humidity Flashcards
define absolute and relative humidity
humidity is the measure of amount of water present in a gas sample
absolute humidity = the mass of water vapour per unit volume (kg/m3) at specified temp and pressure
relative humidity = the percentage of mass of water vapour compared to the max mass of water vapour that could be carried at that temp (SVP)
how does absolute and relative humidity vary with temperature?
absolute - doesnt change
relative - reduces because as the temp increases, the SVP increases.
what is the SVP at 20 degrees and 37 degrees
20 = 17g/m3
37 = 44g/m3
define the dew point…
the point at which the amount of water vapour in the air exceeds the SVP and the water condenses into liquid.
i.e. relative humidity exceeds 100%
e.g. hot day in humid climate - air can hold a lot of water vapour, SVP is high. during the night the temp drops and SVP lowers, the excess water vapour condenses
what happens to the density and pressure of air as humidity increases?
H20 weighs less than N2 - so as humidity increases, density is reduced.
the less dense, the less pressure exerted
how is humidity measured?
measurement can be of relative humidity or absolute
can be divided into electrical and non-electrical
non-electrical methods tend to measure relative humidity e.g hair hygrometer, wet and dry bulb and regnaults hygrometer
electrical / absolute - mass spectrometry, humidity transducer.
how does a hair hygrometer work?
works by the principle that when humid a piece of hair will lengthen.
A hair is attached to a weight and secured at one end. this is attached to a pivot which is attached to a pointer
as the hair lengthens, it pushes the pivot and the pointer moves along a calibrated scale.
works accurately between 30-90% RH
traditionally used human hair, now cellulose or nylon used.
commonly used in theatre
describe how a wet and dry bulb hygrometer works?
2 thermometers used, one with a wet rag around and the other is dry.
the water from the wet rag will evapourate and use latent heat of vapourisation and lower the temp of this thermometer. the amount of evapouration that is possible depends on the humidity (i.e. the more humid, the harder it is to evapourate, less temp drop)
at 100% humidity - the vapour is saturated and no further water can evapourate from wet rag. therefore no difference in temp readings of the 2 thermometers.
the dry thermomometer is used as reference measuring ambient temp.
the difference between the 2 thermometers is read and a calibrated scale (psychometric chart) is used to measure RH.
how does a Regnault hygrometer work?
type of dew hygrometer
cool a sample of air until it starts to condense - note the temp of this dew point. once cooled to this temp it is 100% RH.
use a graph to work out the amount of water present at 100% RH.
can track back to see what the relative humidity would be at that temp
(the mechanism for cooling the sampel is by using an ether and blowing air through so the ether evapourates and uses latent heat to do this and hence cools)
e.g. from graph. the dew point was at 5 degrees. so at 5 degree RH = 100%. can use graph and track to see that this is equivalent to 5g/m3. then can track to see which RH this equates to at 20degrees.
this graph comes up in exam
what are the 2 methods to measure absolute humidity and how do these compare?
mas spec - very accurate and quick however bulky and expensive
transducers - electrical conduction changes with humidity and thus absolute humidity can be calculated. small and rapid.
what is the appropriate level of humidity in theatre and why?
50-60% RH
prevents static electric shock
prevents heat loss from evapouration
higher than this would be uncomfortable and increase risk of microbial growth.
why is it important to humidify inspired medical gases?
important that gases within equiptment e.g. vapouriser/ ventilator are dry - to prevent erosion, water condensing and equiptment malfunctioning
normal breathing includes nose, upper airways which gives time for warming and humidifying. in ET tubed patients this is bypassed and medical air is more dry than normal room air. therefore this would deliver dry gas to patients trachea.
however in the patients airways dry gases can result in irriation and mucosal ulceration and cilia damage. mucus plugging, atelectasis and infeciton. also results in heat and water loss as the patient has to humidify gases themselves.
what are the overall purposes of a humidification device
humidy inspired air to reduce heat and fluid loss from patient and protect patients airways from dry gas.
what is meant by hygroscopic material?
one that attracts moisture from the atmosphere
found inside HME
give an overview of methods of humidification..
active
- hot water bath
- nebuliser
- cascade humidifier
- porrous surface
passive
* HME filter
* soda lime
* bottle humidifier
* cold water bath
in general active ones are more efficient, most around 90% RH with the nebuliser achieving > 100% RH
how does a HME filter work?
contains hygroscopic material in form of filter made of paper/ foam
during expiration the fully saturated expired gas passes over the filter
warms the filter
water condenses onto the filter
during inspiration this heat and water is picked back up to humidify inspiratory gases.
overall no power needed and uses patient own heat and moisture to work
what are the pros and cons of HME filter?
pros - simple mechanism, cheap, portable , disposable. can incorporte a breathing system filter (BSF). efficiency of 70 % RH
however
efficiency reduces with time. only work for up to 24 hours
can become blocked by secretions
increases dead space and resistance to breathing.
infection risk -esp pseudomonas
how does a cold water bath humidifier work? pros and cons
sterile water reservoir at room temp
inspired gases are passed over this or bubbled through
can achieve 40% RH
pros- cheap and easy, no power
cons - not very efficient, can collect microbes in reserovir
how does soda lime act as a humidifier
reaction is exothermic
and produces H20
can produce 70% RH
how does a hot water bath humidifier work - pros and cons?
hot water in a reservoir maintained by a heating element (40-60 degrees). inspired air is passed over it and humidified
gases will leave the hot water bath via tubing and there will be a drop in temp by time it reaches patient.
therefore do get some condensation in tube
can reach RH of 90-100%
thermostat at patient end and in bath for safety
pros - very efficient , good for infection due to 60degree heat bacteria cant survive
cons - electricity, risk of scalding and elctric shocks. bulky. condensation can block tubes and increase work of breahting.
how do nebulisers work as humidifiers?
nebulisers use the venturi effect to draw water vapour in.
there is a narrowing which drops pressure and increases velocity. the pressure drop drives water droplets in.
nebulisers may be gas driven or ultrasonic.
ultrasonic will use a vibrating plate to break droplets into smaller sizes so that they can reach alvoeoli.
what are the pros and cons of nebulisers as a form of humidification?
very efficient esp ultrasonic one.
can also be used to deliver drugs
however can reach >100% humidity and cause pulmonary oedema.
if droplet size too big, wont reach alveoli.
carries infection risk.
ultrasonic is very expensive and bulky.
what is a cascade humidifier?
similar to hot bath humidifier
but gas is bubbled through the hot water through a perforated plate
more efficient as S.A is increased.
however more bulky
what is a porrous surface humidifier?
similar to hot water bath
gas flows over porrous fibres that carry water from hot water bath - due to surface tension - increases the S.A
(a bit like wicks in vapouriser)
what is water vapour?
water in its gaseous form below its critical temperature.
describe the process of humidification in normal physiology
inspired air passes over the nasal epithelium which is highly vascularised.
water evapourates from vessels and humidifies air.
100% RH is achieved by the carina
draw an isotherm for N20
isotherms show relationship between volume and pressure at different temperatures.
above critical temp - substance just a gas - boyles law - hyperbolic relatioshop
below this - the gas can be compressed to liquid without change in pressure, but as soon as all liquidified, sharp rise in pressure.
