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11 Practical Applications of Pyrometers 11 .1 Introduction When describing different types of pyrometers in Chapters 9 and 10 it was assumed that they operate in ideal conditions . Pyrometer readings, which are measurements of the true target temperature, T t , are strongly influenced by real, practical, industrial operating conditions as shown in a simplified way in Figure 11 .1 . These measurements, which must account for target emissivity, E (T t ) or E,L (T), in the environment of the surrounding walls of temperature, T om  with emissivity, cw(T W ), or, s xw (T W ), are taken in the presence of polluted atmosphere of temperature, T a , having the equivalent emissivity, Ea(Ta), or, ska (T a ), and an absorption coefficient, a a (T a) . Referring to Figure 11 .1, the pyrometer readings depend on following factors : " radiation emitted by the target whose temperature is to be measured - (1) influence of target emissivity, " radiation emitted by the body and reflected from surrounding walls - (2) influence of surrounding walls, " radiation emitted by the walls - (3) influence of surrounding walls, " radiation absorption by the atmosphere, " radiation absorption by the sighting window, " proper emission by the polluted atmosphere, "  presence of solid bodies B in the sighting field of the pyrometer ; influence of solid obstacles . The pyrometer readings are also influenced by : "  temporarily covering of the target or its movement, " dispersed radiation from outside of the viewing cone of the pyrometer . Temperature Measurement Second Edition L. Michalski, K. Eckersdorf, J. Kucharski, J. McGhee Copyright © 2001 John Wiley & Sons Ltd ISBNs: 0-471-86779-9 (Hardback); 0-470-84613-5 (Electronic) 210  PRACTICAL APPLICATIONS OF PYROMETERS WALLS, SURROUNDING  T ,E,, ,Ex  , SIGHTING WINDOW TARGET i 3 2 PYROMETER t T t (t)  I . ~  ^~,  4  A Ea t ) ~B ATMOSPHERE E,(T,) A,(T,I,E,(T,), kk* T  EX  (T,) Figure 11 .1 Pyrometric temperature measurement in real operating conditions In practice the main components of the total measurement errors are : "  errors, AT E ,depending on the emissivity of the target to be measured, " errors, AT s , depending on the influence of surrounding walls, "  errors, AT,, due to absorption or proper emission by gaseous media, "  errors, ATi, of the indicating instruments, which may be positive or negative . The total error is given by : Y, AT = AT e + AT, + AT, + AT,  (11 .1) The proper errors of the indicating instrument are usually small and do not depend upon the operating conditions . 11 .2 Influence of Target Emissivity . A presentation of the methods of calculation, limitation and elimination of the influence of target emissivity which are used, will now be given . 11 .2 .1 Calculation of true temperature The relevant formulae for the particular types of pyrometers are given in Chapters 9 and 10 : Disappearing filamentpyrometers, as in equation (9 .16) : T  1 / T + log EA , / 9613 - precise if the spectral emissivity at A e = 0 .65 ltm is well known . INFLUENCE OF TARGET EMISSIVITY  211 Total radiation pyrometers, as in equation (10 .13) : T t =T j V1/8 - not very precise . Band photoelectric pyrometers, as in equation (10 .26) : T t = Ti n 1 / i,Zt _,12 - not very precise . Spectralphotoelectric pyrometers, based on equation (10 .29) : T t =  fl (1/Ti)+(/, e /C 2 )109Ek - precise if the emissivity at X'e is well known . Two-colour and two-wavelengthpyrometers : - no correction needed for grey bodies . Multi-wavelength pyrometers : - no correction needed . Calculated examples of the measurement errors of non-black body targets, for different types of pyrometers, are given in Table 11 .1 . Table 11 .1 Calculated error AT, = Ti - T t at T t = 1300 K ; a = 4 = LX1-a2 = 0 .8 Pyrometer  Equation  Error, AT, (K) Disappearing filament  (9 .16)  -21 Total radiation  (10 .13)  -72 Band photoelectric (n = 6)  (10 .26)  ca -34 Spectral photoelectric Ae = 0 .65 pm  -21 Ae = 2 .3 4m  (10 .29)  -58 -124 'le = 5 .2 prn 212  PRACTICAL APPLICATIONS OF PYROMETERS 11 .2 .2 Methods of approaching black-body conditions 1 .  Place a closed end ceramic tube of Ild ? 6, as described in Section 8 .2, within the investigated medium or inside the furnace chamber, then measure the internal temperature using a pyrometer as shown in Figure 11 .2 . 2 . Apparent increase of emissivity has been reported by Heimann andMester (1975) using an additional reflecting plate of low emissivity placed above the target surface as shown in Figure 11 .3 . This method is also used successfully in temperature measurement of a steel sheet in a continuousprocess of rolling, as shown in Figure 11 .4 . A more detailed description of this method is given by Honda et al (1992) . 3 .  Any type of pyrometer should be directed at the part of the target covered by black, matt varnish of sz 1 . 4 . The pyrometric sensor, which is directed at the inside of a low emissivity cup, described in Section 16 .4, is periodically brought in contact with the surface, whose temperature is to be measured . Due to multiple inside reflections, the interior of the cup approaches a black body . Thus, the measured temperature may also be a reference value for correcting the pyrometer readings . The reflected radiation from outside is also eliminated during periods when the cup is near the target surface . 5 . A further development of the system described above is the Emissivity Enhancer also produced by Land Infrared Ltd (Land Infrared Ltd, undated) . In this device the reflecting cup can be placed about 30 mm from the target surface and can be used with Land System 4 or UNO pyrometers . 6 . Application of a parallel polarising filter . Aiming the pyrometer at an angle of about 45° to the measured surface results in an increase in the apparent emissivity . An optimum angle has to be found experimentally . Walther (1981) has pointed out the really inconvenient necessity of a rigid filter and pyrometer mounting . The filter CERAMIC TUBE PYROMETER 9 Ild>6 C  /1 Figure 11 .2 Pyrometric measurement of the temperature ofa furnace chamber using a sighting tube SCREEN  PYROMETER  STEEL SHEET  PYROMETER TARGET  ROLLING CYLINDER Figure 11 .3 Low emissivity screen for  Figure 11 .4 Pyrometric temperature pyrometric measurement of the temperature  measurement of steel sheet in a rolling process of non-black bodies INFLUENCE OF TARGET EMISSIVITY  213 attenuation decreases the pyrometer output signal, limiting the application range of the method, which can only be applied to metallic surfaces . 11 .2.3 Other methods 1 . In the majority of photoelectric pyrometers, as well as in some total radiation pyrometers it is possible to set the emissivity value of the target by changing the measuring channel gain (Ircon Inc ., 1997) . The relevant element, equipped with emissivity scale, is placed before the lincarising circuit, as shown in Figure 11 .5 . If the true spectral emissivity, ei, differs from the set value, Es, some additional measurement errors, Ad , are observed . These can be determined from Table 11 .2 using the formula (Ircon Inc ., 1993) : E __ E At9=-100 s  c At9 1io (11 .2) Et where A6 1 % is the measuring error occurring at 1 % difference of E S -E t , as given in Table 11 .2 . In the case of emissivity change by x %, the corresponding total error is given by the formula : AO, % =xxA0 1% (11 .3) As shown in Table 11 .2, AYE errors increase with increasing values of effective wavelength, ~, . If the emissivity of the body as a function of temperature is known a priori, such as for induction heated steel or graphite, then the dependence of the emissivity, s (T), can be stored in the memory of the measuring arrangement and taken account of as a correcting signal during the measurements . 2 .  If the condition of constant emissivity is not fulfilled, the pyrometer readings could be less influenced by the emissivity changes by convenient choice of operating infrared wavelength range . Using Planck's law, the spectral heat flux density qk as a function of wavelength,  and of target temperature, T, is : " ' 1 A -5  (11 .4) qz =E, ~ e C2/AT-1 PYROMETER E LINEARIZER HEAD °C M Figure 11 .5 Emissivity setting in a photoelectric pyrometer by gain change 214  PRACTICAL APPLICATIONS OF PYROMETERS Table 11 .2 Emissivity errors, A,9 1% , in °C of spectral and band pyrometers due to 1 % difference between the set £ s and true £ t emissivity values (Ircon Inc ., 1997) Target  Effective wavelength, .l, e (gym) temperature '9 ( °C)  0 .65  0 .9  1 .6  2 .3  3 .4  3 .9  5 .0  8 .0  10 .6 0  0 .03 0 .04 0 .08 0 .12 0 .17 0 .20 0 .26 0 .41 0 .54 100  0 .06 0 .08 0 .15 0 .22 0 .33 0 .37 0 .49 0 .76 1 .0 200  0 .10 0 .14 0 .25 0 .36 0 .53 0 .60 0 .79 1 .2 1 .6 300  0 .15 0 .20 0 .37 0 .53 0 .78 0 .79 1 .2 1 .7 2 .2 400  0 .20 0 .28 0 .51 0 .73 1 .1 1 .2 1 .6 2 .3 2 .9 500  0 .27 0 .37 0 .68 0 .96 1 .4 0 .16 2 .1 3 .0 3 .6 600  0 .35 0 .47 0 .87 1 .2 1 .8 2 .1 2 .6 3 .7 4 .4 700  0 .43 0 .59 1 .1 1 .5 2 .2 2 .6 3 .2 4 .4 5 .2 800  0 .52 0 .72 1 .3 1 .8 2 .7 3 .2 3 .8 5 .2 6 .1 900  0 .63 0 .86 1 .6 2 .2 3 .2 3 .8 4 .4 6 .0 7 .0 1000  0 .74 1 .0 1 .8 2 .6 3 .7 4 .4 5 .1 6 .8 7 .8 1100  0 .86 1 .2 2 .2 3 .0 4 .3 5 .1 5 .8 7 .6 8 .7 1200  0 .99 1 .4 2 .5 3 .4 4 .9 5 .8 6 .6 8 .5 9 .6 1300  1 .1 1 .6 2 .8 3 .9 5 .5 6 .6 7 .3 9 .3 11 1400  1 .3 1 .8 3 .2 4 .4 6 .1 7 .3 8 .1 10 11 1500  1 .4 2 .0 3 .6 4 .9 6 .8 8 .1 8 .9 11 12 1600  1 .6 2 .2 4 .0 5 .5 7 .5 8 .9 9 .6 12 13 1800  2 .0 2 .7 4 .8 6 .5 8 .9 9 .6 11 14 15 2000  2 .4 3 .2 5 .8 7 .7 10 11 13 16 17 2200  2 .8 3 .8 6 .8 9 .0 12 13 15 18 19 2400  3 .3 4 .5 7 .8 10 13 14 16 19 21 2600  3 .8 5 .1 9 .0 12 15 16 18 21 23 2800  4 .3 5 .9 10 13 17 18 20 23 25 3000  4 .9 6 .6 11 15 18 20 22 25 27 where c l and c 2 are Planck constants . Also, using the Stefan-Boltzmann law : q = f o Ea (A)gAd . , = E6 o T 4  (11 .5) where 6 o is the Stefan-Boltzmann constant . Calculating the partial derivatives of the above formulae relative to emissivity and temperature, yields respectively : The heat flux density for wavelength range ~ 1 to /1 2 and x>>4 is given by : q ;, 1 - ~ z = f~ q ;d,,=£6 O T I A2  x  (11 .6) INFLUENCEOF TARGET EMISSIVITY  215 a (6j4)=6j4 ; d- (6, T 4 )=4e6 .T 3 ;  (11 .7) dE  dT dE(E6 .Tx)=Q .T' ; ~ T (Ea .T x )=xEa o T x- ' ;  (11 .8) Finally the ratios of rate of signal change relative to temperature and to emissivity changes are respectively : 4E6,T 3 _ 4E g T4  T  (11 .9) 0 xE6 o T x -1 xE ,T x T =  (11 .10) U Thus, using pyrometers, which operate in a narrower wavelength range, the readings are less influenced by emissivity change, even for bodies of low emissivity (e ; 0 .1 - 0 .3) . The ratio of these readings is about x/4 ; sometimes even as high as x/4 ; zz 4 - 6 . 3 . For a certain assumed target emissivity, the data of Table 11 .2 also indicate that the influence of changing emissivity on spectral and band pyrometer readings can be reduced by using pyrometers operating at the shortest possible effective wavelength, as shown before in Section 10 .4 .3 . 4 . In repeated production processes, measuring the true temperature, Tt, is avoided . Instead, another, more precise method is used to measure the true temperature, T t , at the same time determining the relevant indicated temperature, Ti . This apparent temperature, Ti, may serve as a reference for the repeatability of the process, as long as the target emissivity is constant . 5 . Kelsall (1963) describes a special method of automatic compensation of the influence of target emissivity, which is represented in Figure 11 .6 . The detector is sequentially irradiated with two heat fluxes . One heat flux is the combined effect of the true heat flux due to the temperature of the target with emissivity, el, and another component due to the reflection of the heat flux from the target :, with reflectivity, pl, initially coming from a heating element of emissivity, c2 = 1 . This combined heat flux, (DI, is : ('I =kjEj .f(Tj)+kjE2pl .f(T2)  (11 .11) The direct heat flux from the heating element is : ('2 = k2E2f(T2)  (11 .12) 216  PRACTICAL APPLICATIONS OF PYROMETERS DETECTOR LENS \~\  ROTATING DISK 01  02 1 2 .1, T 2 HEATING ELEMENT 31 E 1 T1  11 TARGET N /, Figure 11 .6 Automatic compensation of the influence of emissivity In the formulae (11 .11) and (11 . 12), Ej and £2 , are the respective emissivities of target and heating element, pj is the reflectivity of the target, k j is a coefficient, depending on the diameter of the aperture in the rotating disk associated with the flux 4)j , k 2 is a similar constant for 02 . Changing and measuring the temperature, T2, of the heating element until the condition (D j = 02 is reached at kj = k2 when TI = T2 and thus regarding that E 2 = 1, the pyrometer readings are correct . 11 .3 Influence of Surrounding Walls The surrounding conditions can exert a marked influence on pyrometer readings . For instance, when measuring the temperature of a charge placed inside a furnace chamber whose wall temperature is different from that of the charge, 6, the pyrometer is aimed at the charge through a sighting window as shown in Figure 11 .7 . Assuming a non-transparent charge, the pyrometer readings depend upon the signal, E c f j (t9 c ) , emitted by the charge surface of temperature, 0, and emissivity, E c , and also upon the signal, E W (1- E,) f2 (t9,,) , emitted by the walls of temperature 6 v , , and emissivity, E W , reflected from the charge . The overall signal, s, determining the pyrometer readings, is then : S=Ed1(0c)+Ew(1-Ec)f2(Ow)  (11 .13) Any non-linear dependence of the pyrometer readings upon the measured temperature, which is represented by the functions, . f j (O c ) and f2 (6w) , depends on the pyrometer INFLUENCE OF SURROUNDING WALLS  217 PYROMETER  HEATING ELEMENTS " 9 v E V SIGHTING  f / / L q k ti~'+~ + I WINDOW CHARGE  'J E, -TEMPERATURE AND EMISSIVITY OF CHARGE 4 .J . -TEMPERATURE AND EMISSIVITY OF WALLS Figure 11 .7 Pyrometric charge temperature measurement in a chamber furnace used . Similarly, the emissivity of walls, e W , and of charge, E c , which can be either total or band or spectral emissivity, also depends upon the pyrometer type . If the charge emissivity is high, E c -~ 1, the error due to radiation reflected from the walls can be neglected, because (1- e c ) ) 0 . Hence, the signal, s, given by equation (11 . 13), only depends upon the charge radiation . When e,, < r9 c measurement errors, due to radiation reflected from the walls, are usually negligibly small, especially when the charge emissivity is correctly set on the emissivity corrector scale of the pyrometer . It is then advisable to use a pyrometer with a short effective wavelength Ae, because the reflected radiation has a mainly long wavelength . When O W = 6, no errors are observed . When 6w > O c , the pyrometer readings are too high, giving errors which increase with increasing wall temperature, Ow, and with decreasing charge emissivity, e c . As the correct setting of the emissivity corrector does not prevent the errors, it is advisable to use pyrometers with a long effective wavelength, Ae . The following methods can be used to reduce the influence of radiation from walls : 1 . Application of a pyrometer of such an Effective wavelength at which the charge emissivity is as high as possible . 2 . A water, air or nitrogen cooled sighting tube protecting the sighted area from wall radiation, as shown in Figure 11 .8 . Water cooled tubes can be even 1 .5 m long (Land Infrared, 1997a) . 3 . Directing of pyrometer sensing tube such that reflected environmental radiation does not influence the readings, as shown in Figure 11 .9 (Ircon Inc ., 1997) . Only in the case when the charge emits dispersed reflected radiation may a small amount arrive at the pyrometer . 218  PRACTICAL APPLICATIONS OF PYROMETERS HEATING ELEMENTS  FALSE  CORRECT POSITIONING POSITIONING PYROM ETER COOLING  CHARGE FURNACE AGENT - CHARGE RADIATION SIGHTING TUBE  CHARGE  WALLS RADIATION Figure 11 .8 Charge temperature measurement in  Figure 11 .9 Elimination of reflected a chamber furnace, using a cooled sighting tube  environmental radiation (Ircon Inc ., 1997) A numerical example illustrates the calculation of the true charge temperature . Numericalexample The temperature of a cast-iron charge was measured by a Siemens AG Ardometer 20 total radiation pyrometer, having the measuring temperature range : 500 to 1000 °C . A charge of total emissivity c c = 0 .5 was placed in a chamber of wall temperature A, = 840 °C and of wall emissivity s ue , = 1 . If the temperature indicated by the pyrometer was 9W = 740 °C, find the true charge temperature 9 c . Solution : Using the pyrometer characteristic shown in Figure 11 .10, the pyrometer output signal corresponding to an indicated temperature of 740 °C was 1 .75 mV, whereas the signal corresponding to 840 °C was 2 .96 mV . Inserting these data into equation (11 .13) gives . 1 .75 = 0 .5xfl(9a) + (1-0 .5)x2 .96 or fi(dc) = (1/0 .5)x(1.75 - 0 .5x2 .96) = 0 .54 mV From the pyrometer characteristic in Figure 11 .8 the corresponding true charge temperature is : 9 c = 565 °C 6 E _5 W 4 z 2,96 3' r- 2-1,75 a 1 ,0,54 0  3~=565°C  4 . 7114  ,A=B 40°C 0 500 600 700 B00 900 1000 TEMPERATURE ,A , "C Figure 11 .10 Determining true charge temperature, 9 c , based upon the characteristic of a total radiation pyrometer (Siemens AG, 1996)