Humidity Flashcards
Define humidity?
Humidity is the amount of water contained within a defined volume of gas
What is the difference between absolute and relative humidity?
Absolute humidity is mass of water vapour present in a given volume of gas at a given temperature and pressure. It is expressed as mg/L or g/m3.
Relative humidity is the ratio of the actual mass of water vapour present in a volume of gas compared to the maximum amount of water vapour that the gas could contain.
What is the formula for relative humidity?
Actual vapour pressure/saturated vapour pressure
What is the relationship between temperature and humidity?
Temperature does not affect absolute humidity as the mass of water vapour will be the same.
However as temperature increases (saturated vapour pressure (SVP) increases. Relative humidity is actual vapour pressure/SVP, therefore as temp increases relative humidity decreases.
At sea level what is the SVP of water at 20 degrees and 37 degrees?
At sea level, 20°C, SVP = 17g/m3
At sea level, 37°C, SVP = 44g/m3
Explain how a wet and dry bulb thermometer can be used to measure humidity?
As water evaporates heat is lost due to latent heat.
In a more humid environment water is less likely to evaporate.
A wet and dry bulb thermometer can be used to calculate relative humidity.
A dry thermometer measures the temp. A wet thermometer with a wick is in water, as the water evaporates the temp of the wet thermometer drops.
Comparing the temp of the wet and dry thermometer you can extrapolate the relative humidity.
How does pressure affect humidity?
Consider Boyle’s law Pressure*Volume is constant.
Therefore if you increase pressure then the volume of a gas will decrease.
If the volume of the gas decreases but the mass of water vapour stays the same the the absolute humidity will increase and therefore the relative humidity will also increase.
What is the dew point?
The temperature to which a given volume of gas must be cooled in order for water vapour to condense out into water – this is the point at which 100% relative humidity is achieved.
The higher the relative humidity, the less temperature drop is required to reach dew point and vice versa.
Remember SVP increases with temp and conversely drops as temp drops.
How does a Regnault’s Hygrometer work?
Air is bubbled through a silver tube containing ether. As air is bubbled through the temperature drops. The temperature at which condensation occurs on the outside of the tube is the dew point i.e. 100% relative humidity.
List the different ways humidity can be measured? (6)
Mass measurement
Temperature change from latent heat of vaporization (wet/dry bulb thermometer)
Pressure changes affecting the partial pressure of water vapour
Effect of moisture on an electrical circuit
Physical change in a porous material such as in the hair hygrometer
Formation of the dew point
Why is humidification important in the respiratory tract?
Moistens secretions
Prevents mucus plugs
Improves gas exchanges
Prevents ciliary dysfunction
Why is humidification an issue in-invasively ventilated patients?
Normally, air entering the upper trachea has 34 g/m3 and is humidified in the oro and nasopharynx.
In invasively ventilated patients the mucosal membranes of the pharynx are bypassed and piped gases are dry and minimally humidified.
What are the methods of humidification?
HME filter
Cold water bath/bubble humidifier
Hot water bath
Heated element humidifier
Cascade humidifier
Nebuliser
How does a HME (humidity and moist exchanger) filter work, what are the advantages and disadvantages?
It has an internal paper/sponge/foam impregnated with a hygroscopic substance (i.e. absorbs moisture).
When water vapour passes through, it condenses and provides latent heat to the HME so that the next inhale will warm the
cooler inspired air and absorb the moisture.
A bacterial and viral filter can be added to the HME.
They are cheap, however they can cause increased resistance leading to higher ventilatory pressures.
How does a cold water bath/bubble humidifier work, what are the advantages and disadvantages?
Most commonly used in the ward setting.
Dry gas is bubbled through water at room temperature.
Advantages: Cheap and easy to run with no power source required.
Disadvantages: There is a loss of heat through latent heat of vaporisation so relative humidity is reduced. It is 30% efficient due to this and the formation of large bubbles.
