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Measuring air temperature in field studies
J. therm. Biol. Vol. 10, No. 1, pp. 55-56, 1985 Printed in Great Britain. All rights reserved
0306-4565/85 $3.00 + 0.00 Copyright ~ 1985 Pergamon Press Ltd
MEASURING AIR TEMPERATURE IN FIELD STUDIES K. A. CHRISTIAN Department of Biology, University of Puerto Rico, Rio Piedras, PR 00931, U.S.A. C. R. TRACY Department o f Zoology and Entomology, Colorado State University, Fort Collins, CO 80523, U.S.A.
(Received 14 December 1983; accepted in revised form 28 March 1984)
Abstract--In order to find a simple technique for reducing errors in measurements of air temperature, experiments were performed in a wind tunnel using different sizes of thermocouples and a variety of shields. Thermocouples with a coat of clean, white enamel paint on the tip were simple and inexpensive, and they resulted in only small errors due to radiation. Key Word Index--Air temperature; environmental temperature; field techniques: solar radiation; thermal radiation.
INTRODUCTION
were read in conjunction with a model TH-65 Wescor (Logan, Utah) thermocouple thermometer (accuracy = 0.2°C; precision = 0.1°C). All sensors were placed l0 cm above the floor of the wind tunnel, with the tips pointing upward. All thermocouples measured equal values of Ta before the lights were turned on. Radiation shields were constructed from Al foil. One shield was simply a flat sheet (25 x 25cm) suspended 17cm above the floor. This shield is referred to as the "A1 umbrella". The remainder of the shields were constructed as tubes (12 cm dia). One tube was simply Al foil ("Al tube"), but the inside surfaces of the other two tubes were spray-painted flat black ("black tubes"). All but one of the thermocouples were dipped in a glossy enamel white paint (Mautz Industrial Enamel No. WP 138900) so that there was a thin coat over the sensing tip. One unpainted thermocouple with a shiny soldered surface was used under some experimental conditions. The experiments were run at wind speeds of 0.36, 1.02 and 2.54m s -~ (as measured with a hot-wire anemometer). The Al tube and one of the black tubes were placed parallel to the windstream. The other black tube was placed perpendicular to the windstream. One group of white-tipped thermocouples was placed in the wind tunnel without a shield. Radiation was brought to 1.054 W m -2 inside the wind tunnel, and the T,~ranged between 24-31°C. The "true" Ta was subtracted from the other values to determine the error due to radiation for the different sized thermocouples at each wind speed and shielding condition (Table I). The unshielded Schultheis thermometer and the unpainted thermocouple both resulted in large errors due to radiation. Large errors resulted from both the Al tube and the perpendicular black tube. However, the Al umbrella, the parallel black tube, and the white unshielded thermocouples all resulted in relatively small errors due to radiation under the conditions tested. The fact that the black parallel tube resulted in
Accurate measures of air temperature (Ta) are important in many field studies. Meteorologists have long recognized the problems associated with accurately measuring Ta, and they often make a considerable effort to avoid such errors (Fuchs and Tanner, 1965; Daniels, 1968; Hadlock et al., 1972). However, we have noticed that there is a lack of uniformity among biologists in the techniques used to measure Ta, and in many cases little effort is made to avoid errors. Both solar and thermal radiation influence the reading from a thermometer. The size of the error due to solar radiation is dependent upon the intensity of solar radiation incident on the thermometer, reflectivity and size of the sensor, and the convective environment (wind speed) around the sensor. Error due to thermal radiation is determined by the intensity of thermal radiation on the sensor, absorptivity and size of the sensor, and wind speed. As Fuchs and Tanner (1965) noted, it is important to consider both solar and thermal radiation when shielding a thermometer. Thus, even if Ta is measured in the shade of a tree, considerable error could result from thermal radiation if rocks and soil in the surrounding environment were very hot (e.g. in a desert), and if the temperature sensor were large (e.g. a bulb thermometer or a large thermistor). In order to determine a simple way to reduce errors due to radiation, we performed an experiment in a closed-circuit wind tunnel (inside dimensions: 65 x 65cm). Eight reflector floedlight bulbs (GE, 150W) were shone through the Plexiglas roof (6.5mm thick) of the wind tunnel. Radiation was measured with a Moll-Gorzinski solarimeter. The "true"T~ inside the wind tunnel was measured with a 24 G thermocouple placed upstream where it was not exposed to the radiation. Temperature sensors included Cu/constantan thermocouples (18, 24, 30 and 36G), and a Schultheis quick-reading Hg thermometer. The thermocouples 55
56
K. A. CHRISTIAN and C. R. TRACY
Table 1. Errors in T~ measurements (C) for different sized thermocouples and different shielding conditions Wind speed (m s -I) 2.54 1.02 0.36 Black tube parallel to wind
18 G 24 G 30 G 36 G
thermocouple thermocouple thermocouple thermocouple
0.4 -0.0 0.0
1.0 0.4 0.2 0.2
1.5 0.7 0.4 0.3
1.4 1.3 1.0 1.0
1.1 1.2 1.2 1.2
3.5 3.5 3.5 3.1
0.8 0.3 0.2 0.7
1.7 0.9 0.8 0.7
2.6 1.5 0.9 0.9
0.3 0.1 0.0 0.2
1.0 0.3 0.2 0.1
1.8 0.9 0.5 0.2
0.6 0.4 0.1 0.1 --
1.4 0.7 0.3 0.2 2.3
2.7 1.3 0.8 0.5 3.5
Black tube perpendicular to wind
18 G 24G 30G 36 G
thermocouple thermocouple thermocouple thermocouple
AI tube parallel to wind
18 G 24 G 30 G 36 G
thermocouple thermocouple thermocouple thermocouple
AI umbrella
!8 G 24 G 30 G 36 G
thermocouple thermocouple ther'mocouple thermocouple
Unshielded white thermocouples
18 G 24 G 30 G 36 G
thermocouple thermocouple thermocouple thermocouple
Schultheis thermometer Unpainted, shin)' 24 G thermocouple
-2.6 3.0 Radiation load = 1,054W m -2. See text for detailed description of shields. smaller errors t h a n the AI tube suggests t h a t some o f the r a d i a t i o n was able to get inside the shields via reflection. A p p a r e n t l y the r a d i a t i o n t h a t went inside the black tube was a b s o r b e d by the black wall, a n d the resulting heat was then ventilated away, whereas r a d i a t i o n t h a t entered the AI tube was re-reflected
until some of it struck the thermocouple. As long as the walls of the shield were adequately ventilated, the black tube was an effective shield for the t h e r m o couples. T h e unshielded white t h e r m o c o u p l e s resulted in relatively small errors, especially the smaller gauges. A l t h o u g h the 3 6 G wire resulted in the smallest errors, this fine wire is easily b r o k e n a n d difficult to use in the field. The 30 G wire resulted in reasonably small errors, a n d we have found it r o b u s t a n d easy to use. Errors from unshielded white t h e r m o c o u p l e s could be further reduced by butt Ag soldering, which reduces the d i a m e t e r of the sensing tip by o n e - h a l f [see A p p e n d i x 2 in Daniels (1968)]. F u r t h e r m o r e , errors could be reduced by using a very highly reflective white p a i n t t h a t c o n t a i n s BaO. However, it is likely t h a t any clean white enamel paint will yield results similar to those reported here. The m a j o r p r o b l e m with tubes is t h a t large errors m a y occur if there is a shift in the direction of the wind so t h a t they are not adequately ventilated. Therefore, with respect to the potential for error, ease, a n d low cost, we suggest that small (30 G) unshielded white-tipped t h e r m o c o u p l e s are a d e q u a t e for m o s t field studies. Care should be taken to keep the tips clean to minimize a b s o r p t i o n o f solar radiation.
REFERENCES
Daniels G. E. (1968) Measurement of gas temperature and the radiation compensating thermocouple. J. appl. Met. 7, 1026-10355. Fuchs M. and Tanner C. B. (1965) Radiation shields for air temperature thermometers, J. appl. Met. 4, 544-547. Hadlock R., Sequin W. R. and Garstang M. (1972) A radiation shield for thermistor deployment in the atmospheric boundary layer. J. appl. Met. 11, 393-399.
0306-4565/85 $3.00 + 0.00 Copyright ~ 1985 Pergamon Press Ltd
MEASURING AIR TEMPERATURE IN FIELD STUDIES K. A. CHRISTIAN Department of Biology, University of Puerto Rico, Rio Piedras, PR 00931, U.S.A. C. R. TRACY Department o f Zoology and Entomology, Colorado State University, Fort Collins, CO 80523, U.S.A.
(Received 14 December 1983; accepted in revised form 28 March 1984)
Abstract--In order to find a simple technique for reducing errors in measurements of air temperature, experiments were performed in a wind tunnel using different sizes of thermocouples and a variety of shields. Thermocouples with a coat of clean, white enamel paint on the tip were simple and inexpensive, and they resulted in only small errors due to radiation. Key Word Index--Air temperature; environmental temperature; field techniques: solar radiation; thermal radiation.
INTRODUCTION
were read in conjunction with a model TH-65 Wescor (Logan, Utah) thermocouple thermometer (accuracy = 0.2°C; precision = 0.1°C). All sensors were placed l0 cm above the floor of the wind tunnel, with the tips pointing upward. All thermocouples measured equal values of Ta before the lights were turned on. Radiation shields were constructed from Al foil. One shield was simply a flat sheet (25 x 25cm) suspended 17cm above the floor. This shield is referred to as the "A1 umbrella". The remainder of the shields were constructed as tubes (12 cm dia). One tube was simply Al foil ("Al tube"), but the inside surfaces of the other two tubes were spray-painted flat black ("black tubes"). All but one of the thermocouples were dipped in a glossy enamel white paint (Mautz Industrial Enamel No. WP 138900) so that there was a thin coat over the sensing tip. One unpainted thermocouple with a shiny soldered surface was used under some experimental conditions. The experiments were run at wind speeds of 0.36, 1.02 and 2.54m s -~ (as measured with a hot-wire anemometer). The Al tube and one of the black tubes were placed parallel to the windstream. The other black tube was placed perpendicular to the windstream. One group of white-tipped thermocouples was placed in the wind tunnel without a shield. Radiation was brought to 1.054 W m -2 inside the wind tunnel, and the T,~ranged between 24-31°C. The "true" Ta was subtracted from the other values to determine the error due to radiation for the different sized thermocouples at each wind speed and shielding condition (Table I). The unshielded Schultheis thermometer and the unpainted thermocouple both resulted in large errors due to radiation. Large errors resulted from both the Al tube and the perpendicular black tube. However, the Al umbrella, the parallel black tube, and the white unshielded thermocouples all resulted in relatively small errors due to radiation under the conditions tested. The fact that the black parallel tube resulted in
Accurate measures of air temperature (Ta) are important in many field studies. Meteorologists have long recognized the problems associated with accurately measuring Ta, and they often make a considerable effort to avoid such errors (Fuchs and Tanner, 1965; Daniels, 1968; Hadlock et al., 1972). However, we have noticed that there is a lack of uniformity among biologists in the techniques used to measure Ta, and in many cases little effort is made to avoid errors. Both solar and thermal radiation influence the reading from a thermometer. The size of the error due to solar radiation is dependent upon the intensity of solar radiation incident on the thermometer, reflectivity and size of the sensor, and the convective environment (wind speed) around the sensor. Error due to thermal radiation is determined by the intensity of thermal radiation on the sensor, absorptivity and size of the sensor, and wind speed. As Fuchs and Tanner (1965) noted, it is important to consider both solar and thermal radiation when shielding a thermometer. Thus, even if Ta is measured in the shade of a tree, considerable error could result from thermal radiation if rocks and soil in the surrounding environment were very hot (e.g. in a desert), and if the temperature sensor were large (e.g. a bulb thermometer or a large thermistor). In order to determine a simple way to reduce errors due to radiation, we performed an experiment in a closed-circuit wind tunnel (inside dimensions: 65 x 65cm). Eight reflector floedlight bulbs (GE, 150W) were shone through the Plexiglas roof (6.5mm thick) of the wind tunnel. Radiation was measured with a Moll-Gorzinski solarimeter. The "true"T~ inside the wind tunnel was measured with a 24 G thermocouple placed upstream where it was not exposed to the radiation. Temperature sensors included Cu/constantan thermocouples (18, 24, 30 and 36G), and a Schultheis quick-reading Hg thermometer. The thermocouples 55
56
K. A. CHRISTIAN and C. R. TRACY
Table 1. Errors in T~ measurements (C) for different sized thermocouples and different shielding conditions Wind speed (m s -I) 2.54 1.02 0.36 Black tube parallel to wind
18 G 24 G 30 G 36 G
thermocouple thermocouple thermocouple thermocouple
0.4 -0.0 0.0
1.0 0.4 0.2 0.2
1.5 0.7 0.4 0.3
1.4 1.3 1.0 1.0
1.1 1.2 1.2 1.2
3.5 3.5 3.5 3.1
0.8 0.3 0.2 0.7
1.7 0.9 0.8 0.7
2.6 1.5 0.9 0.9
0.3 0.1 0.0 0.2
1.0 0.3 0.2 0.1
1.8 0.9 0.5 0.2
0.6 0.4 0.1 0.1 --
1.4 0.7 0.3 0.2 2.3
2.7 1.3 0.8 0.5 3.5
Black tube perpendicular to wind
18 G 24G 30G 36 G
thermocouple thermocouple thermocouple thermocouple
AI tube parallel to wind
18 G 24 G 30 G 36 G
thermocouple thermocouple thermocouple thermocouple
AI umbrella
!8 G 24 G 30 G 36 G
thermocouple thermocouple ther'mocouple thermocouple
Unshielded white thermocouples
18 G 24 G 30 G 36 G
thermocouple thermocouple thermocouple thermocouple
Schultheis thermometer Unpainted, shin)' 24 G thermocouple
-2.6 3.0 Radiation load = 1,054W m -2. See text for detailed description of shields. smaller errors t h a n the AI tube suggests t h a t some o f the r a d i a t i o n was able to get inside the shields via reflection. A p p a r e n t l y the r a d i a t i o n t h a t went inside the black tube was a b s o r b e d by the black wall, a n d the resulting heat was then ventilated away, whereas r a d i a t i o n t h a t entered the AI tube was re-reflected
until some of it struck the thermocouple. As long as the walls of the shield were adequately ventilated, the black tube was an effective shield for the t h e r m o couples. T h e unshielded white t h e r m o c o u p l e s resulted in relatively small errors, especially the smaller gauges. A l t h o u g h the 3 6 G wire resulted in the smallest errors, this fine wire is easily b r o k e n a n d difficult to use in the field. The 30 G wire resulted in reasonably small errors, a n d we have found it r o b u s t a n d easy to use. Errors from unshielded white t h e r m o c o u p l e s could be further reduced by butt Ag soldering, which reduces the d i a m e t e r of the sensing tip by o n e - h a l f [see A p p e n d i x 2 in Daniels (1968)]. F u r t h e r m o r e , errors could be reduced by using a very highly reflective white p a i n t t h a t c o n t a i n s BaO. However, it is likely t h a t any clean white enamel paint will yield results similar to those reported here. The m a j o r p r o b l e m with tubes is t h a t large errors m a y occur if there is a shift in the direction of the wind so t h a t they are not adequately ventilated. Therefore, with respect to the potential for error, ease, a n d low cost, we suggest that small (30 G) unshielded white-tipped t h e r m o c o u p l e s are a d e q u a t e for m o s t field studies. Care should be taken to keep the tips clean to minimize a b s o r p t i o n o f solar radiation.
REFERENCES
Daniels G. E. (1968) Measurement of gas temperature and the radiation compensating thermocouple. J. appl. Met. 7, 1026-10355. Fuchs M. and Tanner C. B. (1965) Radiation shields for air temperature thermometers, J. appl. Met. 4, 544-547. Hadlock R., Sequin W. R. and Garstang M. (1972) A radiation shield for thermistor deployment in the atmospheric boundary layer. J. appl. Met. 11, 393-399.