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
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.
