TECHNICAL NOTE: CONSTRUCTION OF A LOW‐COST BLACK GLOBE THERMOMETER J. L. Purswell, J. D. Davis ABSTRACT. An economical self‐contained globe thermometer was developed for use in field applications. The instrument uses a miniature integral temperature sensor and datalogger in place of a separate sensor and external datalogger. The performance of the miniature sensor/logger units was shown to be satisfactory for use in globe thermometers. Calibration of the units showed a linear behavior with standard errors ranging from 0.02°C to 0.04°C for the span of 15°C to 90°C. The integral sensor/logger performed as well as the thermistor/external logger combination and can be fabricated at a low cost. Keywords. Globe thermometer, Data acquisition, Thermal comfort.
B
lack globe thermometers are widely used to assess the effects of radiant and convective heat transfer on humans and animals. These measurements are typically combined with dry‐bulb or wet‐bulb tem‐ peratures to form an effective environmental temperature. This instrument has been used in studies with dairy (Muller and Botha, 1997; Yamamoto et al., 1994; Buffington et al., 1981) and beef cattle (Bond and Kelley, 1955; Brosh et al., 1998; Mader et al., 1999), swine (Houszka et al., 2001), equine (Schroter et al., 1996; Schroter and Marlin, 1995), and poultry (Bucklin et al., 1993; Baxter et al., 1970). The basic design of the black globe thermometer is straightforward; however, the availability and cost of suitable data acquisition equipment may limit its utility in the field. The construction and measurement criteria for a black globe thermometer are specified in ISO 7243 (1989) and ISO 7726 (2001). The requirements for the measurement device are a measurement range of 20°C to 120°C with an accuracy of ±0.5°C or better for 20°C to 50°C and ±1°C or better for 50°C to 120°C; the type of transducer is not specified. Typical field applications require the transducer to be interfaced with a battery‐powered datalogger which requires, at the very least, some form of weatherproof housing. Additionally, black globe thermometers that are not installed in close proximity to will require separate dataloggers for each unit; this adds considerably to monitoring cost. The objective of this study was to develop a self‐contained low‐cost black globe thermometer capable of long‐term field deployment. Development of this system would facilitate
Submitted for review in February 2007 as manuscript number SE 6893; approved for publication by the Structures & Environment Division of ASABE in December 2007. The authors are Joseph L. Purswell, ASABE Member Engineer, Agricultural Engineer, USDA‐ARS Poultry Research Unit, Mississippi State, Mississippi; and Jeremiah D. Davis, ASABE Member Engineer, Assistant Professor, Department of Biological Engineering, Mississippi State University, Mississippi State, MS. Corresponding author: Joseph L. Purswell, USDA‐ARS Poultry Research Unit, P.O. Box 5367, Mississippi State, MS 39768; phone: 662‐320‐7480; fax: 662‐320‐7589; e‐mail:
[email protected].
multiple measurement sites during field experiments which may be cost‐prohibitive using traditional data acquisition techniques.
CONSTRUCTION AND TESTING The performance requirements for a temperature sensor for use in globe thermometers is specified in ISO 7243 (1989) and call for a device with a measurement range of 20°C to 120°C with an accuracy of ±0.5°C or better for 20°C to 50°C and ±1°C or better for 50°C to 120°C. Small self‐contained units which integrate a datalogger, temperature sensor, and battery power supply are available in a variety of models for different temperature ranges (DS1922, Dallas Semiconduc‐ tor, Sunnyvale, Cal.). Given the performance requirements for a temperature transducer specified in ISO 7243 (1989), one unit (DS1922‐T, Dallas Semiconductor, Sunnyvale, Cal.) was determined to meet these requirements. The manufacturer lists the following specifications for the sensor/logger unit: temperature range of 0 to 125°C, with an accuracy of ±0.5°C. Storage capacity is dependent upon the resolution selected; the unit can record 8192 readings in 8‐bit mode and 4096 readings in 11‐bit mode, e.g. a 15‐min recording interval will result in approximately 42 d of data. Manufacturer 's specifications list battery life for the temper‐ ature ranges to be expected in a black globe sensor as between two and three years, dependent upon sampling frequency. Robert and Thompson (2003) detailed procedures to disas‐ semble the loggers to reduce overall size and weight, and the same procedure can be used to replace batteries but loggers must be repackaged as disassembly destroys the case of the logger. The units were calibrated in a water bath against a NIST‐traceable RTD (DP97, Omega Engineering, Stamford, Conn.) in 2.5°C increments to a range of 15°C to 90°C. Five units were tested, and plots of calibration data are shown in figure 1; the associated regression statistics can be found in table 1.
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2008 American Society of Agricultural and Biological Engineers ISSN 0883-8542
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100 #1 #2 #3 #4 #5
60
T
Logger
80
40 20 0 0
20
40
60
80
100
TRTD Figure 1. Example calibration data of five DS1922T logger units.
Table 1. Regression statistics for calibration of five logger units. #1
#2
R
0.999
0.999
R2 SE Slope[a] Intercept[a]
0.999 0.023 1.000 0.095
0.999 0.043 0.999 0.138
[a]
#3
#4
#5
0.999
0.999
0.999
0.999 0.026 1.000 0.097
0.999 0.033 1.000 0.104
0.999 0.032 1.000 0.099
All regression coefficients were significant at P < 0.0001.
Once the accuracy of the temperature recording units was verified, a globe and mounting fixture were fabricated. The internal diameter of the mounting spud originally installed in the spherical copper tank float (2892K27, McMaster‐Carr, Chicago, Ill.) was too small to pass the logger. A 1.9‐cm (0.75‐in.) pipe nipple was found to accommodate the logger and was used to replace the original mounting spud. The mounting fixture used to secure the logger in the center of the globe was constructed of a pipe cap, PVC material, and wall mount bracket (DS9093S, Dallas Semiconductor, Sunny‐ vale, Cal.). The completed globe and mounting fixture are shown in figure 2.
Figure 3. Three‐dimensional section drawing of globe thermometer as‐ sembly. Components are labeled in figure 3.
The prototype instrument was compared against a system currently being used in field studies at the Iowa Beef Center at Iowa State University. The system deployed in the field consists of an identical globe using a water‐tight cable grip (SHC1001CR, Hubbell Inc., Milford, Conn.) to mount a thermistor probe (PT916, Pace Scientific, Mooresville, N.C.). Readings are recorded using a battery‐powered datalogger (XR440, Pace Scientific, Mooresville, N.C.) mounted in a weatherproof enclosure (1555VGY, Hammond Mfg. Co., Cheektowaga, N.Y.). Both instruments were affixed to a wooden mount attached to a weather station mast and oriented north‐south with a 76‐cm separation (fig. 4).
a
b Logger mount Logger
PVC mast
c Pipe cap for securing logger inside globe shell Globe shell
Figure 2. Globe and temperature recorder mounting fixture.
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Figure 4. Both instruments mounted atop a weather station which in‐ cludes: (a) pyranometer, (b) cup anemometer and wind vane, and (c) tem‐ perature and relative humidity sensor inside a gill radiation shield. The prototype globe thermometer containing the miniature logger is located at left.
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ACKNOWLEDGEMENTS The authors would like to thank John Prisock and Jason Johnson, Engineering Technicians (USDA‐ARS Poultry Research Unit) for their assistance on this project.
External Logger Internal Logger
40
Tglobe (°C)
35 30
REFERENCES
25 20 15 10 5 0 26-Oct
27-Oct
28-Oct
29-Oct
30-Oct
Figure 5. Comparison of single instrument readings for both sensor/log‐ ger combinations.
A time course of the readings from both instruments is presented in figure 5. The readings were compared with a paired t‐test and a significant difference of 0.5°C was found (P < 0.0001), however this difference is within accuracy specifications as set out in ISO 7243 (1989) and ISO 7726 (2001). It should be noted that at higher temperatures the error between the thermistor/logger combination and the prototype instrument increases and care should be taken to calibrate the whole instrument accordingly. Construction costs for both systems are shown in table 2 and a significant cost savings can be realized when using the prototype system; however, it should be noted that the external logger chosen is the most expensive item and costs will vary depending on the model.
SUMMARY An economical self‐contained black globe thermometer was constructed using a miniature temperature sensor and logger. The instruments performed as well as a typical data acquisition system using a thermistor and datalogger, while significantly reducing costs. Table 2. Comparison of construction costs for two black globe temperature recording systems. Integral Sensor/Logger
External Logger
Logger Wall Mount Nipple Plug Float Paint
$40.00 $1.24 $2.50 $2.50 $35.00 $3.50
Logger Thermistor Cable grip Enclosure Float Paint
$499.00 $45.00 $7.00 $59.00 $35.00 $3.50
Total
$84.74
Total
$648.50
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Baxter, D. O., T. E. Maddox, and H. V. Shirley. 1970. Temperature preferences of chicks. Transactions of the ASAE 13(6): 788‐791. Bond, T. E., and C. F. Kelly. 1955. The globe thermometer in agricultural research. Agricultural Engineering 36(4): 251. Brosh, A., Y. Aharoni, A. A. Degen, D. Wright, and B. Young. 1998. Estimation of energy expenditure from heart rate measurements in cattle maintained under different conditions. J. Anim. Sci. 76(12): 3054‐3064 Bucklin, R. A., R. W. Bottcher, G. L. Van Wicklen, and M. Czarick. 1993. Reflective roof coatings for heat stress relief in livestock and poultry housing. Applied Engineering in Agriculture 9(1): 123‐129. Buffington, D. E., A. Collazo‐Arocho, G. H. Canton, and D. Pitt. 1981. Black globe‐humidity index (BGHI) as comfort equation for dairy cows. Transactions of the ASAE 24(3): 711‐714. Houszka, H. M, J. S. Strøm, and S. Morsing. 2001. Thermal conditions in covered creep areas for piglets. Transactions of the ASAE 44(6): 1859‐1863. ISO. 1989. ISO 7243: Hot environments – Estimation of the heat stress on working man, based on the WBGT‐index (wet bulb globe temperature). Geneva, Switzerland: ISO. ISO. 2001. ISO 7726: Ergonomics of the thermal environment – Instruments for measuring physical quantities. Geneva, Switzerland: ISO. Mader, T. L., J. M. Dahlquist, G. L. Hahn, and J. B. Gaughan. 1999. Shade and wind barrier effects on summertime feedlot cattle performance. J. Anim. Sci. 77(8): 2065‐2072. Muller, C. J. C., and J. A. Botha. 1997. Roof height in roofed free‐stall structures in relation to the microclimate and production performance of lactating Freisian cows during summer in a Mediterranean climate. Transactions of the ASAE 40(2): 445‐450. Robert, K. A., and M. B. Thompson. 2003. Reconstructing Thermochron iButtons to reduce size and weight as a new technique in the study of small animal thermal biology. Herpteological Review 34(2): 130‐231. Schroter, R. C., and D. J. Marlin. 1995. An index of the environmental thermal load imposed on excercising horses and riders by hot weather conditions. Equine Vet. J. Suppl. 20: 16‐22. Schroter, R. C., D. J. Marlin, and L. B. Jeffcott. 1996. Use of the Wet Bulb Globe Temperature (WBGT) Index to quantify environmental heat loads during three‐day event competitions. Equine Vet. J. Suppl. 22: 3‐6. Yamamoto, S., F. Nakamasu, T. Matsumoto, B. A. Young, and B. P. Purwanto. 1994. Effect of solar radiation on the heat load of dairy heifers (cattle). Aust. J. Agric. Research 45(8): 1741‐1749.
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