Globe temperature
The globe thermometer was introduced by Vernon in 1930 as a means of assessing the combined effects of radiation, air temperature and air velocity on human comfort.
It consists of a hollow copper sphere painted matt black to absorb radiant heat, with a temperature sensor at its centre. When it reaches a steady state (after 15 minutes or so depending on the size of the globe and the environmental conditions) the heat exchanges by convection and radiation will be in equilibrium, and the temperature recorded by the sensor will be somewhere between the air and radiant temperature. This is referred to as the globe temperature (tg) or black globe temperature and resembles the thermal conditions felt by the human body.
The standard sphere has a diameter of 150 mm. Other sizes do exist, but the smaller the diameter of the globe, the greater the effect of convection on the temperature recorded.
From the temperature recorded, the mean radiant temperature (mrt) in can be calculated:
MRT = tg + 2.42 x air velocity in m/s (globe temperature – air temperature)
(equation ref Novalynx Corporation 2010 - this equation is dependent on the size and emissivity of the globe thermometer, and the units they show for air velocity are incorrect - cm/s, rather than the correct m/s shown above)
Mean radiant temperature is a measure of the average temperature of the surfaces surrounding a body.
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