- Email: [email protected]
A calibration plate to determine the performance of infrared thermometers in field use
Agricultural Meteorology, 26 (1982) 279--283
279
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A CALIBRATION PLATE TO DETERMINE THE PERFORMANCE OF I N F R A R E D THERMOMETERS IN FIELD USE C.J. STIGTER, N.T. JIWAJI and M.M. MAKONDA
Section Agricultural Physics, Physics Department, University of Dar es Salaam, Dares Salaam (Tanzania) (Received December 15, 1981; accepted for publication February 10, 1982)
ABSTRACT Stigter, C.J., Jiwaji, N.T. and Makonda, M.M., 1982. A calibration plate to determine the performance of infrared thermometers in field use. Agric. Meteorol., 26: 279--283. As the testing of infrared thermometers under actual field conditions has been reported to be indispensable, a calibration plate has been constructed for such purposes. A simple way of measuring surface temperatures by contact is shown to yield accurate results for comparison. When the emission coefficient of the calibration plate is known, its temperature can be used to investigate the performance of infrared thermometers in field use.
INTRODUCTION
In applied research in micrometeorology as well as on surfaces involved in harnessing of solar energy and (night) sky cooling, infrared field thermometers have a high practical value in the remote measurement of surface temperatures. However, from reports in the literature (Gates, 1962; Gates, 1968; Idso and Jackson, 1968; Jackson and Idso, 1969; McGinnies and Aronson, 1971) and from our own work (Jiwaji, 1981; Stigter et al., 1982) it appears essential to test commercially available infrared thermometers in controlled field use. This necessitates the development of a 'true temperature' surface (e.g. Idso, 1971, 1974) whose emission coefficient e is known for the wavelengths concerned (e.g. Fuchs and Tanner, 1966; Lowry and Gay, 1970). In order to solve this problem, we developed an absolute calibration plate for field use with a simple 'all wavelengths' infrared thermometer, our instrument being an improved version of the one described by Stoutjesdijk (1974) (Stigter et al., 1982). Use of such a plate improves on existing practice of comparing temperatures of soil or other natural surfaces outdoors for such purposes. It can also be used for wavelength band specific radiometers. The main difficulty to be overcome in this procedure was the accurate determination of the true surface temperature of the calibration plate by contact t h e r m o m e t r y or otherwise (e.g. Idso, 1971). The need to obtain a large isothermal surface at relatively high temperature necessitated the use of a material of good thermal conductivity and limited heat capacity for the construction of the plate.
0002-1571/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
280 MATERIALS AND METHODS We obtained excellent results with a 5 mm aluminium plate (50 × 50 cm) and thermojunctions rolled to a thickness of between 0.06 and 0 . 0 8 m m employing a technique proved not to affect calibration constants of the thermocouples (Stigter, 1968). The plate was heavily insulated with tempex to avoid conductive heat losses. A layer (0.05 mm) of aluminized Mylar tape, a polyester-coated aluminium base layer, was used to insulate the wires from the surface. This material exhibits a high solar radiation reflection and high longwave radiation emission, which makes it a very suitable cover for protective screens against solar radiation in t h e r m o m e t r y (Fuchs and Tanner, 1965; Stigter et al., 1976). Moreover the polyester is a very good electrical insulator. These properties make it a suitable substrate for the rolled thermocouples. To improve the contact and keep the wires stretched we covered the wires with Mylar but left 2 cm at each side of the junctions open. To assess the possible effect of a layer of aluminized Mylar on our thermocouple readings, we determined outdoors the differences between a rolled junction m o u n t e d in the manner described above and one sandwiched between two layers. The former mode of mounting was also compared with a junction on top of two layers of aluminized Mylar. Indoors we studied differences between two junctions both sandwiched between two layers of aluminized Mylar, placing one junction on top of the other, with only one insulating layer between them. Differences were monitored under cooling conditions. The calibration plate was heated up to 70°C and was then allowed to cool to room temperature (ca. 30°C). The horizontal temperature homogeneity was studied under similar conditions. The plate surface, the Mylar surfaces and the exposed wire and thermojunction surfaces were covered with Eppley--Parsons optical black laquer throughout these experiments. Attempts to simplify the process of paint application failed. Satisfactory results were only obtained by following the manufacturer's instructions. This meant one undercoat and two thin topcoats of paint by spraying. We were able to use very simple spray equipment (Preval disposable spray units). Details can be found in Jiwaji (1981) and Makonda (1981). RESULTS AND DISCUSSION The indoor cooling experiments showed that maximum differences of 0.1°C were encountered between sandwiched thermojunctions, the higher temperature being registered by the thermojunctions closest to the plate. For junctions at different positions on the plate, values varied by less than 0.01°C. Differences in response times to temperature changes, due to variations in cloud cover or wind speed, were shown to exist in the open air. It was observed that the thermocouples responded faster than the infrared thermometer in all cases. The fastest response was obtained from couples m o u n t e d on top of the Mylar and 'painted into the surface'. Under stable
281 TABLE I Temperature differences between time averaged thermocouple measurements as described and infrared thermometer results at different distances between an inclined calibration plate (no shades) and the remote thermometer. Distance (cm)
Average (°C)
15
25
35
45
+0.08 +0.42 --0.09 --0.01 --0.58
+0.16 --0.07 --0.26 +0.31 +0.22
+0.43 --0.22 --0.15 -0.24 +0.58
+0.19 --0.36 +0.14 +0.22 --0.15
--0.04
+0.07
+0.08
+0.01
insolation conditions differences between two junctions 3 0 c m apart were smaller than 0.05°C. After we had obtained our own indoor calibration facilities we successfully tested the infrared t h e r m o m e t e r indoors on a 0°C water surface with floating ice and a known e of 0.965 +0.005 (Stigter et al., 1982). From laboratory investigation and f r om field experiments using the calibration plate it appeared necessary to wrap the radiometer in foam insulation with an aluminized Mylar coating to minimize internal gradients. By having the plate inclined, shades could be prevented (Idso and Cooley, 1972). Under these conditions the time-averaged surface temperatures of the calibration plate in full sunlight as recorded by the t herm ocoupl es and the infrared t h e r m o m e t e r over e x t e n d e d periods of time were in excellent agreement. Table I shows such differences between averages over a measuring period o f 6 min. For every calibration plate/infrared t h e r m o m e t e r separation we investigated, the average of five such measuring periods gave rise to a difference of less than 0.1°C. In the calculation, e of the black plate was taken to be 0.985 +0.005 for a surface t e m p e r a t u r e of around 100°C. (Eppley--Parsons optical black laquer leaflet). The emission values given by the man u f actu r er were confirmed by Idso (1971) for r o o m temperature (0.98). The overall average difference for the four infrared t h e r m o m e t e r / plate distances used was f ound to be only 0.03°C. A value of e = 1 would have yielded 0.40°C. The error in a single 6 min average is in the order of 0.5°C due to differences in time constants and apparent sky t em perat ure (Idso and Jackson, 1968). Identical results obtained for half-hourly averages for the four plate/ t h e r m o m e t e r distances investigated also confirm the earlier findings of a high degree of temperature h o m o g e n e i t y of the calibration plate. With a viewing angle of 12 ° the diameter of the field of view at 15 and 4 5 c m distance is ap p r o x i m at el y 4 and 10 cm respectively. Shorter distances could introduce t o o large an influence of the f r ont of the instrument as seen by the measured surface part. Using the view factor approach (Reifsnyder, 1967) it can be shown that at a distance of 1 5 c m , less than 5% of the
282
surroundings radiating to the measured plate surface is occupied by the front of the infrared thermometer. At 45 cm this fraction amounts to only 1%. Distances smaller than 15 cm are allowed in principle if corrections are made for the part of the radiating surroundings occupied by the instrument and its front surface temperature is approximately known. Using a difference between smallest and largest distance of more than 30 cm would have introduced too large an influence from the absorption and emission of air between plate and instrument (e.g. Lorenz, 1966; Idso and Cooley, 1971). Larger distances are only permitted if corrections for such atmospheric absorption and emission are applied. We have proved that our calibration plate is the right instrument for field testing of infrared thermometers. Normal Mylar rather than aluminized Mylar could have been used knowing how well the paint covers the surfaces. As shown by Idso (1971, 1972, 1974) such plates can also be used to callbrate (net) radiometers and heat flux plates. An investigation of the influence of a higher reflectivity for solar radiation of other measured surfaces on infrared thermometers could be undertaken with this type of plate. A white paint would have to be used. If the long wave e of such a surface is not known, it has to be determined separately (e.g. Idso et al., 1976; Schurer, 1976). ACKNOWLEDGEMENTS
The equipment was obtained under the project DTH/UH/MV/Agr.Phys., supported by the Directorate General of International Cooperation, Ministry of Foreign Affairs, The Netherlands, with approval from the Treasury, Government of the United Republic of Tanzania. The project is technically assisted by an advisory group of the Department of Physics and Meteorology, Agricultural University, Wageningen, The Netherlands. We are grateful to Dr. M. Friedeberg of our Department for improving the English of a draft of this paper.
REFERENCES Fuchs, M. and Tanner, C.B., 1965. Radiation shields for air temperature thermometers. J. Appl. Meteorol., 4: 544--547. Fuchs, M. and Tanner, C.B., 1966. Infrared thermometry of vegetation. Agron. J., 58: 597--601. Gates, D.M., 1962. Energy Exchange in the Biosphere. Harper and Row, New York, NY, 151 pp. Gates, D.M., 1968. Sensing biological environments with a portable radiation thermometer. Appl. Opt., 7: 1803--1809. Idso, S.B., 1971. A simple technique for the calibration of long-wave radiation probes. Agric. Meteorol., 8 : 235--243. Idso, S.B., 1972. Calibration of soil heat flux plates by a radiation technique. Agric. Meteorol., 10: 467--471. Idso, S.B., 1974. The calibration and use of net radiometers. Adv. Agron., 26: 261--275.
283 Idso, S.B. and Jackson, R.D., 1968. Significance of fluctuations in sky radiant emittance for infrared thermometry. Agron. J., 60: 388--392. Idso, S.B. and Cooley, K.R., 1971. The vertical location of net radiometers I. The effects of the underlying air layer. J. Meteorol. Soc. Jpn., 49: 343--349. Idso, S.B. and Cooley, K.R. 1972. The vertical location of net radiometers. II. The effects of the net radiometer's shadow. J. Meteorol. Soc. Jpn., 50: 49--58. Idso, S.B., Jackson, R.D. and Reginato, R.J., 1976. Determining emittances for use in infrared thermometry : a simple technique for expanding the utility of existing methods. J. Appl. Meteorol., 15: 16--20. Jackson, R.D. and Idso, S.B., 1969. Ambient temperature effects in infrared thermometry. Agron. J., 61: 324--325. Jiwaji, N.T., 1981. Comparisons between a class-A and a new East African standard pan: a physical approach. M.Sc. Thesis, Univ. Dar es Salaam, 322 pp. Lorenz, D., 1966. The effect of the long-wave reflectivity of natural surfaces on surface temperature measurements using radiometers. J. Appl. Meteorol., 5: 421--430. Lowry, W.P. and Gay, L.W., 1970. Errors in infrared thermometry and radiometry. J. Appl. Meteorol., 9: 429--432. Makonda, M.M., 1981. Simple outdoor surface temperature measurements: their problems and calibration. M.Sc. Thesis, Univ. Dar es Salaam, 179 pp. McGinnies, W.J. and Aronson, R.C.W., 1971. Effects of environment on an infrared field thermometer. Agron. J., 63: 813--814. Reifsnyder, W.E., 1967. Radiation geometry in the measurement and interpretation of radiation balance. Agric. Meteorol., 4: 255--265. Schurer, K., 1976. A method for measuring infrared emissivities of near-black surfaces at ambient temperatures. Infrared Phys., 16: 157--163. Stigter, C.J., 1968. On the possibility of determining thermal properties from contactsurface temperatures. Physica, 39: 229--236. Stigter, C.J., Birnie, J. and Jansen, P., 1976. Multi-point temperature measuring equipment for crop environment, with some results on horizontal homogeneity in a maize crop. I. Field results. Neth. J. Agric. Sci., 24: 223--237. Stigter, C.J., Makonda, M.M. and Jiwaji, N.T., 1982. Improved field use of a simple infrared thermometer. Acta Bot. Neerl., 31 (in press). Stoutjesdijk, P., 1974. An improved simple radiation thermometer. Acta Bot. Neerl., 23: 131--136.
279
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A CALIBRATION PLATE TO DETERMINE THE PERFORMANCE OF I N F R A R E D THERMOMETERS IN FIELD USE C.J. STIGTER, N.T. JIWAJI and M.M. MAKONDA
Section Agricultural Physics, Physics Department, University of Dar es Salaam, Dares Salaam (Tanzania) (Received December 15, 1981; accepted for publication February 10, 1982)
ABSTRACT Stigter, C.J., Jiwaji, N.T. and Makonda, M.M., 1982. A calibration plate to determine the performance of infrared thermometers in field use. Agric. Meteorol., 26: 279--283. As the testing of infrared thermometers under actual field conditions has been reported to be indispensable, a calibration plate has been constructed for such purposes. A simple way of measuring surface temperatures by contact is shown to yield accurate results for comparison. When the emission coefficient of the calibration plate is known, its temperature can be used to investigate the performance of infrared thermometers in field use.
INTRODUCTION
In applied research in micrometeorology as well as on surfaces involved in harnessing of solar energy and (night) sky cooling, infrared field thermometers have a high practical value in the remote measurement of surface temperatures. However, from reports in the literature (Gates, 1962; Gates, 1968; Idso and Jackson, 1968; Jackson and Idso, 1969; McGinnies and Aronson, 1971) and from our own work (Jiwaji, 1981; Stigter et al., 1982) it appears essential to test commercially available infrared thermometers in controlled field use. This necessitates the development of a 'true temperature' surface (e.g. Idso, 1971, 1974) whose emission coefficient e is known for the wavelengths concerned (e.g. Fuchs and Tanner, 1966; Lowry and Gay, 1970). In order to solve this problem, we developed an absolute calibration plate for field use with a simple 'all wavelengths' infrared thermometer, our instrument being an improved version of the one described by Stoutjesdijk (1974) (Stigter et al., 1982). Use of such a plate improves on existing practice of comparing temperatures of soil or other natural surfaces outdoors for such purposes. It can also be used for wavelength band specific radiometers. The main difficulty to be overcome in this procedure was the accurate determination of the true surface temperature of the calibration plate by contact t h e r m o m e t r y or otherwise (e.g. Idso, 1971). The need to obtain a large isothermal surface at relatively high temperature necessitated the use of a material of good thermal conductivity and limited heat capacity for the construction of the plate.
0002-1571/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
280 MATERIALS AND METHODS We obtained excellent results with a 5 mm aluminium plate (50 × 50 cm) and thermojunctions rolled to a thickness of between 0.06 and 0 . 0 8 m m employing a technique proved not to affect calibration constants of the thermocouples (Stigter, 1968). The plate was heavily insulated with tempex to avoid conductive heat losses. A layer (0.05 mm) of aluminized Mylar tape, a polyester-coated aluminium base layer, was used to insulate the wires from the surface. This material exhibits a high solar radiation reflection and high longwave radiation emission, which makes it a very suitable cover for protective screens against solar radiation in t h e r m o m e t r y (Fuchs and Tanner, 1965; Stigter et al., 1976). Moreover the polyester is a very good electrical insulator. These properties make it a suitable substrate for the rolled thermocouples. To improve the contact and keep the wires stretched we covered the wires with Mylar but left 2 cm at each side of the junctions open. To assess the possible effect of a layer of aluminized Mylar on our thermocouple readings, we determined outdoors the differences between a rolled junction m o u n t e d in the manner described above and one sandwiched between two layers. The former mode of mounting was also compared with a junction on top of two layers of aluminized Mylar. Indoors we studied differences between two junctions both sandwiched between two layers of aluminized Mylar, placing one junction on top of the other, with only one insulating layer between them. Differences were monitored under cooling conditions. The calibration plate was heated up to 70°C and was then allowed to cool to room temperature (ca. 30°C). The horizontal temperature homogeneity was studied under similar conditions. The plate surface, the Mylar surfaces and the exposed wire and thermojunction surfaces were covered with Eppley--Parsons optical black laquer throughout these experiments. Attempts to simplify the process of paint application failed. Satisfactory results were only obtained by following the manufacturer's instructions. This meant one undercoat and two thin topcoats of paint by spraying. We were able to use very simple spray equipment (Preval disposable spray units). Details can be found in Jiwaji (1981) and Makonda (1981). RESULTS AND DISCUSSION The indoor cooling experiments showed that maximum differences of 0.1°C were encountered between sandwiched thermojunctions, the higher temperature being registered by the thermojunctions closest to the plate. For junctions at different positions on the plate, values varied by less than 0.01°C. Differences in response times to temperature changes, due to variations in cloud cover or wind speed, were shown to exist in the open air. It was observed that the thermocouples responded faster than the infrared thermometer in all cases. The fastest response was obtained from couples m o u n t e d on top of the Mylar and 'painted into the surface'. Under stable
281 TABLE I Temperature differences between time averaged thermocouple measurements as described and infrared thermometer results at different distances between an inclined calibration plate (no shades) and the remote thermometer. Distance (cm)
Average (°C)
15
25
35
45
+0.08 +0.42 --0.09 --0.01 --0.58
+0.16 --0.07 --0.26 +0.31 +0.22
+0.43 --0.22 --0.15 -0.24 +0.58
+0.19 --0.36 +0.14 +0.22 --0.15
--0.04
+0.07
+0.08
+0.01
insolation conditions differences between two junctions 3 0 c m apart were smaller than 0.05°C. After we had obtained our own indoor calibration facilities we successfully tested the infrared t h e r m o m e t e r indoors on a 0°C water surface with floating ice and a known e of 0.965 +0.005 (Stigter et al., 1982). From laboratory investigation and f r om field experiments using the calibration plate it appeared necessary to wrap the radiometer in foam insulation with an aluminized Mylar coating to minimize internal gradients. By having the plate inclined, shades could be prevented (Idso and Cooley, 1972). Under these conditions the time-averaged surface temperatures of the calibration plate in full sunlight as recorded by the t herm ocoupl es and the infrared t h e r m o m e t e r over e x t e n d e d periods of time were in excellent agreement. Table I shows such differences between averages over a measuring period o f 6 min. For every calibration plate/infrared t h e r m o m e t e r separation we investigated, the average of five such measuring periods gave rise to a difference of less than 0.1°C. In the calculation, e of the black plate was taken to be 0.985 +0.005 for a surface t e m p e r a t u r e of around 100°C. (Eppley--Parsons optical black laquer leaflet). The emission values given by the man u f actu r er were confirmed by Idso (1971) for r o o m temperature (0.98). The overall average difference for the four infrared t h e r m o m e t e r / plate distances used was f ound to be only 0.03°C. A value of e = 1 would have yielded 0.40°C. The error in a single 6 min average is in the order of 0.5°C due to differences in time constants and apparent sky t em perat ure (Idso and Jackson, 1968). Identical results obtained for half-hourly averages for the four plate/ t h e r m o m e t e r distances investigated also confirm the earlier findings of a high degree of temperature h o m o g e n e i t y of the calibration plate. With a viewing angle of 12 ° the diameter of the field of view at 15 and 4 5 c m distance is ap p r o x i m at el y 4 and 10 cm respectively. Shorter distances could introduce t o o large an influence of the f r ont of the instrument as seen by the measured surface part. Using the view factor approach (Reifsnyder, 1967) it can be shown that at a distance of 1 5 c m , less than 5% of the
282
surroundings radiating to the measured plate surface is occupied by the front of the infrared thermometer. At 45 cm this fraction amounts to only 1%. Distances smaller than 15 cm are allowed in principle if corrections are made for the part of the radiating surroundings occupied by the instrument and its front surface temperature is approximately known. Using a difference between smallest and largest distance of more than 30 cm would have introduced too large an influence from the absorption and emission of air between plate and instrument (e.g. Lorenz, 1966; Idso and Cooley, 1971). Larger distances are only permitted if corrections for such atmospheric absorption and emission are applied. We have proved that our calibration plate is the right instrument for field testing of infrared thermometers. Normal Mylar rather than aluminized Mylar could have been used knowing how well the paint covers the surfaces. As shown by Idso (1971, 1972, 1974) such plates can also be used to callbrate (net) radiometers and heat flux plates. An investigation of the influence of a higher reflectivity for solar radiation of other measured surfaces on infrared thermometers could be undertaken with this type of plate. A white paint would have to be used. If the long wave e of such a surface is not known, it has to be determined separately (e.g. Idso et al., 1976; Schurer, 1976). ACKNOWLEDGEMENTS
The equipment was obtained under the project DTH/UH/MV/Agr.Phys., supported by the Directorate General of International Cooperation, Ministry of Foreign Affairs, The Netherlands, with approval from the Treasury, Government of the United Republic of Tanzania. The project is technically assisted by an advisory group of the Department of Physics and Meteorology, Agricultural University, Wageningen, The Netherlands. We are grateful to Dr. M. Friedeberg of our Department for improving the English of a draft of this paper.
REFERENCES Fuchs, M. and Tanner, C.B., 1965. Radiation shields for air temperature thermometers. J. Appl. Meteorol., 4: 544--547. Fuchs, M. and Tanner, C.B., 1966. Infrared thermometry of vegetation. Agron. J., 58: 597--601. Gates, D.M., 1962. Energy Exchange in the Biosphere. Harper and Row, New York, NY, 151 pp. Gates, D.M., 1968. Sensing biological environments with a portable radiation thermometer. Appl. Opt., 7: 1803--1809. Idso, S.B., 1971. A simple technique for the calibration of long-wave radiation probes. Agric. Meteorol., 8 : 235--243. Idso, S.B., 1972. Calibration of soil heat flux plates by a radiation technique. Agric. Meteorol., 10: 467--471. Idso, S.B., 1974. The calibration and use of net radiometers. Adv. Agron., 26: 261--275.
283 Idso, S.B. and Jackson, R.D., 1968. Significance of fluctuations in sky radiant emittance for infrared thermometry. Agron. J., 60: 388--392. Idso, S.B. and Cooley, K.R., 1971. The vertical location of net radiometers I. The effects of the underlying air layer. J. Meteorol. Soc. Jpn., 49: 343--349. Idso, S.B. and Cooley, K.R. 1972. The vertical location of net radiometers. II. The effects of the net radiometer's shadow. J. Meteorol. Soc. Jpn., 50: 49--58. Idso, S.B., Jackson, R.D. and Reginato, R.J., 1976. Determining emittances for use in infrared thermometry : a simple technique for expanding the utility of existing methods. J. Appl. Meteorol., 15: 16--20. Jackson, R.D. and Idso, S.B., 1969. Ambient temperature effects in infrared thermometry. Agron. J., 61: 324--325. Jiwaji, N.T., 1981. Comparisons between a class-A and a new East African standard pan: a physical approach. M.Sc. Thesis, Univ. Dar es Salaam, 322 pp. Lorenz, D., 1966. The effect of the long-wave reflectivity of natural surfaces on surface temperature measurements using radiometers. J. Appl. Meteorol., 5: 421--430. Lowry, W.P. and Gay, L.W., 1970. Errors in infrared thermometry and radiometry. J. Appl. Meteorol., 9: 429--432. Makonda, M.M., 1981. Simple outdoor surface temperature measurements: their problems and calibration. M.Sc. Thesis, Univ. Dar es Salaam, 179 pp. McGinnies, W.J. and Aronson, R.C.W., 1971. Effects of environment on an infrared field thermometer. Agron. J., 63: 813--814. Reifsnyder, W.E., 1967. Radiation geometry in the measurement and interpretation of radiation balance. Agric. Meteorol., 4: 255--265. Schurer, K., 1976. A method for measuring infrared emissivities of near-black surfaces at ambient temperatures. Infrared Phys., 16: 157--163. Stigter, C.J., 1968. On the possibility of determining thermal properties from contactsurface temperatures. Physica, 39: 229--236. Stigter, C.J., Birnie, J. and Jansen, P., 1976. Multi-point temperature measuring equipment for crop environment, with some results on horizontal homogeneity in a maize crop. I. Field results. Neth. J. Agric. Sci., 24: 223--237. Stigter, C.J., Makonda, M.M. and Jiwaji, N.T., 1982. Improved field use of a simple infrared thermometer. Acta Bot. Neerl., 31 (in press). Stoutjesdijk, P., 1974. An improved simple radiation thermometer. Acta Bot. Neerl., 23: 131--136.