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

· Humidity

· Normal values

· Hygrometer types

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

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