For thermal comfort research, globe thermometers have become the de facto tool for mean radiant temperature, tr, measurement. They provide a quick means to survey the radiant environment in a space with nearly a century of trials to reassure researchers. However, as more complexity is introduced to built environments, we must reassess the accuracy of globe measurements. In particular, corrections for globe readings taking wind into account rely on a forced convection heat transfer coefficient. In this study, we investigate potential errors introduced by buoyancy driven flow, or free convection, induced by radiant forcing of a black globe’s surface to a temperature different from the air. We discovered this error in an experimental radiant cooling system with high separation of air to radiant temperature. Empirical simulations and the data collected in a radiant cooling setup together demonstrate the influence of free convection on the instrument’s readings. Initial simulation and data show that tr measurements neglecting free convection when calculating tr from air temperatures of 2 K above tr could introduce a mechanism for globe readings to incorrectly track air temperatures. The experimental data constructed to test this hypothesis showed the standard correction readings are 1.94 ± 0.90 °C higher than the ground truth readings for all measurements taken in the experiment. The proposed mixed convection correction is 0.51 ± 1.07 °C higher than the ground truth, and is most accurate at low air speeds, within 0.25 ± 0.60 °C. This implies a potential systematic error in millions of measurements over the past 30 years of thermal comfort research. Future work will be carried out to experimentally validate this framework in a controlled climate chamber environment, examining the tradeoffs between accuracy and precision with globe thermometer measurements.
In a conventional indoor environment, thermal comfort is supplied by an air based distribution system. This system is controlled by an air temperature (and occasionally humidity) sensor, and the role of radiation in thermal comfort is often overlooked. In a typical indoor environment, slightly less than half of the heat occupants shed to maintain thermal comfort is lost to convection. The other portion is lost to radiation. We have developed the Scanning Mean Radiant Temperature (SMART) Sensor to fully characterize radiant heat transfer in the built environment. Combining surface temperatures and geometry allows us to produce 3D thermal point clouds which may be meshed to produce watertight surfaces. The view factor between occupants and environmental surfaces may be calculated, allowing us to accurately model radiative heat transfer. Additionally, this may be calculated for any location in the space, allowing us to map spatial variation of the mean radiant temperature from a single scan. In this paper, we use the SMART sensor to calculate the spatial distribution of mean radiant temperature over a range of environmental conditions. Its performance is validated using a net radiometer. The sensor demonstrates that there is frequently significant spatial variation of mean radiant temperature in typical indoor environments up to 4 °C.
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