The air velocities in the wind tunnel were measured using a portable digital thermo-anemometer Itan with an air velocity scale between 0. To measure the temperature of the spheres, the mercury thermometer was inserted up to the centers of the spheres. The hysteresis and thermal inertia of the spheres determined in the wind tunnel were used to define the black globe temperature T bg considering a sphere of 15 cm diameter as standard.
These measurements were performed under field conditions at hourly intervals, from 10 am to 5 pm, to characterize the errors between the readings and their possible causes. Thus, the black globe thermometers were evaluated under indoor environment conditions, i. Thereafter, they were evaluated under outdoor environment conditions, i. The air velocities and temperatures and black globe temperatures T bg were measured with the same instruments used for the wind tunnel measurements.
However, the air temperature was determined using a sling psychrometer rotating handle. The hysteresis and thermal inertia were defined in function of the repetition of measurements and by the sensitivity in the definition of this value after three repetitions for each velocity. The response variable was the black globe temperature T bg and the schedules, the blocks. The type of sphere in use is specified in the graphs. The graphs show similar behavior for all spheres, with the maximum heating achieved as a function of the velocity of the heated air.
The values of thermal inertia were approximately 24, 20, and 17 min for the hollow copper sphere; 19, 18, and 16 min for the hollow brass sphere; 23, 20, and 19 min for the hollow plastic sphere with For hysteresis, it was found that it required a long time for the spheres to reach the initial temperature by cooling and the condition was rarely achieved, i. The values of hysteresis for the hollow copper sphere were approximately 28, 25 and 20 min with respect to air velocities of 1.
The values obtained were approximately 22, 22, and 18 min for the hollow brass sphere; approximately 26, 24, and 20 min for the hollow plastic sphere with Figure 3 shows the variation of the black globe temperature T bg with respect to time under indoor environment conditions, i.
Under these conditions, the globe thermometers presented small variations, of less than 1. It was confirmed that the black globes with the evaluated spheres presented identical readings during the course of the day. Figure 4 shows the variation of the black globe temperature T bg with respect to schedule under outdoor environment conditions, i. It can be verified that the black globe measurements follow a similar behavior with respect to external air temperature variations.
There were no statistical differences among the black globe measurements only at 1 pm and 5 pm. The measurements conducted by BG-Sbco commercially available black globe thermometer instrument with digital display differed statistically from those of the black globe standard BG-Ssta at 10 am and 4 pm. The other alternatives to the black globe thermometer, i.
Under turbulent wind conditions, the black spheres Sbco, Spbw and Spbp cool faster compared to Ssta, which could lead to erroneous interpretations and readings for the analysis if the measurements are conducted at that moment.
Much of this analysis of the measurements of T bg , in particular, the different response times, can be related only with the thermal inertia and hysteresis of the spheres, variables obtained at temperature and velocity of the air controlled. Hysteresis is the ability of a body to preserve a deformation effected by a stimulus and thermal inertia is the ability of a material to store heat and to restore slowly. Comparing and contrasting the measurements conducted by the standard black globe thermometer, assuming it to be a reference point, the correction of the values measured by the other spheres is necessary.
Similar results were obtained by Coelho et al. When assessed in the wind tunnel, black spheres of different materials and diameters presented different response times, as indicated by the determination of thermal inertia and hysteresis. The hysteresis using the copper sphere of 15 cm diameter as reference was evaluated as 28 min. Under indoor environmental conditions, i. However, under outdoor environment conditions, i. The J. Thorsson, S. Different methods for estimating the mean radiant temperature in an outdoor urban setting.
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We thank all of the staff and researchers involved in this project, particularly the facilities staff at United World College Southeast Asia for the support in kind of our radiant cooling experiments in the tropics. The original content was unpublished and covered a limited scope of the modeling presented in this current paper. You can also search for this author in PubMed Google Scholar. All authors reviewed the manuscript. Correspondence to Eric Teitelbaum. Authors E. Reprints and Permissions.
Globe thermometer free convection error potentials. Sci Rep 10, Download citation. Received : 08 May Accepted : 29 January Published : 14 February Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.
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Abstract For thermal comfort research, globe thermometers have become the de facto tool for mean radiant temperature, t r , measurement. Introduction We have discovered that millions of mean radiant temperature measurements made using globe thermometers could be systematically flawed, impacting thermal comfort research, building controls and modeling. Results Simulation results Comparing the contributions from both free and forced convection to the globe temperature, as the air velocity increases, forced convection will dominate the reading for t g.
Figure 1. Comparison between heat lost due to free and forced convection. Full size image. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Discussion The results are analyzed further in Fig. Figure 7. Full size table. Figure 8. Figure 9. Figure Methods Empirical model To assess the basic contributions of forced and free convection about a black globe, Nusselt number correlations were obtained. References 1. Google Scholar 2. Google Scholar 4. Google Scholar 7. Google Scholar 9. Article Google Scholar CAS Google Scholar Google Scholar Article Google Scholar Download references.
Acknowledgements We thank all of the staff and researchers involved in this project, particularly the facilities staff at United World College Southeast Asia for the support in kind of our radiant cooling experiments in the tropics. View author publications.
Ethics declarations Competing interests Authors E. About this article. With the recent trend of data-driven agriculture, there has been increasing demand for a thermometer that can be easily used outside. Conventional thermometers for outside use require a radiation shield with ventilation to avoid the impact of solar radiation.
When measurements are performed without them due to the limit of space or mains power availability, direct solar radiation will affect the outcome to become higher than the actual value. However, mains power is not always available in rural areas, such as farmlands and forests. There had been a strong demand for a thermometer that can be used in such places. To solve this problem, NARO has developed a three-globe thermometer which does not require a radiation shield or ventilation.
Instead, a simple calculation is used to obtain the proper air temperature by eliminating the effect of solar radiation based on the temperatures of multiple spheres with different sizes. The formula used in the calculation had been derived from heat balance and heat transfer theory for a spherical surface in the external flow.
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