• Equipment 4
• humidity
• absolute
• mass of water vapour per unit volume of air at a specified temperature and pressure (g/m³ or mg/L - same value!)
• measurement
• transducer
• measures change in resistance as a substance absorbs water
• UV light absorption
• mass spectrometry
• relative
• partial pressure of water / SVP of water at certain temperature and pressure (%)
• 50% in theatre
• higher is uncomfortable
• lower increases risk of sparks
• measurement
• hair hygrometer
• most accurate at 30 - 90% humidity
• 5 minute lag
• wet and dry bulb hygrometer
• require air movement
• dew point hygrometers
• Regnault’s
• air bubbled through a tube containing ether, temp at which dew forms = SVP
• SVP at dew point / SVP at ambient temp = relative humidity
• electronic
• electronic sensors
• capacitative
• resistive
• increasing temperature decreases relative humidity but doesn’t change absolute
• humidifiers
• passive
• cold water
• max humidity = 10g/m³
• soda lime
• max humidity = 29g/m³
• HMEF
• hydrophobic
• aluminium core
• hydroscopic
• insulator impregnated with hydroscopic salts
• more efficient
• combined
• modern
• most efficient
• increase resistance by 0.1 - 2 cm H20
• max humidity = 30g/m³
• active
• warm water (45 C)
• max humidity = 40g/m³
• gas nebulised
• max humidity = 60g/m³
• venturi effect
• 5uM droplets (fall in trachea)
• ultrasound nebulised
• max humidity = 90g/m³
• 2-3 MHz vibration produces water droplets of 1uM (can reach alveoli)
• Airways
• Room air 20C = 17g/m3
• Upper airways at 34C = 34g/m3
• Lower airways at 37C = 44g/m3