2008B09 What is humidity and how can it be measured?

Definitions

·      Humidity: an index of the water vapour content of a gas mixture

·      Absolute humidity: is amount of water / volume of air

·      Relative humidity: is actual partial pressure / saturated vapour pressure (at that temp)

Importance

(no marks for this last time)

·      Prevent water loss (by evaporation)

·      Prevent heat loss (latent heat of vapourisation)

·      Prevent crusting of secretion

·      Maintain mucociliary function hence sterility

Temperature (°C)

Absolute (g/m³)

Relative

Room air

20

7

40%

Nasopharynx

30

30

95%

Lungs

37

43

100%

Hair hygrometer

·      Hair under constant tension

·      Hair is hygroscopic = absorbs moisture

·      ↑ humidity -> ↑ hair length -> movement of dial

·      Most accurate between 20-90% relative humidity

·      Pros: simple and cheap

·      Cons: slow response, difficult to couple to electrical circuit

Wet and dry bulb hygrometer

·      Dry thermometer measures ambient temp

·      Wet thermometer surrounded by a wick

·      Evaporation causes cooling (latent heat)

·      Ambient temperature and wet-dry temp difference are used to estimate relative humidity from a reference table

·      Accurate between 30-90% relative humidity

·      Pros: simple and cheap

·      Cons: slow response, evaporation accelerated if airflow

Regnault’s hygrometer

·      Silver tube for high thermal conductivity

·      Bubbling air evaporates ether

·      Ether temp falls

·      Dew point: temp at which relative humidity of surrounding air is 100% and condensation occurs on tube

·      SVP for different temperatures is known in tables.

·      Relative humidity = actual vapour pressure / SVP ambient = SVP dew point / SVP ambient

·      Tables used to correlate temperature, absolute and relative humidity

·      Pros: simple and cheap

·      Cons: ether pollution

Electrical hygrometers

·      Multiple devices

·      e.g. resistance, capacitance, electronic dewpoint

·      Most measure both absolute and relative humidity