9 Manually Operated Pyrometers 9 .1 Disappearing Filament Pyrometers 9 .1 .1 Principle of operation The first disappearing filament pyrometer was built in 1901 by L . Holborn and F . Kurlbaum (Holborn and Kurlbaum, 1903) . Disappearing filament pyrometers are spectral pyrometers where the brightness of a lamp filament is changed by adjusting the lamp current until the filament disappears against the background of the target, whose temperature is to be measured . In pyrometers of this kind, the eye of the observer is itself the detector . In another seldom applied type of pyrometer the brightness match is achieved by the attenuation of the target brightness, using a neutral grey filter . In the first type shown in Figure 9 .1, the observer sees the filament of the lamp against the target background, through the eyepiece and red filter . The lamp current is adjusted by the resistor until the filament picture disappears when the brightness, or radiance, L, of equation (8 .23) of filament and target are identical . The measured value of temperature is read from the ammeter, A, calibrated in temperature units . A comparison of the radiance, L, of the filament and the target occurs at one wavelength . A grey filter can be used to increase the measurement range . Disappearing filament pyrometers, which are calibrated for black-body targets, have a lower limit of temperature range of about 700 °C, determined by the long wave visibility limit of the human eye . TARGET GREY FILTER LAMP RED FILTER LENS .C EYE PIECE ET _ A INDICATOR ADJUSTING RESISTOR Figure 9 .1 Principle of the disappearing filament 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) 164 MANUALLY OPERATED PYROMETERS 9 .1.2 Red filter A red filter is used for the following reasons and advantages . The spectral transmissivity, ZX, of a red filter and the spectral relative sensitivity, Vj,, of a standard human eye are displayed in Figure 9 .2 as a function of wavelength, X . . It may be clearly seen that there is a well defined small wavelength band, in which the brightness of the filament and that of the target can be compared . This wavelength band is narrow enough to assume that the comparison occurs at one wavelength, called the effective wavelength, X,, of a disappearing filament pyrometer . All of the pyrometer readings should be referred to this value of A, . Thus, as the comparison of the brightness of the lamp filament and that of the target takes place only at one colour, the subjective estimation of colour by different observers cannot influence the measurement results . As shown in Figure 9 .2 the effective wavelength ;L e , which can be found in a graphical way, has a value of 0 .65 ltm given by the abscissa of the centre of gravity of the common cross-hatched area under the curves zX(A) and VX(A) . Because of the small width of the utilised wavelength band, the effective wavelength, Xe, is nearly constant at all measured temperatures . In practice when applying filters such as the Scholl RG2 and Jena 4512, A e does not change by more than 0 .003 ltm in the measured temperature range of 1300 to 3600 K . Forsythe (1941) points out that the value of Xe, which is also a function of the filter temperature, may be regarded as nearly constant . More detailed theoretical background and methods of experimental and calculative determination of /1 e , may be found in Henning (1951) and Righini et al . (1972) . As there is a high percentage of red colour in the emitted thermal radiation, in the temperature range below 800 °C, the application of a red filter is not advised . The resulting increase in observed radiation intensity consequently also increases the overall precision of the measurements . The total emitted radiant intensity, W ., following the Stefan-Boltzmann law given in equation (8 .16), and the spectral radiant intensity, W o, ,t = 0 _ 65 , following Planck's law of equation (8 .7), which are displayed against the absolute temperature of a black body in Figure 9 .3, are both relative values of radiant intensity referred to those at 1000 K . From -EFFECTIVE WAVELENGTH 1,Q > V t - r E 0,6 r/i Ln z 0 " 4 _ . 02 as ~n Vi 0,4 45 0 r 6 1 . 0,7 WAVELENGTH a , ym Figure 9 .2 Spectral transmissivity, r A , of red filter and spectral relative sensitivity, V, of standard human eye versus wavelength, DISAPPEARING FILAMENT PYROMETERS 165 this figure it can be seen that the steepness of the curve W,, ,z -0 .65 = PT) is far greater than that of the curve W o =f (T) . Weichert (1976) shows that the spectral radiance, L o, ,, of a black body is directly proportional to the spectral radiant intensity so that : L o, .a = CW" , .z (9 .1) where C is a constant . The same dependence is also valid for the total radiance L o . From the above considerations and also from Figure 9 .3, it follows that the spectral radiance difference corresponding to a unit temperature difference at A = 0 .65 pm is far greater than for the total radiance . In summary, the reasons for the application of a red filter are : " Comparison of the filament and target brightness takes place only at one wavelength or colour so eliminating the influence of any subjective colour estimation by different observers . " The effective wavelength /1 e = 0 .65 pm, which is still within the visible spectrum range, adjoins the infrared radiation band thus permitting the lowest possible temperatures to be measured . " At Xe = 0 .65 pm the pyrometer sensitivity is higher than for the total radiation . " It is relatively easy to produce good filters of A e = 0 .65 pm which are stable in time . " At the assumed wavelength, about A e = 0 .65 pm, the smallest colour changes as a function of wavelength are observed . 10 5 8 0 3 10 / 3 f- 10 3 / 10 4 Z 4 0 1 :0 .65 W C d' 1 0 / g 1 a 1000 1400 1800 TEMPERATURE T , K Figure 9 .3 Spectral radiant intensity Woa-o .65 and total radiant intensity, W o versus black body temperature . Both values relative to radiant intensities at 1000 K 166 MANUALLY OPERATED PYROMETERS 9 .1 .3 Scale defining equation for black bodies Disappearing filament pyrometers are calibrated for black bodies, whose spectral radiant intensity at the temperature, T t , follow Wien's law of equation (8 .9) in the manner : W .'), = CI X-5 e -c 2 /AT, (9 .2) Conforming to equation (9 .1) the spectral radiance is : LO'X = Cc, X -5 e -c z /"T, (9 .3) where C is a conversion factor . If a black body is observed by a human eye, through a red filter of spectral transmissivity, a x , the physiological feeling of brightness will be given by : Lo'~ = Cc1VATAX -5 e -C2 /ATt (9 .4) where T t is the true temperature and Vx is the relative spectral sensitivity of a standard human eye . Assuming that the spectral emissivity of the filament is Elq , then, observing the filament through the same red filter, results in the feeling of brightness given by : Lox = Cc 1 E f V,a,X-5e-`z l 1Tf (9 .5) where T f is the filament temperature . In equations (9 .4) and (9 .5) the negligibly small lens and eyepiece attenuation of incident radiation does not need to be considered . At the moment of reading the measured temperature value, the brightness of the filament and of the target are equal . Combining equations (9 .4) and (9 .5), yields : L O ' ' A = L0~ I (9 .6) For A = fe , it follows that : e-c2 /A,eTt =eaee-c2 /i1eTf (9 .7) or : _ c2 _ c2 + 1nEfA e (9 .8) -~eTt -eTf DISAPPEARING FILAMENT PYROMETERS 167 and finally : 1_ 1 + .'~eInE£,a' (9 .9) T £ T t c2 For any givenpyrometric lamp, the filament temperature, T f , is a function of the lamp current, I, so that : Tf = f (I) (9 .10) or more conveniently : If =f2(Tf) (9 .11) From equations (9 .9) and (9 .11) it follows that : I = h(TO (9 .12) This allows direct calibration of the ammeter of the pyrometer in temperature units . In BS 1041 the temperature found in that way is called the radiance or luminance temperature of the target at the wavelength, A, . The scale divisions of the temperature scale of a disappearing filament pyrometer, which is not linear, increase at higher temperatures . 9 .1 .4 Temperature measurement of non-black bodies When measuring the temperature of non-black bodies, the pyrometer readings are too low . The radiance temperature of a target at a given wavelength is the temperature of a black body which exhibits the same spectral radiance, as the considered target . For a non-black body of spectral emissivity, E, , ~ , at the temperature, T t , and at the wavelength, X, , the spectral radiance is given by : L k = CEZec,Ae5e-cz I1,,T, (9 .13) The pyrometer readings are then Ti, for which the radiancetemperature of a black body is : L,a = Cc, A ;5e-cz l .l,T ; (9 .14) Equating (9 .13) and (9 .14) yields : _1 _ 1 + Ae 1n k (9 .15) T t Ti C 2 168 MANUALLY OPERATED PYROMETERS or T t (9 .16) (1 / T ;) + (A e / c2 )lnE4 The true temperature, T,of a non-black body of emissivity, EA . , can easily be calculated from equation (9 .16), when the indicated temperature, T ;, is known . Substituting c2 = 1 .4388x10 -2 m-K from equation (8 .7) and A e = 0 .65 pm, then (9 .16) becomes : 1 Tt _ (9 .17) (1 / T l ) + (loge, / 9613) Numerical example Measuring the temperature of a body of EA, = 0 .7, a disappearing filament pyrometer indicated T ; = 1300 K (1027 °C) . Calculate the true temperature of the body . Solution : Inserting values into equation (9 .17) gives : _ 1 Tt (1 / 1300) + (Iog0 .7 / 9613) _ 1327 K When measuring the temperature of non-black bodies the corrections to the readings can be read directly from the diagram of Figure 9 .4 . The necessary values of E,f for different metals at A = 0 .65 ltm, are given in Table XIX . As the values of E,~ are known, but with an uncertainty of ±10 % to ±20 %, the resulting errors of the corrections AO can be estimated from Figure 9 .4 . Moreover, as the steepness of the curve W o,A=0 .65 =f (T) is far greater than that W o = f (T) in Figure 9 .3, errors in the temperature measurement of black bodies using disappearing filament pyrometers are smaller than those for total radiation pyrometers . In equation (8 .25) it has been proved that the radiance of black bodies does not depend on the viewing angle, (p . This is also true for non-black bodies, where only insignificant radiance changes are observed for tp > z / 4 . Taking into account the actual values of E,k , it follows, that the disappearing filament pyrometers can be directed at any viewing angle . A method of measuring the temperature of metallic surfaces, using a polarising filter also exists . At higher temperatures, metallic surfaces emit radiation which is polarised parallel to the surface at an angle 7r/5 to their normal, where the radiating surface approaches a black body . The pyrometer is then calibrated together with a polarising filter which is introduced in front of the lens . In that way the measured radiance temperature nearly equals the true value of the surface temperature (Pepperhoff, 1960 ; Tingwaldt, 1960 ; Murray, 1972 ; Walter, 1981) . DISAPPEARING FILAMENT PYROMETERS 169 1, ~ """ 1" 300 I _ 800 1000 1200 1400 1600 IBM INDICATED TEMPERATURE OC Figure 9 .4 Correction, A9, to readings of disappearing filament pyrometers of non-black bodies versus indicated temperature values, 9i . e stands for , ' ; A- O65- 9 .1 .5 Extension of measurement range The tungsten filament of a pyrometer lamp can only be used up to 1400'C . At higher temperature tungsten sublimes, the filament resistance increases and a dark deposit is formed on the glass surface, gradually changing the lamp characteristic . To extend the pyrometer measurement range up to 2000 'C a grey filter which is placed between the -d . . uces the target radiance without influencing that of the possible . determine the dependence between the black-body temperature, T t , equal to the indicated value, Ti, without the grey filter, and the true temperature, T t , of another black body - " filter . In b . . .g same . From - . - ctral radiance is : . . 17 0 MANUALLY OPERATED PYROMETERS where r t is the spectral transmissivity of the grey filter . e For an equal feeling of brightness in both cases, corresponding to Lo ; = Lo' it follows that : + " e In 1 (9 .20) T i T t c2 Denoting : Le In 1 = A (9 .21) c2 z4 from equations (9 .20) and (9 .21) it follows that : 1 1 _ = A (9 .22) Ti Tt ' and thus : T = 1 (9 .23) A+(1/T) The coefficient, A, describes the radiance reducing factor of the grey filter . Following equation (9 .23) it is possible to calibrate the disappearing filament pyrometer above the maximum filament temperature . Griffith (1947) describes a method, used in the past, for extension of the measurement range, which employs rotating disk with apertures . 9 .1 .6 Applications and construction Disappearing filament pyrometers, which are used for the spot measurement of steady or slowly changing temperatures, are also especially suitable for small size targets . Operating at a wavelength about ;Le= 0 .65 pm, they are useful in measurement of the temperatures of non-black bodies as the emissivity, ER=0 .65, of many materials is known . Typical applications are : " Comparison measurements in calibration of total radiation pyrometers . " Temperature measurement of small size targets (even about 0 .1 mm) . " Temperature measurement in research laboratories . " Comparison measurements of temperature of non-black bodies . DISAPPEARING FILAMENT PYROMETERS 171 " Measurement of temperature uniformity inside furnace chambers, to judge if the application of total radiation and photoelectric pyrometers is possible since these two types indicate the average value . As an example, the Mikro-Pyrometer, PV 11 by Keller GmbH (1998), shown in Figure 9 .5, will be described . The technical data of this pyrometer are as follows : " temperature range : 700 to 3500 oC divided into 6 subranges, " accuracy : - 1 .5 % of reading in the range 700 to 800 °C, - 0 .6 % of reading in the range 800 to 2000 °C, - 2 .0 % of reading in the range 2000 to 3500 °C, " precision of current adjusting as an average depending on the operator : - 1 .5 °C at 1000 °C, - 5 .0 °C at 2000 °C, - 10 .0 °C at 3000 0 C, " target size : - 0 .3 mm at 1 m distance, - 0 .1 mm at 0 .2 m distance when used with a supplementary lens, " focusable optic, " emissivity to be set from 0 .1 to 1 in 0 .001 steps, " effective wavelength : - 0 .5 to 0 .67 N .m in the lower temperature range, - 0 .6 to 0 .67 pm in the medium temperature range, - 0 .65 to 0 .67 gm in the higher temperature range, " extreme distance ratio : 1 mm at l = 5 m, " dimensions : l00x100x450 mm, " display : 4 digit, LCD, " output : RS 232 . In recent times, the application of disappearing filament pyrometers in industry has become less frequent . They are being replaced by other pyrometers, still operating at ;Le 0 .65 pm . so Figure 9 .5 The Mikro Pyrometer PV I 1 disappearing filament pyrometer (Courtesy of Keller GmbH) 172 MANUALLY OPERATED PYROMETERS 9 .2 Two-Colour Pyrometers 9 .2 .1 General information A two-colour or ratio pyrometer, which measures temperature from the ratio of spectral radiances emitted by the object at two different wavelengths, is calibrated for grey bodies and gives correct readings for grey and black bodies . If the utilised wavelengths are placed within the visible range of the radiation spectrum, the name of two-colour pyrometer is precisely correct . Nevertheless the samename is used sometimes for automatic pyrometers working outside the visible spectrum range . The working principle of two-colour pyrometers, in which the ratio of spectral radiances at two wavelengths is estimated by the human eye, is thoroughly discussed in the papers of Forsythe (1923), Haase (1933), Naeser (1935/36) and Schmidt (1924/25) . A simplified diagram of a two-colour pyrometer given in Figure 9 .6 shows that it is composed of a lens, eyepiece and a two-colour filter, in most cases red/green . The observer adjusts the filter position so that the target to be measured appears to be grey . This position corresponds to equal spectral radiances or spectral brightnesses, as they are felt by a human eye, in two supplementing colours . With increasing target temperature, the percentage of green colour radiation increases while the red one decreases, so that each temperature corresponds to a definite filter position . The measured temperature, called the colour temperature, T c , can be read from the pointer position on a scale . The target to be measured is observed through a two-colour graded sliding filter . At a position, where the target turns grey, the measured temperature is read directly on a scale . The error limit of the colour slider is about ±20 to ±30 °C over its measurement range of 1200 to 2000 °C . Modern two-colour pyrometers, which are mostly automatic with the human eye replaced by photoelectric detectors, are considered in Section 10 .5 . 9 .2 .2 Scale defining equation Following Wien'slaw given in equation (8 .9), the spectral radiant intensity, W~l emitted at the wavelength, /11, and at the temperature T t by a body with emissivity e k , is given by : s -Cz /A, r TARGET G RED-GREEN FILTER i R LENS oC EYE PIECE Figure 9 .6 Principle of two-colour pyrometer