Humidity & Measurement
Absolute Humidity
- Actual amount of water vapour contained in
a given volume of gas at a given temperature & pressure
- Expressed in grams of water vapour per
cubic metre gas (eg mg H2O/l gas)
Relative Humidity
- Actual amount of water present in the gas
expressed as a percentage of the amount of the same volume of gas would
contain at the same temperature & pressure if it were fully saturated
- The amount of water vapour required to
fully saturate a given volume of gas increases with temperature — eg air
which is fully saturated at 20°C will only be 40% saturated when warmed to
37°C (usually the additional water vapour to fully saturate inspired air
is added during its passage through the upper airways & nose)
Wet & Dry
Bulb Thermometer
- 2 mercury-in-glass thermometers are mounted
side-by-side
- One bulb is exposed to air, the other is
surrounded by a small wick that dips into a water reservoir
- The temperature of the wet bulb depends on
the rate of evaporation and hence the humidity of the air
- When humidity is high, the rate of
evaporation is less and the temperature difference between the two bulbs
is less
- The relative
humidity is determined from tables which relate the dry & wet bulb
temperatures to humidity