Niepewność pomiarów średniej temperatury promieniowania za pomocą termometrów kulistych = Uncertainty of mean radiant-temperature measurements using globe thermometers

Abstract

In the analysis of the human heat balance, the biggest problem is to precisely determine the amount of energy absorbed by a human organism under dynamically changing solar conditions and while being surrounded by many different surfaces that can absorb, emit or reflect solar radiation in different ways. Therefore, in thermal comfort studies some simplifications of the radiation field around human body are necessary and thus the concept of mean radiant temperature (Mrt) is most often used. Mrt values can be derived either from direct instrumental measurements or calculated by one of the microclimate models. The most accurate way to determine the Tmrt outdoors is by integral radiation measurements and the calculation of angular factors (i.e. the proportion of radiation received by the human body from different directions). This method involves simultaneous measurements of long- and short-wave radiation flux densities in six directions, which requires three sets of net radiometers. Although this method of Mrt determination describes the radiation environment accurately and independently of the variable wind field, it requires the use of expensive and complex measurement devices. Cheap and easy to operate alternative offers globe thermometer. In steady state conditions, readings from the air temperature sensor inside the globe thermometer (tg) will reflect the convective and radiative heat exchange around the globe thermometer. Knowing tg, as well as globe emissivity and diameter, the Mrt may be calculated. In this research three different globe thermometers were used to determine Mrt: 15-cm standard globe thermometer (SGT) and two 4-cm custom made acrylic globe thermometers (AGT) painted in black and grey. The measurements were conducted in the summer in Warsaw, on two selected green areas, under sunny and warm weather conditions. In each location Mrt measurements were taken in two spots – in the shade of trees and in the open grassy spaces, with high SVF. Our results demonstrate, that globe thermometers, although being relatively cheap and less complex instruments for determining Mrt, can be used to a limited extend, due to high instability of their indications under significant radiative loads.