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