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Mean Radiant Temperature
Mean Radiant Temperature Article 039 c/o
Dr. Andrew Marsh email:support@squ1.com,
Web:
http://www.squ1.com
"Even though not in direct contact with the body, hot
or cold objects still greatly affect our perception of
temperature. This is because they emit and absorb radiant energy
which activates the same sensory organs as conducted or convected heat. The net exchange of radiant energy between two
objects is proportional to their temperature difference
multiplied by their ability to emit and absorb heat. Mean
radiant temperature (MRT) is simply the area weighted mean
temperature of all the objects surrounding the body. It will be
positive when surrounding objects are warmer than the average
skin temperature and negative when they are colder.
Technically, MRT is defined as 'the uniform temperature of a
surrounding surface giving off blackbody radiation (emissivity e
= 1) which results in the same radiation energy gain on a human
body as the prevailing radiation fluxes which are usually very
varied under open space conditions'.
MRT is the most important parameter governing human energy
balance, especially on hot sunny days. MRT also has the
strongest influence on thermophysiological comfort indexes such
as PET (Physiological Equivalent Temperature) or PMV (Predicted
Mean Vote) which are derived from heat exchange models. The
activities of sweat glands and blood vessels in the skin are
fundamental to the maintainance of internal body temperature.
One square inch of skin contains up to 4.5m of blood vessels,
the contents of which is heated or cooled before flowing back to
influence the deep body temperature. Thus the close relationship
between radiant energy and thermal comfort.
As evidence of the importance of radiant heat exchange to the
body’s thermal equilibrium, physiologists have discovered that
living human skin has extraordinarily high absorptivity and
emissivity (0.97), greater than almost any other known
substance, matte-black metals included. Consequently, we are
highly responsive to changes in mean radiant temperature.
Measuring Radiant Temperature Radiant temperature is usually
measured with what it known as a globe thermometer. This is
simply a normal dry bulb thermometer encased in a 150mm diameter
matte-black copper sphere whose absorptivity approaches that of
the skin. Hence MRT is sometimes referred to as globe
temperature (GT) as they are nearly equivalent.
Figure 2 - A pretty crude globe thermometer used by the
author as a portable first take on environmental conditions.
Related Links INNOVA Thermal Comfort booklet
http://www.innova.dk/books/thermal/thermal.htm
Dr. Andreas Matzarakis, Estimation and Calculation of the
Mean Radiant Temperature within Urban Structures
http://www.mif.uni-freiburg.de/rayman/description.htm
David Pearlmutter - Street Canyon Geometry and Microclimate
http://www.bgu.ac.il/CDAUP/plea/plea-paper.html
Surface Area of Human Skin
http://hypertextbook.com/facts/2001/IgorFridman.shtml
Mean Radiant Temperature Website
http://www.meanradianttemperature.com/about_mrt.htm
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