eLFH - Humidity Flashcards
Humidity definition
Amount of water contained within a defined volume of gas
Why does humid climate feel more hot
High levels of water in atmosphere inhibit the evaporation of sweat
Absolute humidity definition
Mass of water vapour present in a given volume of gas at a given temperature and pressureU
Units of absolute humidity
g/m^3
or
mg/L
Both are numerically the same
Absolute humidity equation
Absolute humidity = Mass of water vapour present / Mass or Volume of gas
Relative humidity definition
Ratio of actual mass of water vapour in a gas compared to the maximum amount of water vapour that a gas could contain at the same temperature and pressure
Units of relative humidity
Percentage
or
Vapour pressure
Relative humidity equation and working out
Mass is proportional to number of moles n
n = PV / RT
Therefore is temp and pressure are constant, number of moles is directly proportional to pressure
Therefore:
Relative humidity = Actual vapour pressure / Saturated vapour pressure
Dew point definition
100% relative humidity
At this temperature, air is fully saturated with water vapour so condensation occurs
Frost point definition
When dew point falls below freezing, called frost point
Relationship between temperature and absolute humidity
Increasing temp does not affect absolute humidity as mass of water vapour is the same
Relationship between temperature and relative humidity
Relative humidity decreases with increasing temperature as the maximum possible absolute humidity to cause saturation increases with increased temp
Approximate maximum absolute humidity at 20 degrees Celsius at sea level air saturated with water vapour
17 g//m^3
Approximate maximum absolute humidity at 37 degrees Celsius (body temperature) at sea level air saturated with water vapour
44 g//m^3
Effect of increased humidity on temperature
The greater the atmospheric humidity, the less likely water is to evaporate and therefore less likely to cause drop in temperature reading as a result of latent heat of vaporisation
Effect of pressure on humidity in a closed system
At full saturation, partial pressure of water vapour = SVP
SVP is unaffected by changes in pressure
When not in full saturation, ideal gas law applies in a closed system
PV = nRT
As volume of air is compressed, water vapour increases as per Dalton’s law so relative humidity increases as SVP is unchanged
Effect of pressure on humidity in an open space
Relative humidity is directly proportional to barometric pressure until relative humidity reaches 100%, then condensation occurs
How to measure dew point
Regnault’s hygrometer
Silver tube containing Ether
Air bubbled through causing cooling
Temperature at which condensation forms on tube is the Dew point
Benefit of measuring Dew point
Relative humidity can be derived from Dew point as SVP is known
Methods of measuring humidity
Mass measurement
Temp change from latent heat of vaporisation
Pressure changes affecting partial pressure of water vapour
Moisture effect on electrical circuit
Physical change in a porous material
Formation of Dew point
Physical change in a porous material example
Hair hygrometer
Functions of airway humidification - what does it provide?
Moist pulmonary secretions
Increased ciliary activity
Improved gas exchange
Functions of airway humidification - what does it prevent?
Inflammatory change in pulmonary epithelium
Mucus plugging
Heat loss
Impaired mucociliary escalator
Atelectasis
Ideal droplet size for humidification of airways
1 micron - these can reach alveoli
