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Thermometry Understanding and using the Infrared

Advances in electronic and detector the curve increases, increasing the area () technology have resulted in a variety of Infrared is part of the electromagnetic beneath it, and (2) the associated spectrum which includes , with the peak energy (highest point of the curve) non-contact infrared (IR) for , visible , , gamma- shifts to the shorter wavelength end of the scale. industrial and scientific use. It is important and X-rays (Figure 2). These various forms of to understand their major differences in energy are categorised by or This relationship is described by Wien’s order to select the proper unit for a given wavelength.* Note that visible light extends from Displacement Law: 3 application. .4 to .7 micron, with ultraviolet (UV) shorter than λmax = 2.89 x 10 /T .4 micron, and infrared longer than .7 micron, where λmax = wavelength of peak Infrared Theory extending to several hundred microns. In energy in microns Energy is emitted by all objects having a practice, the .5 to 20 micron band is used for IR T = in degrees temperature greater than . This . energy increases as the object gets hotter, For example, the wavelength for peak energy permitting measurement of temperature by Planck’s Law emitted from an object at 2617 degrees measurement of the emitted energy, particularly The amplitude (intensity) of radiated energy can (2890 degrees Kelvin) is: the radiation in the infrared portion of the be plotted as a function of wavelength, based on 3 spectrum of emitted radiation. Figure 1 shows Planck’s law. Figure 3 shows the radiation λmax = 2.89 x 10 /2890K = 1.0 µm a typical infrared radiation thermometer. emission curves for objects at two different or . By convention, longer Another illustration involves heating a steel billet.

Dare shownAmp to the right on IR graphs, reverse of At about 600°C (1100°F), a dull, glow is Lens or mirror electromagnetic spectrum charts, such as Figure emitted from the steel. As the temperature D Amp 2. The area under each curve represents the total increases, the colour changes from red to Output to Signal controller, energy radiated atConditioner the associated temperature. and as the peak passes into the Output to Signalrecorder, etc. controller, Conditioner visible light spectrum. Finally, the energy emitted recorder, etc. Note that two changes occur simultaneously as throughout the entire is at such Figure 1: Block diagram, infrared temperature is increased: (1) the amplitude of a high level that white light is given off by the steel at about 1650°C. Because the peak of the curve shifts as temperature increases, selection ed owaves

oadcast oadcast; ed α owaves elevision Infrar Visible Light Ultraviolet X-Rays -Rays

Longwave Radio AM Radio Br FM Radio Br T Micr of the optimum portion of the spectrum is

The IR to UV spectrum 10 KHz 1 MHz 100 MHz 100 µm 1 µm thermometer performance. is explained as shown.

The IR to UV spectrum is explained as shown. een ellow

Infrared Red Y Gr UV Emissivity is defined as the ratio of the energy 20 µm .7 µm .4 µm radiated by an object at a given temperature to

een the energy emitted by a perfect radiator, or ellow

Infrared Red Y Gr Blue Violet UV blackbody, at the same temperature. The 20 µm .7 µm .4 µm emissivity of a blackbody is 1.0. All values of Figure 2: Electromagnetic spectrum and light spectrum *Wavelength (λ) is inversely related to frequency (f): λ = c/f, where c = velocity of the ; λ is measured in microns (µm); 1 µm = 10-6m. emissivity fall between 0.0 and 1.0.

Emissivity (E), a major but not uncontrollable factor in IR temperature measurement, cannot be ignored. Related to emissivity are reflectivity (R), a measure of an object’s ability to reflect 700C infrared energy, and transmissivity (T), a measure of an object’s ability to pass or transmit IR 25C

Relative Radiation Intensity energy. Since all radiation must be either transmitted, reflected or absorbed:

l 1 µm 10 µm 20 µm A + R + T = 1.0 Figure 3: Radiation intensities derived from Planck’s Law (Blackbody radiation curves)

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk

R

E Hot Source T

Pyrometer Consider the example in Figure 4a. Object X is a hot block of material, Y is colder; therefore, 1.0 will be radiated from X to Y. Some heat will be absorbed by Y, some reflected, and some transmitted through Y. The three dispositions 5

must equal 100%, represented as 1.0 for ransmission T coefficients of absorption, reflection, and transmission. If A = 1.0, all the heat is Wavelength 0 in microns absorbed; if R = 1.0, then A = T = 0. Usually 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (µm) some combination exists: Figure 5: IR transmission through

A = .7 (70% absorbed) window to pass a high percentage of the IR 2. Increase the size of object B until it fills R = .2 (20% reflected) energy at that wavelength. the thermometer’s FOV. T = .1 (10% transmitted) 3. Decrease the emissivity compensator Atmospheric Absorption (described later) to compensate for the Sum = 1.0 (100% energy radiated from One of the first considerations in selecting the loss of energy. X to Y) spectral response (wavelength range at which 4. Get a thermometer with a smaller FOV. an instrument is sensitive to IR) of a device is atmospheric absorption. Certain components Field of view is described either by its angle or

1. Heat a sample of the material to a known temperature as determined by a precise sensor in an oven, and measure the temperature of the object with the IR instrument. Use the emissivity compensator Wavelength in microns (µm) adjustment to force the indicator to display 1 2.2 8 14 20 Figure 7: Distribution of energy as received by filtered IR detectors the correct temperature. Use this value of

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk Energy Curve 2 present.

λ λ 1 2 Two factors limit how short the wavelength can be: (1) the lowest temperature which must be measured; as can be seen from blackbody radiation curves, the shorter the wavelength, the less energy is available at that wavelength,

wavelengths, producing more accurate e, ° F heat sources is not a problem), well-oxidised measurements when emissivity variations are λ = 2.2 µm 1600

1.0 1400 Indicated temperatur

.5 1200 Emissivity .2 .4 .6 .8 1.0 Target material emissivity 0 1 µm 10 µm 20 µm Figure 10. Readings obtained on IR with centre wavelengths of .8 µm and 2.2 µm as actual emissivity of tar- get material varies; target temperature is 1100°C, emissivity Figure 9: Typical metal: variation of emissivity with wavelength is 1.0.

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk 2000 λ = .8 µm

1800 e, ° F

λ = 2.2 µm 1600

1400 Indicated temperatur

1200 .2 .4 .6 .8 1.0 Target material emissivity Critical Specifications 100 In addition to optics, spectral response, emissivity, temperature range, and mounting (fixed-mount vs portable), the following list of items should be considered in selecting an

ransmission 50

%T : 1. Response time: The instrument must respond quickly enough to process changes for 2 3 4 5 6 7 8 10 12 14 16 proper recording or control of temperature. Figure 11: line represents transmission characteristics of 25 µm polyethylene film (typical of plastics wtih strong 3.43 µm IR thermometers are usually faster than most absorption) while dashed line illustrates 7.9 µm absorption of polyester film other temperature measurement devices, polypropylene, nylon, polystyrene) are opaque steam and water vapour can be guaranteed with typical response times in the 100 ms to at 3.43 µm; other plastics (polyester, (due to the 5.5 to 7.5 µm ); 7.9 1 s range. polyurethane, Teflon, FEP, cellulose, polyamide) µm is ideal for surface measurement, with no 2. Environment: The instrument must function are opaque at 7.9 µm (Figure 11). Some films reflectance. within the range of ambient temperatures to are opaque at both. In the latter case, a choice which it will be exposed. Special provisions may be based on spectral reflectivity, instrument Spectrum for Flame Measurement/ must be made to protect the instrument price and availability, or whether quartz heaters Combustion Optimising from dirt, dust, flames, and vapours. are used in the process (because these heaters While most IR instruments can be used to Intrinsically safe or explosion-proof may cause severe interference at wavelengths measure “dirty” flame temperatures, a clean instruments may be required. shorter than 5 µm). For plastics opaque only at flame (one with no particulate or smoke) can be 3. Physical mounting limitations: The sensing head must fit in the space available to sight 3.43 µm it may be possible to use the weaker, measured at 4.5 µm where CO2 and NOx are secondary 6.86 µm wavelength to avoid quartz opaque, provided these by-products are the object. If this is a hazardous location, heater interference. present and the IR pathlength through the risk can be minimised by using a head which flame exceeds 25 cm. The same instrument contains the fewest parts (i.e., detector and Spectrum for Glass can also assist in combustion optimising, even ambient sensor only) so that a catastrophic The glass industry is one in which the different for smaller flames, because relative readings loss does not require replacement of the factors involved in IR measurement, particularly can be used (absolute readings are not entire instrument. This type of instrument reflection and transmission, must be well required). typically uses a remotely located electronics understood for optimum results. Figure 12 box containing most of the circuitry, which shows the relationship of transmission to Fixed Mount and Portable IR Thermometers can be mounted a safe distance away from wavelength. In general, pane glass is opaque Fixed mount instruments are generally installed a hazardous location. Alternatives include beyond 5 µm, and becomes progressively in one location to continuously monitor or control use of fibre optics, sight tubes, or front- transparent at shorter wavelengths (as a given process. They operate from the local surface to direct IR energy to the evidenced by the human ). The .8 µm power source (110/220 V AC or 24 V DC), are detector. instrument measures several centimetres into aimed at a single point or scan an area by a 4. Viewing port or window applications: If a molten glass, 2.2 µm about 75 to 100 mm. mechanical aiming device. Often they are vacuum chamber, special atmosphere, or Instruments using 3.8 µm will measure no supplied with a portable case and can be moved other process requires measuring further than 25 or 50 mm, depending on the from one location to another. In manufacturing, temperatures through windows into vessels, type of glass, so this wavelength is excellent for a process can be studied by monitoring several care must be taken to ensure that the averaging “gob” temperatures. (These figures points at different intervals. The sensing head window will pass energy at wavelengths are for non-pigmented glass, and it must be can be mounted on a , and the signal measured by the instrument. Glass will pass remembered that glass nearest the surface will output fed to a chart recorder or data logger for wavelengths shorter than 3 µm, quartz .5 to contribute the most to the temperature reading; later analysis. 4.5 µm, selenide from 2 to 15 µm, pigmented glass will be more opaque, even at 4 to 14 µm. Irtran, a series of short wavelengths.) For panes, , and If a truly portable unit is needed, battery- materials manufactured by Kodak, is other thin-wall glass, the longer wavelengths operated IR thermometers are available to available in several different band pass must be used. Reflection becomes critical at 8 match the features of nearly all fixed-mount wavelengths from .5 to 20 µm. to 14 µm; reflectivity averages 15%. This band instruments except control functions. One of the If visible sighting is required as well as can be used with emissivity settings of .85 with limitations of these units is the need for periodic infrared, a window material which transmits good results. Reflectivity is negligible between battery replacement. Generally, their uses have visible energy as well as infrared must be 5 to 8 µm but 5.1 µm is preferred as most of the been maintenance diagnostics, quality control used. The temperature range of temperature sensed is from a few mils beneath functions, periodic spot measurements of measurement dictates the longest the surface, reducing the cooling effect of temperature critical processes, and energy wavelength to be passed, since peak energy surface convection currents. The 5 to 7 µm surveys. wavelengths increase as temperature band is discouraged unless the absence of decreases. 5. Signal processing: Various signal processing devices are integrated to produce outputs to

1.0 interface with displays, recorders, controllers, data loggers, and computers. Displays,

.5 alarm set points, and PC Interfces are ransmission

T commonly an integral part of the IR 0 thermometer. 1 µm 3 µm 5 µm 10 µm 20 µm

Figure 12: Transmission spectrum of glass (typical 3 mm pane glass)

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk Signal processing features include: Integration of reflected energy Various accessories are available to make Maximum reading: a stored value for the compensation: allows calculation based on IR thermometers convenient to use and highest temperature measured. discrete input for unwanted energy received reduce installation costs. For portable Minimum reading: a stored value for the by instrument. instruments, accessories include: carrying lowest temperature measured. case, wrist strap and source. Difference: maximum minus minimum. Output formats: Fixed instrument accessories include: sight Average temperature: the mean of all mV linear or nonlinear tube, air purge collar, water-cooled housing, temperatures measured in a given time mA linear mounting bracket, swivel bracket and period. equivalents alignment light spots. Variable time constant: enables smoothing RS-485 displayed temperature or output in rapidly USB changing temperature measurements. Contact closures for preset alarm points Self-test or diagnostic outputs.

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk