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
Most common method of airway humidification in intubated patients
HME filter
Heat and moisture exchange
Mechanism of HME
Internal paper, sponge or foam is impregnated with a hygroscopic substance (absorbs water from air)
Water vapour passing through condenses and provides latent heat to the HME
Next breath is warmed and absorbs moisture collected by HME
Advantages of HME
Disposable
Cheap
Can incorporate a bacterial filter
Disadvantages of HME
Secretions collected in mesh can increase resistance in breathing circuit
Efficiency of HME
~70%
Most commonly used humidification system on wards
Cold water bath / Bubble humidifier
Mechanism of cold water bath / bubble humidifier
Dry gas is bubbled through water at room temperature
Advantages of cold water bath / bubble humidifier
Cheap
Easy to run
Gas driven - no power source required
Disadvantages of cold water bath / bubble humidifier
Loss of heat through latent heat of vaporisation reduces maximum obtainable humidity
Efficiency of cold water bath / bubble humidifier and why
~30% due to large bubbles and heat loss
Commonly used humidification system on ITU
Hot water bath
Mechanism of hot water bath
Dry gas bubbled through water but water is heated
Requires thermostatic control at both the humidifier and the patient to prevent scalding
Temperature of hot water bath
40 - 45 degrees Celsius
Advantages of hot water bath
Greatly improved efficiency
No heat loss via respiratory system
Disadvantages of hot water bath
Risk of scalding
Hyperthermia possible in young children
Water vapour can condense in ventilation tubing increasing resistance in breathing circuit
Efficiency of hot water bath
~90%
Mechanisms of Nebulisers
Gas driven
Ultrasonic
Mechanism of gas driven nebuliser
High flow gas ejected close to a tube filled with nebuliser fluid
Dorp in pressure created by gas (Bernoulli effect) leads to entrainment of the fluid
Fluid stream hits an ‘anvil’ causing droplets to divide
Mechanism of ultrasonic nebulisers
Fluid dropped onto vibrating plate at ultrasonic frequency, producing tiny droplets
Advantages of nebulisers
Full humidification
Disadvantages of nebulisers
Some droplets vaporise using latent heat so heater should be added
Efficiency of nebulisers
100%
Highest with ultrasonic nebulisers
Two other humidifiers rarely used now
heated element humidifiers
Cascade humidifier
Mechanism of heated element humidifier
Water vaporised by dripping onto heated element at 100 degrees Celsius
High risk of burns
Advantages of heated element humidifier
High temp ensures sterility
Disadvantages of heated element humidifier
High risk of burns
heat may alter chemical properties of anaesthetic / medicinal gases
Efficiency of heated element humidifier
100%
Mechanism of cascade humidifier
Gas bubbled through a perforated screen at the base of a tube within a heated water bath causing mixing within the tube
Refined hot water bath system
Advantages of cascade humidifier
Large surface area ensure full humidification
Avoids risk of burns
Disadvantages of cascade humidifier
Bulky equipment - reduces practical use
Efficiency of cascade humidifier
100%
