TEMPERATURE CALIBRATION OF AN INFRARED THERMOGRAPHIC CAMERA AND ITS UNCERTAINTY OF MEASUREMENT

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1 Proceedings of COBEM 007 Copyright 007 y ABCM 19th International Congress of Mechanical Engineering Novemer 5-9, 007, Brasília, DF TEMPERATURE CALIBRATION OF AN INFRARED THERMOGRAPHIC CAMERA AND ITS UNCERTAINTY OF MEASUREMENT Orlando, A.F., afo@mec.pc-rio.r PUC-Rio, R. Marqês S.Vicente 5, Rio de Janeiro, RJ, , Brazil Teixeira, Renato Nnes, rnteixeira@inmetro.gov.r INMETRO, Av. N.S. das Graças 50, Xerém, Dqe de Caxias, RJ, Brazil Hamani, Marlon M., marlonmax@mec.pc-rio.r PUC-Rio, R. Marqês S.Vicente 5, Rio de Janeiro, RJ, , Brazil Santos, Palo R.F., prsantos@inmetro.gov.r INMETRO, Av. N.S. das Graças 50, Xerém, Dqe de Caxias, RJ, Brazil Costa, Hmerto Silvino A., hmerto.costa@light.com.r Light, R. Marechal Floriano 168, Rio de Janeiro, RJ, Brazil Astract. A methodology was developed to calirate the temperatre otpt from a thermographic camera, sed for maintenance planning prposes. Initially, a device was designed to calirate the thermographic camera in PUC-Rio, starting from the knowledge of the emissivity of a lack srface, as part of the pper ase of a rass cylindrical lock, and located 3 mm aove the liqid srface of a temperatre controlled caliration ath. Its temperatre was measred y three calirated thermocoples, inserted in radial holes ellow the lack srface. Ths, the indicated temperatres of the thermographic camera was compared to the measred temperatre. Then, the camera was calirated against a lackody in INMETRO, with its ncertainty estimated in the 0,54 C to 0,6 C range. Joining the two pieces of information, the emissivity of the lack srface cold e determined from amient p to 150. Its ncertainty was estimated in ± 0,0. When the thermographic camera measres the temperatre of a target, the ncertainty of measrement was estimated in the 0,9 C to,7 C range, inclding the ncertainty of emissivity measrement and of srface temperatre niformity, matching the manfactrer s specification. Keywords: Thermographic camera, caliration, lack ody, maintenance, emissivity 1. INTRODUCTION Temperatre monitoring of electricity prodction eqipments in hydroelectric power plants is a tool to e sed for maintenance planning prposes. When the temperatre gets higher than the manfactrer s specification vale, or even the designer s one, it is assmed that a shift in its performance has occrred, and, therefore, a maintenance procedre mst e sed to recover its original operating characteristics. The sooner the prolem is oserved, the smallest will e the conseqences of the interrption of the electric energy spply, and the smallest the maintenance costs will e. Therefore, a continos temperatre monitoring is an important follow p tool. Traditionally, thermocoples and platinm resistance thermometers have een sed for the jo. However, they reqire many times that the eqipment e de-energized, so that the sensors can e installed. Lately, the remote temperatre measrement technology has een developed to a point to ecome an important tool for operation diagnosis. The thermographic camera analyzes the infrared radiation emitted from the target, and interprets its signal, relating it to temperatre, according to the laws of heat transfer y radiation. The existing eqipment measres temperatre with a reasonale accracy if the srface emissivity of the target is known, and if the temperatre is niform. Therefore, the accracy of temperatre measrement y the thermographic camera depends on its caliration and on the signal interpretation at different operating conditions. In this work, an infrastrctre was developed to calirate a thermographic camera, and to adeqately interpret the infrared signal at different operating conditions. A methodology was developed to calirate it at ideal conditions (known emissivity and niform temperatre) and, then, to measre the target emissivity. Tipically, electricity prodction eqipments operate in the 0 C to 150 C temperatre range. The measrement tolerance is sally admitted as %. The thermographic camera that was sed in this experiment has an accracy of ± C or %, whichever is greater, according to the manfactrer. Therefore, in this work, it was decided to calirate the camera in the 0 C to 150 C temperatre range. The methodology consists in calirating the camera against a lackody in INMETRO (Institto Nacional de Metrologia, Normalização e Qalidade Indstrial, Brazil)), which is traceale to NIST (National Institte for Standards and Technology, USA). Then, the camera is calirated in PUC-Rio against a lack srface of a cylindrical rass lock placed in a temperatre controlled caliration ath. Using the radiation heat transfer relationships, the emissivity of the

2 srface can e determined. The contrition of this work is to develop a methodology to measre the emissivity of a srface as a fnction of temperatre with the thermographic camera, and then to measre the temperatre of a target, having the same srface emissivity, at different operating conditions.. CALIBRATION OF THE THERMOGRAPHIC CAMERA AGAINST A BLACK BODY AT INMETRO The caliration eqipment consists of a temperatre controlled lackody, with known radiative properties. The camera views the lackody srface at a measred temperatre, and compares with the indicated vale, ths performing a caliration y the comparison method. A MIKRON M315X8 lackody, traceale to NIST, was sed to calirate the thermographic camera at INMETRO. The spectral emissivity of the lackody, as a fnction of the wavelength, was measred y NIST and shown in Fig.1. Mikron Infralack 3 - Spectral Emissivity Test performed y NIST: March ,95 0,9 0,85 Emissivity 0,8 0,75 e Mean 0,7 0,65 0, Wavelength (m) Figre 1 : Spectral emissivity of the MIKRON M315X8 lackody, as measred y NIST In the µm < λ < 18µm wavelength range the spectral emissivity (ε λ ) varies in the 0,94 ± 0,0 range with a confidence level of 95,45%, reslting in a total emissivity along the whole spectrm of ε = 0,96. The indicated temperatre (T c ) is otained y setting an emissivity vale of 0,96 in the camera, which can e related to the lackody temperatre (T ) y the least sqare fit method (Orlando et allii,1985), (Orlando et alli, 003), (Holman, 1971) and (Benedict, 1977), reslting in a root mean sqare deviation ( fit ) : T = A + B.T c (1) 1 fit =. () N N ( ) A + B. Tc, i T, i i= 1 The camera also displays the root mean sqare of the measred temperatre ( d ), which can e interpreted as the temperatre niformity of the target srface measred y the camera. The staility of the lackody temperatre is very good. It is estimated that its expanded ncertainty (95,45 %) of measrement is U = ± 0,01 C, or = 0,005 C for its standard ncertainty. In order to check the repeatiility of temperatre measrement y the camera, each lackody temperatre (1) was measred ten (10) times. The fitting coefficients A and B, together with the root mean sqare deviation ( fit ), were calclated sing all measred vales (10). Then, the standard ncertainty of temperatre caliration along the operating range of the camera ( c ), inclding the crve fit, if only one measrement is allowed, following (ISOGUM, 1995), can e estimated as : = + + (3) c fit ( B. d ) U c =. c (4)

3 Proceedings of COBEM 007 Copyright 007 y ABCM 19th International Congress of Mechanical Engineering Novemer 5-9, 007, Brasília, DF 3. EMISSIVITY MEASUREMENT AT PUC-Rio When the thermographic camera views a non lack srface with emissivity ε at a measred temperatre (T ), an indicated camera temperatre (T c ) is read, adjsting the emissivity for ε=1. Then, an ive temperatre (T ) is calclated from the caliration crve, following Eq. (1) : T = A + B. T c (5) The srface emissivity can ths e calclated from Eq. (6) : 4 T ε = (6) T A device was designed to measre the emissivity at PUC-Rio. Alternatively, it can e sed to calirate the thermographic camera, provide that the srface emissivity is known. It consists of a cylindrical rass lock with 75 mm diameter and 110 mm height, inside a temperatre controlled caliration ath, having its pper srface painted lack and placed aot 3 mm aove the liqid srface of the ath. The ath temperatre was measred six (6) times for each set temperatre with a Rosemont 195 standard platinm resistance thermometer (SPRT) to test repeatiility. An HP3440A mltimeter was sed to measre the thermometer resistance with an eqivalent ncertainty of ± 0,05 C. The lack srface temperatre was measred y three type T thermocoples, gage 4, sing an HP34401A mltimeter to measre its emf, with a maximm eqivalent ncertainty of ± 0,3 C. They were placed inside three radially drilled holes, immediately elow the lack srface. The device is shown in Fig.. Figre : Device for calirating the thermographic camera at PUC-Rio The thermocople caliration was made y comparison with the otpt from the standard platinm resistance thermometer (SPRT) in the Pressre and Temperatre Laoratory of PUC-Rio, which is accredited y the National Caliration Network (RBC) of Brazil, in a temperatre controlled ath. Six (6) measrements were made, each one at every a 5 minte time interval, to check the repeatiility of reslts. The reference jnction of each thermocople was kept in an ice ath. The otpt from the thermocoples, in mv, was converted to C (I ITS ), sing the eqations from ITS-90 (International Temperatre Scale), and compared to the otpt from the SPRT (T ). A third degree polynomial, chosen y the least sqare fit method, minimized the root mean sqare deviation of the fit, among other polynomials. The ncertainty of measrement was estimated taking into accont the standard thermometer ncertainty, the ncertainty of the fit (root mean sqare deviation) and the niformity of the caliration ath. Tale 1 shows the fit coefficients (C 1, C, C 3, C 4, C 5 ), together with the ncertainty of temperatre measrement (U) at 95,45 % confidence level.

4 T = C + (7) C. TITS + C3. TITS + C4. TITS C5. TITS Tale 1 : Crve fitting coefficients and ncertainty of measrement for type T thermocoples Thermocoples 1 3 C 1 0,608 0,989-0,69686 C 0, , ,0550 C 3,6977E-5,181E-4-1,4954E-3 C 4 0-7,6167E-7 1,445E-5 C ,506E-8 U ( ) 0,5 0,15 0,30 The average temperatre of the lack srface (T ) was estimated as an arithmetic mean of the three thermocople readings (T 1, T, T 3 ), as indicated y Eq. (6) Uncertainty of average srface temperatre (U ) was calclated y Eq. (8), sing the indicated vale of the expanded ncertainty of each thermocople in Tale 1, respectively, U 1 = ± 0,5, U = ± 0,15 and, U 3 = ± 0,30. T1 + T + T3 T = (8) 3 U1 + U + U 3 U = = 0,14 C (9) 3 U = = 0,07 C (10) However, de to the non niformity of the srface temperatre, a type B expanded ncertainty U can e chosen to represent the maximm difference etween any of the three measrement thermocoples (T 1, T, T 3 ) and the average vale, if it is larger than the vale in Eq. (9). The standard ncertainty ( ) can e calclate as = U / 3. The lack srface indicated temperatre measrement y the thermographic camera (T c ) is made y setting an emissivity vale of ε=1. The camera also displays the root mean sqare of the measred temperatre ( d ), which can e interpreted as the temperatre niformity of the target srface measred y the camera. The ive temperatre (T ) can e calclated sing the caliration crve, Eq. (5) T = A + B. T c (5) The vale to e sed for the standard ncertainty of the ive temperatre ( ) is the same as c, otained dring the caliration against a lackody at INMETRO, Eq. (3). For interpolation prposes, Eq. (11) can e sed. + + = (11) fit ( B. d ) where fit is taken from Eq. (), and = 0,005 C, for INMETRO s lackody temperatre standard ncertainty. The camera displays the root mean sqare of the measred temperatre ( d ). The srface emissivity (ε) can e calclated from Eq. (6), sing temperatre vales of Eq. (5) and (8). Uncertainty of emissivity measrement ( ε ) can e calclated from Eq. (1) ε = ε (1) T T The expanded ncertainty (U ε ) of emissivity measrement (95,45%) can e calclated from Eq. (13). U ε =. ε (13)

5 Proceedings of COBEM 007 Copyright 007 y ABCM 19th International Congress of Mechanical Engineering Novemer 5-9, 007, Brasília, DF 4. TEMPERATURE MEASUREMENT WITH THE THERMOGRAPHIC CAMERA The measrement of temperatre y the thermographic camera can e made y pointing it to the target, after having set an emissivity vale of ε=1, reslting in an indicated temperatre (T c ). Then, the ive temperatre (T ) can e calclated from the caliration crve, Eq. (5). The target temperatre (T ) can, therefore, e calclated from Eq. (14), sing the same srface emissivity (ε) vale measred in a similar srface at PUC-Rio. T T = * (14) 4 ε The ncertainty of temperatre measrement with the thermographic camera can e estimated from the knowledge of the srface emissivity and its ncertainty, determined as efore. In many practical applications the target temperatre can only e measred once. If a smaller ncertainty is desired, several measrements have to e made. The vale to e sed for the standard ncertainty of the ive temperatre ( ) is the same as c, otained dring the caliration against a lackody at INMETRO, Eq. (3). For interpolation prposes, Eq. (11) can e sed. = + B + (11) fit (. d ) where fit is taken from Eq. (), and = 0,005 C, for INMETRO s lackody temperatre standard ncertainty. The camera displays the root mean sqare of the measred temperatre ( d ). The srface emissivity and its ncertainty were determined y Eq. (6) and (1). The ncertainty of the target temperatre ( ) can e determined y Eq. (15) = T. ε 4. ε + T (15) U = (16) 5. RESULTS 5.1 Caliration of the thermographic camera against a lackody at INMETRO Tale : Caliration of the thermographic camera against a lackody at INMETRO, for one measrement TEMPERATURE ( C) UNCERTAINTY ( C) Camera Blackody Blackody Fit Camera Expanded T c T fit d U 1 35,1 34,90 0,005 0,156 0,5 0,58 45,4 45,10 0,005 0,156 0,6 0, ,43 55,10 0,005 0,156 0,4 0, ,65 65,10 0,005 0,156 0,5 0, ,79 75,10 0,005 0,156 0,3 0, ,87 85,10 0,005 0,156 0,3 0, ,89 95,00 0,005 0,156 0, 0, ,98 105,07 0,005 0,156 0, 0, ,19 115,10 0,005 0,156 0,5 0, ,30 15,10 0,005 0,156 0,6 0, ,47 135,10 0,005 0,156 0,4 0, ,50 145,10 0,005 0,156 0,7 0,6

6 Ten measrements for each of the 1 lackody temperatres in the 5 C to 150 C range were made to test the repeataility of the camera. Their averages are shown in Tale. The lackody has a temperatre ncertainty (95,45%) of ± 0,01 C. The fitting coefficients are in shown in Eq. (17), (18), (19). A = 0,1639 C (17) B = 0, (18) fit = 0,156 C (19) It can e seen that the ncertainty of temperatre measrement (95,45%) is in the 0,54 C to 0,6 C range. Also, the most important contrition to it is the temperatre niformity of the target srface measred y the camera. 5. Emissivity determination at PUC-Rio Tale 3 : Black srface temperatre measred y thermocoples Black Srface Temperatre ( ) T 1 T T 3 T U max U average U 5,01 4,98 4,94 4,98 0,04 0,14 0,14 0,07 34,00 33,94 34,00 33,98 0,04 0,14 0,14 0,07 44,49 44,40 44,56 44,48 0,08 0,14 0,14 0,07 54,11 53,96 54,16 54,08 0,1 0,14 0,14 0,07 63,70 63,69 64,0 63,81 0,1 0,14 0,1 0,1 73,69 73,49 73,93 73,70 0,3 0,14 0,3 0,13 83,06 83,0 83,85 83,37 0,48 0,14 0,48 0,8 9,9 9,67 93,67 93,08 0,58 0,14 0,58 0,34 8,77 8,67 8,88 8,77 0,10 0,14 0,14 0,07 9,18 9,13 9,56 9,9 0,7 0,14 0,7 0,15 101,05 100,97 101,84 101,9 0,55 0,14 0,55 0,3 110,1 110,17 111,35 110,55 0,80 0,14 0,80 0,46 10,40 10,31 11,66 10,79 0,87 0,14 0,87 0,50 10,5 10,50 11,87 10,96 0,91 0,14 0,91 0,5 130,04 19,86 131,35 130,4 0,93 0,14 0,93 0,54 139,91 139,70 140,81 140,14 0,67 0,14 0,67 0,39 Tale 4 : Emissivity determination as a fnction of temperatre Black Srface Thermographic Camera Emissivity (Tale ) Indicated Effective (ε=1) T T c d T ε ε U ε 4,98 0,07 4,7 0,40 4,66 0,43 0,996 0,006 0,01 33,98 0,07 33,51 0,35 33,35 0,38 0,99 0,005 0,010 44,48 0,07 43,79 0,31 43,5 0,34 0,988 0,004 0,009 54,08 0,07 5,58 0,33 5,1 0,36 0,977 0,004 0,009 63,81 0,1 6,15 0,34 61,67 0,37 0,975 0,005 0,009 73,70 0,13 71,6 0,37 71,03 0,40 0,970 0,005 0,009 83,37 0,8 80,84 0,9 80,15 0,33 0,964 0,005 0,009 93,08 0,34 89,6 0,3 88,84 0,35 0,954 0,005 0, ,9 0,07 98,97 0,59 98,07 0,60 0,966 0,006 0, ,55 0,15 108,09 0,71 107,09 0,7 0,964 0,007 0,015 10,79 0,3 117,88 0,80 116,77 0,81 0,960 0,009 0, ,4 0,46 17,3 0,90 16,0 0,90 0,957 0,010 0, ,14 0,50 137,46 0,85 136,14 0,85 0,96 0,009 0,019

7 Proceedings of COBEM 007 Copyright 007 y ABCM 19th International Congress of Mechanical Engineering Novemer 5-9, 007, Brasília, DF It can e seen that the emissivity has an approximately constant vale of ε = 0,96 for temperatre vales higher than 70 C, with an ncertainty of ± 0, Temperatre measrement with the thermographic camera A simlation was carried on for three sets of temperatre measrement, to check repeatiility. Tales 5, 6 and 7 shows the reslts. It can e seen from Tales 5, 6 and 7, that the ncertainty of temperatre measrement (95,45%) y the thermographic camera is in the 0,9 C to,7 C range, matching the manfactrer s declaration, that is ± C, or %, whichever is greater. The ncertainty of temperatre measrement with the thermographic camera is estimated in Tales 5, 6 and 7 for sitations where only one measrement is allowed. If smaller ncertainty vales are reqired, several measrement (n) mst e made. Then, the average of the measred vales represents the tre temperatre. Its ncertainty can e calclated y dividing the expanded ncertainty, estimated for only one measrement, y n. Finally, the non niformity of target srface temperatre has the greatest contrition to its measrement. Tale 5 : Uncertainty of temperatre measrement with the thermographic camera. Data Set 1. T c d T ε ε T U Camera Camera Eq. (3) Eq. (11) Tale 3 Tale 3 Eq. (16) Eq.(17) Eq. (18) 4,75 0,37 4,69 0,40 0,996 0,006 5,00 0,59 1, 33,49 0,35 33,33 0,38 0,99 0,005 33,96 0,54 1,1 43,64 0,31 43,37 0,34 0,988 0,004 44,33 0,49 1,0 5,34 0,33 51,97 0,36 0,977 0,004 53,83 0,5 1,0 6,0 0,3 61,54 0,8 0,975 0,005 63,68 0,48 1,0 71,50 0,8 70,9 0,3 0,970 0,005 73,59 0,53 1,1 81,04 0,8 80,35 0,3 0,964 0,005 83,57 0,54 1,1 90,03 0,31 89,4 0,34 0,954 0,005 93,49 0,60 1, 99,14 0,54 98,5 0,56 0,966 0, ,46 0,83 1,7 108,14 0,71 107,14 0,7 0,964 0, ,60 1,04,1 117,86 0,78 116,76 0,79 0,960 0,009 10,77 1,18,4 17,31 0,90 16,10 0,90 0,957 0, ,50 1,37,7 137,63 0,76 136,30 0,77 0,96 0, ,31 1,7,5 Tale 6 : Uncertainty of temperatre measrement with the thermographic camera. Data Set. T c d T ε ε T U Camera Camera Eq. (3) Eq. (11) Tale 3 Tale 3 Eq. (16) Eq.(17) Eq. (18) 4,88 0,34 4,8 0,37 0,996 0,006 5,13 0,57 1,1 33,49 0,35 33,33 0,38 0,99 0,005 33,96 0,54 1,1 43,86 0,6 43,58 0,30 0,988 0,004 44,55 0,46 0,9 5,93 0,31 5,55 0,34 0,977 0,004 54,4 0,51 1,0 6, 0,31 61,74 0,34 0,975 0,005 63,88 0,5 1,0 71,65 0,37 71,06 0,40 0,970 0,005 73,74 0,58 1, 80,78 0,9 80,09 0,33 0,964 0,005 83,31 0,54 1,1 90,3 0,8 89,44 0,3 0,954 0,005 93,69 0,59 1, 98,89 0,59 98,00 0,60 0,966 0, ,1 0,86 1,7 108,06 0,61 107,07 0,6 0,964 0, ,5 0,97 1,9 117,89 0,80 116,79 0,81 0,960 0,009 10,80 1,0,4 17,07 0,77 15,86 0,78 0,957 0, ,6 1,9,6 137,65 0,85 136,3 0,85 0,96 0, ,33 1,3,6

8 Tale 7 : Uncertainty of temperatre measrement with the thermographic camera. Data Set 3. T c d T ε ε T U Camera Camera Eq. (3) Eq. (11) Tale 3 Tale 3 Eq. (16) Eq.(17) Eq. (18) 4,54 0,38 4,48 0,41 0,996 0,006 4,79 0,59 1, 33,55 0,9 33,39 0,33 0,99 0,005 34,0 0,51 1,0 43,88 0,5 43,60 0,9 0,988 0,004 44,57 0,46 0,9 5,48 0,9 5,11 0,33 0,977 0,004 53,97 0,49 1,0 6,0 0,34 61,7 0,37 0,975 0,005 63,86 0,54 1,1 71,70 0,5 71,11 0,9 0,970 0,005 73,79 0,51 1,0 80,71 0,9 80,0 0,33 0,964 0,005 83,4 0,54 1,1 88,61 0,3 87,83 0,35 0,954 0,005 9,07 0,61 1, 98,87 0,58 97,98 0,59 0,966 0, ,19 0,86 1,7 108,06 0,61 107,07 0,6 0,964 0, ,5 0,97 1,9 117,88 0,67 116,78 0,68 0,960 0,009 10,79 1,11, 17,30 0,86 16,09 0,86 0,957 0, ,49 1,35,7 137,11 0,85 135,79 0,85 0,96 0, ,79 1,3,6 6. CONCLUSIONS An experimental methodology was developed to calirate thermographic cameras for remote temperatre measrement. The calclated ncertainties match the manfactrers specification, that is, ± C, or %, whichever is greater, for sitations where only one measrement is allowed. Frthermore, the methodology allows the determination of the srface emissivity to within ± 0,0. The methodology is simple and allows a temperatre caliration laoratory to follow it with jst a small investment. Finally, if a smaller ncertainty is reqired, several measrements shold e made. 7. AKNOWLEDGEMENTS The athors are indeted to Light, Serviços de Eletricidade S.A., for having financed this research. 8. REFERENCES Benedict, R.P, 1977, Fndamentals of Temperatre, Pressre, and Flow Measrements, New York : Wiley Interscience. Holman, J.P., 1971, Experimental Methods for Engineers, New York : McGraw-Hill ISOGUM, 1995, Gide to the Expression of Uncertainty in Measrement, International Organization for Standardization, Switzerland. Orlando,A.F. et alli, 1985, Determinação Experimental da Emissividade de Sperfícies, in VIII Congresso Brasileiro de Engenharia Mecânica, VIII COBEM, São José dos Campos Orlando,A.F. et alli, 003, Caliração de m TermômetroPt-100 como Padrão de Traalho de m Laoratório de Serviços Metrológicos. In Metrologia 003. Recife