small or moving in and out of the field of view. Energy received from two-color instruments may be attenuated up to 95% and still provide accurate temperature measurement. Two-piece fiber optic units, where the cable can snake around the Figure 2. Optical charts help determine the spot size at a specific distance. The smallest spot this instrument can measure is 0.25 inches at a distance of 8 inches. In variable focus instruments, the spot size can be adjusted to match the distance. Figure 1. For accurate temperature measurement the target should be greater than the instrument s field of view, or spot size. If the instrument s spot size is larger than the target, the instrument will also measure energy emitted from the background or surrounding objects. obstructions, may be a good choice if a direct line of sight between the instrument and the target is otherwise impossible. Optical resolution Optical resolution is specified by the D:S ratio, which is determined by comparing the distance from the object to the sensor (D) with the size of the spot being measured (S). For example, a 1-inch spot on a target being measured at a distance of 10 inches has a D:S ratio of 10:1. Infrared sensors on the market today have D:S ratios ranging from 2:1 (low optical resolution) to more than 300:1 (high optical resolution). The higher the optical resolution, the more expensive the instrument optics tend to be. The choice of D:S ratio really depends on the size of the object to be measured and the distance the sensor will be from the target. For example, high resolution is needed for high temperature applications (e.g., heat treating) where the sensor must be mounted far away from the target but still must measure a small spot. Infrared temperature sensors are available with both fixed- and variable-focus lenses. The instrument's focal point is the smallest spot it can measure. On a fixed-focus instrument, there is a single focal point at a set distance. While it is possible to accurately measure temperature at a distance closer to or further from the focal point, the spot size will be larger than at the focal point. Variable focus instruments have a minimum focal point that can be adjusted to correspond to the distance from the target. Target material The target material's emissivity and surface characteristics determine the spectral response or wavelength needed in a sensor. Highly reflective metals with different alloy compositions tend to have low or changing emissivities. Thus, the optimum 14

15 Increase value & Celsius/Fahrenheit (C/F) button Laser on/off button Laser, Setup, & Fast LEDs wavelength for measuring high-temperature metal is the near infrared, around 0.8 to 1.0 micron. Because some materials are transparent at certain wavelengths, choose a wavelength at which the material is opaque. For example, 5 microns is a good choice for surface measurement of glass. Plastic films have transmission coefficients that vary according to the wavelength and thickness of the materials. Choosing 3.43 microns for polyethylene or polypropylene or 7.9 for polyester allows measurement of thin films (less than 10 mils). The typical spectral response for low temperature applications is 8 to 14 microns. If there is any doubt, the manufacturer can test a sample of the material to determine the optimum spectral band to use. If processes are run with different target materials, select an instrument with adjustable emissivity. Fixed emissivity instruments are sufficient for some materials, especially in low temperature applications. Fast response time Infrared temperature sensors reach 95% of the final temperature reading a common definition of response time much faster than contact temperature sensors (e.g., thermocouples). This is particularly important when measuring quickly heated or moving objects. New infrared sensors on the market have response times selectable down to one millisecond. However, a fast response time is not desirable for all applications, especially in those where a fast sensor may exceed the capability of existing control instruments. Also, when there is significant thermal lag in heating a process, speed in the instrument may be unimportant. Signal processing needs vary Discrete processes such as parts manufacturing, as opposed to continuous processing, require instruments with signal processing (e.g., peak or valley hold and averaging). Peak hold may be used, for example, to measure the temperature of glass bottles on a conveyor belt with temperature output fed into a controller. Without peak hold, the temperature sensor would read the lower temperature between the bottles and respond by increasing the process temperature. With peak hold, the instrument response time is set slightly longer than the time interval between bottles so there will always be at least one bottle represented in the temperature measurement. A sensitive control system can be fine tuned by averaging the temperature output. Ease of use important C/F Setup/Fast mode selector button Display LAS SET FST Infrared temperature systems should be easy and intuitive for plant operators to use. Today, user interfaces may be located directly on the sensor, on a remote monitor panel, or through a software program. Sensors with a built-in display and user interface are easy to install and set up. A separate, more accessible monitor is appropriate for ongoing temperature monitoring when sensors are installed in hardto-reach locations. The simplest monitors provide a remote display of the current temperature. Additional features include adjustable setpoints that generate an alarm or process correction. Digital displays, which are replacing traditional analog displays, provide averaging and trend plotting, and help minimize operator error. LED displays are easier to read in low light, but may be C S/F LAS ON/OFF MODE F PKH AVG Decrease value button C/F LEDs Mode LEDs Mode selector button Figure 3. The built-in display and user interface of the Marathon Series sensors, facilitates installation for process operators. difficult to see in bright light. Graphical displays that plot temperature data over time are also available. Infrared smart sensors house microprocessors and support bi-directional, serial communications between a sensor on the plant floor and a PC. Software available with smart temperature sensors, often running on the familiar Windows platform, makes it easy to remotely monitor temperature data and modify sensor parameters from the safety of the control room. Environmental considerations Sensors are specified for performance within certain ambient temperature ranges. Dust, gases, or vapor can cause inaccuracies in measurement and/or damage sensor lenses. Noise, electromagnetic fields, or vibration are other conditions that should also be considered before installation begins. A protective housing, air purging and/or water cooling can protect the sensor and ensure accurate measurements. These accessories are available from most manufacturers. In choosing accessories, consider the cost of bringing services (e.g., power, air, and water) to the unit. When possible, choose accessories that require standard services to minimize installation costs. Cable lengths will be specified by the manufacturer and all cables must be rated for the required ambient environment. 15