omega.com HOME

A Historical Perspective
IR Through the Ages
From Newton to Einstein
Today's Application's
 
Theoretical Development
Radiation Basics
Blackbody Concepts
From Blackbodys to Real Surfaces
 
IR Thermometers & Pyrometers
The N Factor
Types of Radiation Thermometers
Design & Engineering
 
Infared Thermocouples
Thermocouple Basics
Self-Powered Infared Thermocouples
Instalation Guidelines
 
Fiber Optic Extensions
Fiber Advantages
Fiber Applications
Component Options
 
Linescanning & Thermography
Infared Linescanners
2-D Thermographic Analysis
Enter the Microprocessor
 
Calibration of IR Thermometers
Why Calibrate?
Blackbody Cavities
Tungsten Filament Lamps
 
Products & Applications
Alternative Configurations
Application Guidelines
Accessories & Options
 
Information Resources
Emissivity Tables
Glossary
Index
List of Figures
 
Technical Learning Home

Application Guidelines
For first level sorting, consider speed of response, target size (field of view), and target temperature. Once the list of possible candidates for the application has been narrowed, consider things like band pass and sensitivity of the detector, transmission quality of the optical system and transmission quality of any windows or atmosphere in the sighting path, emissivity of the target, ambient conditions, and the process dynamics (steady state variations or step changes). These are shown graphically in Figure 8-1.

Figure 8-2: Sighting on a Specular Surface

  If 90% response to a step change in temperature is required in less than a few seconds, pyrometers with thermal detectors may not be suitable, unless you use thermopiles. A pyrometer with a photon detector may be a better choice.
  Thermometers with targets of 0.3 to 1 inch diameter with a focal distance of 1.5 to 3 feet from the lens are common. If a target size in this range is needed to sight on a large target through a small opening in a furnace, a pyrometer in which target size increases rapidly with distance beyond the focal plane may be fit the bill. Otherwise, a thermometer with more sophisticated optics and signal conditioning may be required.
  If the temperature to be measured is below 750°F (400°C) a more sophisticated pyrometer with optical chopping can improve performance.
  If the surroundings between the thermometer and the target are not uniform, or if a hot object is present, it is desirable to shield the field of view of the instrument so that these phenomenon have minimal effect on the measurement.
  Any radiation absorbed or generated by gases or particles in the sighting path will affect measured target temperature. The influence of absorbing media (such as water vapor) can be minimized by proper selection of the wavelengths at which the thermometer will respond. For example, a pyrometer with a silicon detector operates outside the absorption bands of water vapor and the error is nil. The influence of hot particles can be eliminated by ensuring they do not enter the sighting path, or by peak or valley picking, if they are transiently present. A open ended sighting tube, purged with a low temperature gas can provide a sighting path free of interfering particles.
  Thermometers selected to measure transparent targets, such as glass or plastic films, must operate at a wavelength where the transmission of these materials is low so hot objects behind the target do not interfere with the measurement. For example, most glass is opaque at wavelengths above 5 microns if it is 3 mm or thicker. The emittance of glass decreases at higher wavelengths above 8 microns because of its high reflection, so measurement at higher wavelengths is not as desirable. If the incorrect band is picked, the thermometer will sight through the glass and not read the surface temperature.
  Imagine, for example, two thermometers measuring the surface temperature of a lightbulb. One thermometer operates in the 8 to 14 micron range, and the other operates at 2 microns. The 8 to 14 micron device reads the surface temperature of the bulb as 90°C. The 2-micron device, sees through the surface of the glass, to the filament behind, and reads 494°C.
  Other parameters to consider when selecting a non-contact temperature sensor include:
  Target material--The composition of a target determines its emissivity, or the amount of thermal energy it emits. A blackbody is a perfect emitter, rated 1.0 or 100%. Other materials are somewhat lower; their emissivity can be anywhere from 0.01 to 0.99, or 0-99%. Organic materials are very efficient, with emissivities of 0.95, while polished metals are inefficient, with emissivities of 20% or less. Tables only give the emissivity of an ideal surface, and cannot deal with corrosion, oxidation or surface roughness. In the real world, emissivity variations range from 2 to 100% per 100°F temperature change. When in doubt, obtain an appropriate instrument and measure the emissivity exactly.

Figure 8-3: Use of Shielding and Cooling

  Temperature range--The emissivity and the range of expected temperatures of the target determine the wavelengths at which the target will emit efficiently. Choose a sensor that is sensitive at those wavelengths. Accuracy is listed as a percent of full scale or span, so the closer the temperature range to be measured can be specified, the closer sensor match, and the more accurate the final measurements.
  Wavelength choice--Manufacturers typically lists their products with a given temperature range and wavelength, with wavelengths listed in microns. Note that more than one wavelength can apply in any given application. For example, to measure glass, a wavelength of 3.43, 5.0 or 7.92 microns can be used, depending on the depth you want to measure, the presence of tungsten lamps, or to avoid reflections. Measuring plastic films presents the same problems. You may want to use a broad spectrum to capture most of the radiant emissions of the target, or a limited region to narrow the temperature range and increase accuracy. In many applications, various conditions and choices may exist. You may want to consult with your supplier.
  Atmospheric interference--What is present in the atmosphere between the sensor and the target? Most non-contact temperature sensors require an environment that has no dust, smoke, flames, mist or other contaminants in the sensor's line of sight. If contaminants exist, it may be necessary to use a two-color sensor. If there is an obstructed line of sight, it may be necessary to use a fiber optic probe to go around the obstacles.
  Operating Environment--Into what kind of environment will the sensor itself be installed? If it is hazardous, hot, humid, corrosive or otherwise unfriendly, it will be necessary to protect the instrument. Lenses and cases are available to withstand corrosives; air purge systems can protect lenses from process materials; and various cooling systems are available to cool the lenses, optics and electronics.
  If the surrounding temperature is the same as the target temperature, the indicated temperature from a radiation thermometer will be accurate. But if the target is hotter than the surroundings, it may be desirable to use a device with a high N° value to minimize the emissivity error and minimize radiation from the surroundings reflected into the thermometer. Two approaches can be used when the target is at a lower temperature than the surroundings. The first method, Figure 8-2, is possible if the target is fixed, flat, and reflects like a mirror. The thermometer is arranged so that it sights perpendicular to the target.
  To measure the temperature of a target with a matte surface, you must shield the field of view of the thermometer so that energy from hot objects does not enter. One approach, shown in Figure 8-3, involves sighting the thermometer through an open ended sighting tube. The other approach is to use a cooling shield. The shield must be large enough so that D/H ratio is 2 to 4. This method can not be used for slowly moving or stationary targets. An uncooled shield can be used to block out radiation from a small, high temperature source that will not heat it significantly.
  A closed end sight tube is an accessory that can be used to protect optics and provide a clear sight path for broadband thermometers. The one end of the tube reads the same temperature as the target (it may be touching the target or very close to it), while cooling can be used to protect the thermometer itself, at the other end of the tube, from high temperatures. A closed or open end sight tube can prevent attenuation of emitted radiation by water vapor, dust, smoke, steam and radiation absorptive gases in the environment.
  Industrial applications invoke either surface temperature of objects in the open, or temperatures inside vessels, pipes and furnaces. The target may need to sight through a window in the latter case. The thermometer, if permanently installed, can be mounted to an adjacent pedestal, or attached to the vessel. Hardware is available from manufacturers to accomplish this. The thermometer housing may need to be protected from excessive heat via a cooling mechanism, and/or may require a continuous clean gas purge to prevent dirt accumulation. Hardware is optionally available for both needs.

Figure 8-4: Accessories for Furnace-Wall Installation

  The accessories needed for difficult applications, for example, to permanently install a radiation thermometer on the wall of a furnace, can easily escalate the cost of an infrared thermometer into the thousands of dollars, doubling the price of the standard instrument. In Figure 8-4, for example, the thermocouple sensor head and its aiming tube are mounted inside a cooling jacket. The coolant flow required depends on the actual ambient conditions which exist. Also shown are an air purge assembly, and a safety shutter. The latter allows the furnace to be sealed whenever the radiation thermometer must be removed.
  If the target and the surroundings are not at the same temperature, additional sensors, as shown in Figure 8-5 need to be supplied. This configuration allows automatic compensation in the radiation thermometer electronics for the effects of the surroundings on the target temperature reading.

Figure 8-5: Compensation for Elevated
Ambient Temperatures

  There is a lot to consider when selecting and installing a non-contact sensor to measure a critical process temperature. And to the unfamiliar, the task can seem mind boggling. How do I get emissivity data? Which wavelength(s) is best for my application? What options do I really need? .... and a thousand other questions easily come to mind. But help is available. For example, many manufacturers have open Internet sights that contain an abundance of helpful information to assist the first time user in getting started. (See list of resources, p. 68.) In addition, there are consultants, as well as the manufacturers themselves, who can supply all the assistance needed to get up and running quickly.

Industrial Applications
In most cases, at least one of the sensors we've discussed can be used to measure temperature in any kind of application, from -50 to 6,500°F . The key is to identify the sensor that will do the best job. This can be a very simple or an extremely difficult choice. Perhaps some of the applications listed below will give you a few ideas on how to use a non-contact temperature sensor in your plant.
  Airplane Checkout--The sheer size and height of a widebody 747 aircraft makes it very difficult for technicians to check the operation of various devices, such as pitot tubes and heating tapes used to warm pipes, water and waste tanks in various parts of the aircraft. Before, a technician had to climb a 25-ft ladder and touch the surfaces to see if the devices were working properly.
  Now, a radiation thermometer is used during final assembly to check the operation of various heating elements. The technician stands on the ground, and aims the thermometer at each pitot tube or heating element. Boeing reports saving 4-5 construction hours on each jet. Asphalt--Asphalt is very sensitive to temperature during preparation and application. Thermocouples normally used to measure asphalt temperature usually have severe breakage problems because of the abrasiveness of the material. Infrared thermocouples are an ideal replacement.
  The sensor can be mounted so that it views the asphalt through a small window in the chute, or slightly above for viewing at a distance. In either case, the sensor should have an air purge to keep the lens clean from vapor or splashes. Plus, it can be connected to the control system as if it was a thermocouple.
  Electrical System Maintenance-- Infrared scanning services are becoming widely available. Typically, a scanning service brings in a portable imaging processor and scanner twice a year to check a building's switchgear, circuit breakers, and other electrical systems. The service looks for hot spots and temperature differences.
  Between visits, maintenance personnel can perform spot checks and verify repairs with an inexpensive radiation thermometer. Attaching a data logger lets a technician determine heating trends of switchgear during peak periods, and identify the parts of system that suffer the most when electrical consumption goes up.
  Flame Cutting--In flame cutting, before a computer cuts various shapes from steel plate, the steel surface has to be heated by a natural gas or propane flame. When a "puddle" of molten metal is detected by the operator, oxygen is injected into the gas stream. This blows the molten metal through the plate and the cutting cycle begins. If oxygen is injected prematurely, it makes a defective cut, leaving an objectionable rough and wide pit-like depression in the plate.
  A fiber optic sensor can be mounted on the torch and aimed to look through the gas stream at the plate surface. It will detect the proper plate temperature for puddling, and inform the operator.
  Glass--An infrared thermometer is ideal for measuring the temperature of soda-lime-silica glass, predominantly used in making sheet, plate, and bottles. The biggest problem is that glass has relatively poor thermal conductivity, so temperature gradients exist at various depths. The three most commonly used wavelengths for measuring glass--3.43, 5.0, and 7.92 microns--each see a different distance into the glass. A sensor with 7.92 microns sees only the surface, while a 3.43 micron sensor can see up to 0.3 in. into the glass.
  The trick is to select a thermometer which is not adversely influenced by thickness variations. Your best bet may be to send samples of glass products to the thermometer manufacturer, and let them advise you on what device to use.

Table 8-2: Successful Radiation Thermometer Applications
  MOUNTED PORTABLES
2 H L 2 H L
 Cement Kilns    
   Burning zones, preheaters            
 Energy Conservation        
   Insulation and heat flow studies, thermal mapping            
 Filaments        
   Annealing, drawing, heat treating            
 Food        
   Baking, candy-chocolate processing, canning freezing, frying,            
   mixing, packing, roasting            
 Furnaces    
   flames, boiler tubes, catalytic crackers            
 Glass
   Drawing, manufacturing/processing bulbs, containers,            
   TV tubes, fibers            
 Maintenance        
   Appliances, bearings, currentoverloads, drive shafts, insulation,            
   power lines, thermal leakage detection            
 Metals (ferrous and nonferrous)    
   annealing, billet extrusion, brazing, carbonizing, casting,            
   forging, heat treating, inductive heating, rolling/strip mills,            
   sintering, smelting            
 Metals, Pouring          
 Quality Control
   printed circuit boards, soldering, universal joints,            
   welding, metrology          
 Paint            
   Coating, ink drying, printing, photographic emulsions, web profiles        
 Paper            
   Blow-molding, RIM, film extrusion, sheet thermoforming, casting        
 Plastic            
   Blow-molding, RIM, film extrusion, sheet thermoforming, casting      
 Remote Sensing            
   Clouds, earth surfaces, lakes, rivers, roads, volcanic surveys        
 Rubber            
   Calendaring, casting, molding, profile extrusion, tires, latex gloves    
 Silicon            
   Crystal growing, strand/fiber, wafer annealing, epitaxial deposition        
 Textile            
   Curing, drying, fibers, spinning        
 Vacuum Chambers            
   Refining, processing, deposition            
2=2-color sensor H=High Temperature L=Low Temperature

  During installation, select the aiming point so that the instrument doesn't see any hot objects behind the transparent glass, or any reflected radiation from hot objects in front of the glass. Aim the sensor at an angle that avoids reflections, or install an opaque shield to block the reflections at the source. If neither is possible, use either of the higher wavelength sensors, because they are not affected as much by reflections.
  Be careful of applications where the glass is heated with high intensity, tungsten filament quartz lamps. These generate radiation levels that interfere with thermometers operating below 4.7 microns. In this case, use a 7.92 micron sensor.
  Glass Molds--The temperature of the mold or plunger used to make glass containers is critical: if too hot, the container may exit the mold and not retain its shape; if too cool, it may not mold properly. Molds must be measured constantly to ensure that cooling is proceeding correctly.
  An infrared thermometer can be used to take mold measurements. A few suggestions: Don't measure new molds. They are usually shiny and clean, so they are reflective and have low emissivity. As they get older, they get dull and non-reflective, and the emissivity becomes higher and more repeatable. Use a radiation thermometer with a short wavelength, such as 0.9 microns, or a two-color instrument.
  Humidity--An infrared thermocouple can be used to measure relative humidity in any situation where there is a convenient source of water and flowing air. Aim the device at a wet porous surface with ambient air blowing across. When air moves across a wet surface, water cools by evaporation until it reaches the wet-bulb temperature, and cooling stops. The sensor can be connected to a display that records the lowest temperature, which is the wet-bulb temperature, and can be used to calculate the relative humidity.
  Immersion Thermowells--Thermowells protrude into a high-pressure vessel, stack, pipe or reactor, allowing a temperature sensor to get "inside" while maintaining process integrity. An infrared thermocouple or fiber optic sensor can be positioned outside the thermowell looking in, rather than being mounted inside the thermowell. Conventional sensors subjected to constant high temperatures suffer metallurgical changes that affect stability and drift. But the non-contact sensors, because they are outside, do not suffer such problems. They also respond more quickly; essentially, the response time of a radiation sensor is the same as the thermowell. Also, since the sensor is outside, it will survive much longer in a very high temperature environment than a conventional sensor will.

Table 8-3: Typical Application Temperature Ranges
APPLICATION TEMP. RANGES
General purpose for textile, printing, food, rubber, thick plastics, paints,
laminating, maintenance
-50 to 1000°C
-58 to 1832°F
Life sciences, biology, zoology, botany, veterinary medicine, heat loss and research 0 to 500°C
32 to 932°F
Thin film plastic, polyester, fluorocarbons, low temperature glass 50 to 600°C
122 to 1112°F
Glass and ceramic surfaces, tempering,annealing, sealing, bending and laminating 300 to 1500°C
572 to 2732°F
See-through clean combustion flames and hot gases. Furnace tubes 500 to 1500°C
932 to 2732°F
Medium to high temperature ferrous and non-ferrous metals. See-through glass 250 to 2000°C
482 to 3632°F
Hot and molten metals, foundries, hardening, forging, annealing, induction heating 600 to 3000°C
1112 to 5432°F

  To install a radiation thermometer in a thermowell, mount it so it is aimed directly into a hollow thermowell, and adjust its distance so that its "spot size" is the same diameter as the thermowell. This way, the sensor will monitor temperature at the thermowell tip. If the thermowell has a sight glass, select a sensor that can see through it.
  Induction Heating--Measuring the temperature of an induction heating process can be accomplished with infrared thermocouples, thermometers or fiber optic sensors.
  An infrared thermocouple will operate in the very strong electrical field surrounding induction heaters. Make sure the sensor's shield wire is attached to a proper signal ground. The preferred method is to view the part between the coil turns or from the end. If there is excessive heating on the sensor, use a water cooling jacket (you can use the same water source used to cool the induction coil).
  Fiber optic sensors should be mounted so the viewing end is placed close to the target. The tip of the fiber can be positioned between the induction coils. Replaceable ceramic tips can be used to minimize damage and adverse effects from the radio frequency field. If the fiber won't fit, use a lens system to monitor the surface from a distance. Fiber optic sensors are not normally affected by induction energy fields, but if the noise is excessively high, use a synchronous demodulation system. The demodulator converts the 400 Hz ac signal from the detector head to dc, which is more immune to noise.
  Plastic Film--A film of plastic or polymer emits thermal radiation like any other material, but it presents unique measuring problems for any sensor, including a radiation thermometer. As with glass, when measuring film temperature, it's important to install it so the instrument doesn't see any hot objects behind the transparent film, or any reflected radiation from hot objects in front of the film.
  For films of 1, 10 or 100 mil thicknesses, a wavelength of 3.43 or 7.92 microns will work for cellulose acetate, polyester (polyethylene terephthalate), fluoroplastic (FEP), polymide, polyurethane, polyvinyl chloride, acrylic, polycarbonate, polymide (nylon), polypropylene, polyethylene and polystyrene.
  As with glass, be careful of applications where the film is heated with high intensity, tungsten filament quartz lamps. These generate radiation levels that interfere with thermometers operating below 4.7 microns. In this case, use a 7.92 micron sensor.
  Web Rollers--Infrared sensors can be used to measure the temperature of rollers used in various web processes, even if they are chrome plated. Uncoated metal or chrome rollers are difficult for an IR sensor to see, because they have low emissivity and the sensor sees too many environmental reflections. In such a case, paint a black stripe on an unused portion of the roller and aim the device directly at the stripe.
  Dull metal rollers often provide reliable signals. Emissivity can shift if the rollers get covered with dirt, moisture or oil. If in doubt, simply paint a stripe. Non-metallic surfaced rollers provide a reliable signal no matter where the device is pointed.

Table 8-4: Application Wavelengths (Microns)
TYPICAL
APPLICATIONS
0.65 0.9 1.0 0.7-1.08
and
1.68
2-color
1.55
and
1.68
2-color
1.65 2.0 3.43 3.9 5.0 7.9 8-14
Aluminum                
Asphalt                    
Automotive      
Appliances                
Ammunition                  
Batteries                    
Cement        
Construction Materials                  
Fiberglass          
Food Processing                  
Foundry              
Glass-Melting              
Glass-Flat                    
Glass Bottles                
Heat Treating              
Induction Heating              
Kilns          
Metalworking              
Mining                      
Non-ferrous Metals                
Ovens  
Paper                  
Pharmaceutical                      
Plastics                  
Plastic Films                    
Rubber                    
Semiconductors          
Steel            
Textiles                
Utilities                    

Top of Page    

Next Chapter: Products & Applications Continued