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15.3.2 Personal Insulation Personal insulation clothing - The thermal resistance of clothing is
expressed as a Clo value, where 1 Clo = 0.155Km
2
W. Personal insulation tends to be self-regulating in that people tend to add or remove clothing
according to their own feelings of comfort. Tables of typical Clo values for clothing assemblies are available for reference.
Clothing Clo Value
Naked Shorts
0.1 Light summer
clothes 0.5
Typical Indoor clothes
1.0 Heavy Suit
1.5 Polar Clothing
3-4 Practical
Maximum 5
15.3.3 Duration of Exposure
The duration of exposure to a thermal situation can in many cases, be varied either voluntarily or by means of workrest regimes thus reducing the risk of
prolonged exposure to heat or cold. The rest period should preferably be taken in an environment that is less extreme.
15.3.4 Dry Bulb Temperature
Dry bulb temperature air temperature is measured by a thermometer with the sensor kept dry and shielded from radiant heat.
Simple thermometer - Thermal expansion of fluid mercury or alcohol in a fine capillary tube. Inexpensive and can be accurate. Limited temperature ranges,
fragile, can be slow to respond. Electrical devices - e.g. thermistor or thermocouple, can be robust accurate
and convenient. Often built into anemometer devices or thermal meters.
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15.3.5 Mean Radiant Temperature
Mean radiant temperature is the hypothetical temperature of a uniform black enclosure which would exchange the same amount of radiant heat with the
body as the non-uniform enclosure. Pyrometers or thermopiles - Directional devices which when pointed at a
surface of known emissivity can be used to determine radiant temperature of that surface. With sufficient data the mean radiant temperature can be
calculated. Globe thermometer - A black copper sphere with a simple thermometer
projecting into its centre. When the air temperature and air velocity are known the mean temperature of surroundings can be calculated using basic
calculations, or with the help of nomograms.
15.3.6 Air Velocity
Heat will be removed from the body by convection when an air current is passed over it unless the air temperature is higher than the temperature of the
skin. Air movement will also affect the rate of evaporation of moisture from the skin unless the air is 100 saturated, or its vapour pressure is greater than
that on the surface of the skin. Vane anemometer - propeller type. directional, electrical or mechanical. Can
be used for measuring fluctuating, uni-directional air flows Resistance anemometer - sensitive, fragile, uni-directional device.
Kata thermometer - an alcohol in glass thermometer with a large silvered bulb at its base and a small bulb to the top. It is heated until the liquid expands into
the top bulb, heat is then removed to allow the airflow to cool it. As the liquid contracts back into the lower bulb its fall is timed between two marks on the
stem. The air velocity can be calculated from this cooling time. Tracer smoke - extremely valuable for visualising air flow and measuring very
low velocities.
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15.3.7 Moisture Content
