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HUMIDITY
www.propofology.com
Dr. David Lyness
@Gas_Craic
Absolute humidity
The absolute amount of water vapour in a gas expressed in either mg/L of gas mixture or mmHg (partial pressure).
The absolute amount of water vapour in a gas expressed in either mg/L of gas mixture or mmHg (partial pressure).
MEASUREMENT
Wet and dry bulb hygrometer
A system using two thermometers, one with a wet and the other a dry bulb. Air movement over the wet bulb causes evaporative cooling, generating a difference in temperature readings. This difference relates to the rate of airflow over the wet bulb and the relative humidity. Tables are used to look up the relative humidity from the two temperatures.
Regnault’s (dewpoint) hygrometer
Using a precisely cooled shiny plate, the user observes the temperature at which condensation first occurs. At this temperature, the gas is fully saturated with water, hence both the water content and the relative humidity at any other temperature can be ascertained from a vapour pressure table.
Mass spectroscopy
Mass spectroscopy can be very accurate but only if condensation does not occur in the sample line. This is the best technique for assessing "in-circuit" humidity, as it can assess breath by breath changes.
Humidity transducers
Transducers are available in which the electrical conductivity of a membrane changes with water vapour pressure.
A system using two thermometers, one with a wet and the other a dry bulb. Air movement over the wet bulb causes evaporative cooling, generating a difference in temperature readings. This difference relates to the rate of airflow over the wet bulb and the relative humidity. Tables are used to look up the relative humidity from the two temperatures.
Regnault’s (dewpoint) hygrometer
Using a precisely cooled shiny plate, the user observes the temperature at which condensation first occurs. At this temperature, the gas is fully saturated with water, hence both the water content and the relative humidity at any other temperature can be ascertained from a vapour pressure table.
Mass spectroscopy
Mass spectroscopy can be very accurate but only if condensation does not occur in the sample line. This is the best technique for assessing "in-circuit" humidity, as it can assess breath by breath changes.
Humidity transducers
Transducers are available in which the electrical conductivity of a membrane changes with water vapour pressure.
Hair Hygrometer
Principle that the length of the hair increases with increasing humidity. Accurate between 30 and 90%.
Relative humidity
Amount of water vapour in a gas expressed as a % of that which could be held by the gas if it were fully saturated at the same temperature, i.e. R.H. = Actual water content / Water content fully saturated %, or
R.H. = Actual vapour pressure / Saturated vapour pressure %
Amount of water vapour in a gas expressed as a % of that which could be held by the gas if it were fully saturated at the same temperature, i.e. R.H. = Actual water content / Water content fully saturated %, or
R.H. = Actual vapour pressure / Saturated vapour pressure %
PARTICLE SIZES
20μm = deposited in the tubing or upper respiratory tract
5μm = fall out in the tracheal region
1μm = pass through to alveoli and are deposited - not very useful
Relative Humidity in a theatre should be between 50-70%
Hot Water Baths- have many disadvantages, including cost, storage requirements, servicing, circuit complexity, water rain-out leading to monitor malfunction or tubing occlusion, extra risk of disconnection, infection hazard, potential for burns and drowning, and altered circuit compliance.
HME filters prevent particulate matter entering the patient but also humidify the air.
The HME is warmed by the latent heat of water condensing on it.
This heat is also released during subsequent inspiration.
Particles >1mcm will not pass through the pores.
Below this and <0.5mcm - electrostatic, inertial impact and diffusional interception will prevent movement of those particles. Pore size is 0.3 microns.
