Abstract
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.
Highlights
For thermal comfort research, globe thermometers have become the de facto tool for mean radiant temperature, tr, measurement
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
The magnitude is smaller than air speeds above 0.1 m/s, the plateau shape of the free convection response presents certain challenges for interpreting the true tr from globe readings in a free convection dominated scenario
Summary
Globe thermometers have become the de facto tool for mean radiant temperature, tr, measurement. Local climate dictates air conditioner or furnace sizing based on the humidity and air temperature based loads These systems fail to engage with radiation as an independent mechanism for comfort, despite equal radiative and convective fluxes in typical office environments[4]. We propose a reevaluation of the way in which radiant exchange and mean radiant temperature are measured using globe thermometers in the built environment to improve contemporary building thermal comfort and efficiency analyses. Globe thermometers operate on an assumed principle that a thin, near mass-less blackbody sphere will interact radiatively with all surfaces surrounding it and arrive at an equilibrium temperature near or equal to the mean radiant temperature of its surrounding environment This type of measurement technique rose to prominence in 1934 when a method of correcting measurements for wind speed was published[9], and has remained unchanged. Other researchers have previously noted potential sensitivities of black globe readings to the emissivity of their often visibly black coating, along with an inability to resolve spatial or directional variations in the readings and a high sensitivity to the air velocity measurements that can turn the device into more of an anemometer than a radiant temperature sensor[11]
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