Humidity Flashcards
What is humidity?
Humidity is the amount of water contained in a volumes of gas.
How is precipitation related to humidity?
The higher the humidity, the higher the chance of precipitation
I.e. more water = more chance of condensation etc.
What is absolute humidity?
The mass of water/ volume of gas
At a given temp and pressure
Measured in g/m3 or mg/L
What is relative humidity?
The relative amount of humidity present compared to the maximum amount of humidity there could be e.g. actual vapour pressure/SVP
How does mass of water compare to vapour pressure?
Mass = moles
Ideal gas law for moles would be n = PV divided by RT
For humidity, volume, R and temperature are constant and therefore moles are directly proportional to pressure
What is the effect of temperature on humidity?
Increasing temperature does not alter absolute humidity as the volume of water and volume of gas remains the same.
However, increasing temperature does increase SVP and therefore relative humidity will fall.
How does humidity effect temperature?
A change in humidity will cause a change in temperature through latent heat of vaporisation
I.e. the greater the humidity, the less likely more water is to evaporate and cause a drop in temperature
How do we measure the effect of humidity on temperature?
A wet and dry bulb thermometer.
One dry thermometer measures temp.
Another wet bulb has a with coming from water, as more evaporates the temp of the water will drop and we can measure the temperature different between the two thermometers.
At 100% SVP there will be no more drop
How are humidity and pressure related?
Humidity is inversely proportional to pressure e.g.the higher the pressure the lower the humidity.
(Think that at higher pressure, molecules are closer together and therefore there’s no space for water)
(Cloud experiment on YouTube)
Does hotter air hold more or less water?
Hotter holds more water
(Think hotter = particles are further apart = more space for water)
How does pressure effect relative humidity in terms of Dalton’s law?
Pressure has no effect on SVP. However, increasing pressure reduces volume of gas, which results in increased partial pressure of water. Therefore relative humidity will increase.
What is the dew point?
The temperature to which a volume of gas must be cooled to allow water vapour to condense out into water. E.g. the point where 100% relative humidity is achieved.
What is the dew point called if it is below freezing?
Frost point
Does a higher relative humidity require a higher or lower temperature drop to reach due point?
Higher relative humidities require less of a temperature drop to reach dew point.
What is the dew point in Britain during summer?
Around 8 degrees Celsius
What can we use to deduce the dew point?
A regnaults hygrometer
This has a silver tube with ether inside, which air is bubbled through to reduce its temp. When condensation forms on the outside, this is the dew point (100 relative humidity)
Other than a Regnaults hygrometer, what can we use to measure humidity?
Mass measurement
Temp changes from LHV
Formation of dew point
Hair hygrometer (hair porous and length increases with humidity). Hair attached to a pointer.
What is the benefit of increased respiratory tract humidity?
Less epithelial damage
Moist secretions
Prevention of mucous plugs
Improved ciliary activity etc.
What is the normal air humidity when it enters the respiratory tract vs. room air humidity?
34g/m3 vs 17 for room air.
What dehumidification droplet sizes reach different parts of the airway?
1 micron = alveoli
5 micron = trachea
20 microns = condenses on tubing
What are different methods of humidifying intubated patients airways?
HME filter
Cold water bath/bubble humidifier
Hot water bath
Nebuliser
Heated element humidifier
Cascade humidifier
How does an HME filter work?
Positives?
Negatives?
Internal paper/sponge/foam impregnated with hygroscopic substance (tending to absorb moisture). When water vapour passes through it condenses and gives latent heat to filter. Next breath is then warmed and humidified.
Positive = cheap, anti-bacterial, max = 35g/m3
Negative: increases resistance to breathing with secretions making it 70% effective
How does an cold water bath/ bubble humidifier work?
Positives?
Negatives?
Dry gas bubbles through room temp water.
Positive = cheap and easy with no power needed
Negative = loss of heat through latent heat of vaporisation, so relative humidity decreased. 30% effective. Max 10g/m3
How does an hot water bath work?
Positives?
Negatives?
Gas bubbled through heated water bath (variable to keep temp the same) preventing LHV loss. Kept at 40-45 deg.
Positive = 90% effective, 40g/m3.
Negatives = scalding and hyperthermia in kids. Water vapour can condense in the tubing, increasing resistance.
