Development of globe thermometers for measuring radiation and air speed

Abstract

Rapid urbanisation results in more complex and diverse alterations in city morphology. The urban microclimate is strongly influenced by terrestrial structures, which result in significant spatial non-uniformity and temporal fluctuations of the outdoor thermal environment. The study of outdoor thermal comfort has become an appealing academic topic alongside conventional research on the urban environment. A fundamental approach in the investigation of local outdoor phenomena is the fixed-point field measurement conducted according to the ‘Euler’ concept. Unfortunately existing meteorological sensors are either extremely cumbersome or single-function. There is a lack of convenient and economical measurement techniques, which greatly restricts high-density and massive field experiments.

This study is motivated by a desire to facilitate the on-site observation of outdoor micrometeorological parameters that play the most important role in human thermal sensation. The globe thermometer has the potential to be developed into an integral instrument, as it measures the mean radiant temperature by responding to the combined influence of multiple environmental parameters. There have been a few impressive early attempts at its function extension. But being a simple apparatus originally invented for use indoors, its application in outdoor settings has sparked wide controversy in terms of accuracy. This thesis conducts a systematic investigation of how to obtain the correct globe temperature according to a comprehensive understanding of its principle and how to interpret implicit information using the wavelet method to extend the function of a globe thermometer.

The thermal response of a globe thermometer is determined by environment parameters and influenced by its configuration. To ascertain the impact of intrinsic factors, time-domain observations were conducted in both static indoor conditions and the dynamic outdoor environment. The frequency-domain coupling between globe temperature and environmental parameters were tested by continuous wavelet transforms. The wind and radiation patterns which influence the globe’s thermal response were identified by discrete wavelet decomposition and correlation analysis.

The effects of surface colour (emissivity), globe size, and material were generalised. The experimental results challenge the suitability of the commonly used acrylic globe thermometer. This study puts forward an emendation of the conventional calculation equation by redefining it in the frequency space directly related to radiant factors. The globe temperature couples with radiation and wind speed in distinctly different spaces of the frequency-domain, which suggests the causality between them exists in separate frequencies. The implicit information hidden in the fluctuant globe temperature is thus extracted, and it is used for empirical estimation of local radiant fluxes and the mean wind speed.

By clarifying the essential configurations, this study corrects some common mistakes in the outdoor application of globe thermometers. For the first time, time-series of globe temperatures were analysed by wavelet transforms, based on which the accuracy of estimated mean radiant temperature is improved. An empirical method for estimating the local shortwave and longwave fluxes and wind speed by normal-sized, non-heated globe thermometers is proposed. It provides a heuristic method for future development of globe thermometers for multi-parameter outdoor observations.