Biological Physics: Water Vapour in the Air

  • Water Vapour in the Air

    • Important in thermodynamics.

  • Key Objectives of Lecture 4

    • Understand quantities for water vapour-air mixtures:
    • Dew-point temperature.
    • Moisture content and vapour pressure relationship.
    • Use psychrometric chart for property measurement.

  • Evaporation and Body Temperature Regulation

    • Sweat production can reach up to 3 liters/hour.
    • Evaporation utilizes latent heat of water.
    • High atmospheric vapour pressure can impede sweat evaporation.

  • Dew Point Temperature

    • Defined as the temperature for water condensation.
    • Occurs when partial pressure equals saturated vapour pressure at a lower temperature.

  • Vapour Pressure of Water

    • Approximately doubles every 10 °C between 0 °C and 30 °C.

  • Moisture Content of Air

    • Defined as mass of moisture per kg of dry air (absolute humidity).
    • Applies ideal gas law to calculate using:
    • m<em>w=M</em>wPwRTm<em>w = M</em>w \frac{P_w}{RT}
    • Ratio with dry air: m<em>wm</em>a=M<em>wP</em>wM<em>aP</em>a\frac{m<em>w}{m</em>a} = \frac{M<em>w P</em>w}{M<em>a P</em>a}
    • Utilizes partial pressures to determine moisture content.

  • Relative Humidity (RH)

    • Defined as ratio of actual vapour pressure to saturated vapour pressure.
    • Measured using psychrometric chart coordinates of air temperature and moisture content.

  • Examples on Application

    • Example 20-1: Calculate moisture content at given dew point and air pressure.
    • Example 20-2: Determine RH from given dew point and dry-bulb temperature.
    • Example 20-3: Avoid mould growth by controlling wall temperature based on RH.

  • Psychrometric Chart

    • Composite graph displaying relationships among properties of moist air:
    • Dry-bulb, wet-bulb temperature, and RH.
    • Assumes standard atmospheric pressure (101.3 kPa).