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All about humidity |
 • Humidity theoretical
approach
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All about humidity
Humidity theoretical approach
HUMIDITY TERMS, DEFINITIONS AND UNITS
| Term |
Definition |
Unit (IS) |
Comment |
| Mixing Ratio |
Mass, Water vapor
Mass, Dry air |
kg/kg |
also in g/kg or
lb/lb |
| Absolute Humidity |
Mass, Water vapor
Volume, |
kg/m3 |
also in g/m3 or
grains/ft3 |
| RELATIVE HUMIDITY |
Actual water vapor pressure
Saturation water vapor pressure |
% |
Can be also the masse ratio. |
| Dew Point |
Temperature of condensation
(saturation) |
K |
also °C or °F |
The pressure of a gas can be defined as the sum of the partial pressures created by each of the components entering in the gas composition. This is DALTON's law.
Ptotal = P1 + P2 + ..........+ Pi.
with
P1 : pressure created by the component 1 alone
P2 : pressure created by the component 2 alone and so one...
The partial pressure of a pure body is linked only with its temperature, if it is in equilibirum with the liquid phase.
Generally, a good approximation is obtained by using the partial pressure equation, named Antoine's equation :
log 10(p)=A-B/(t+C)
with p in mm Hg, t in °C, and A, B and C as a function of each particular body.
The partial pressure as defined above is the saturation vapor pressure.
This pressure value is the image of the maximum quantity of a body under its gaseous form which can be found in a gaseous mixture for a given temperature.
One can compare the partial vapor pressure which is found without the associated liquid phase relatively to the saturation vapor pressure.
The RELATIVE HUMIDITY is defined as the ratio of the partial pressure vapor to saturation vapor pressure at a given temperature.
By measuring the relative humidity, we compare the actual quantity of water vapor to the maximum quantity of water vapor which can be found in these conditions of ambient temperature and pressure.
Relative Humidity (RH) is the most commonly used measurement in the humidity arena.
However, other ways can be used in order to quantify or describe the humidity parameter :
DEW POINT is the temperature at which the wet gaseous atmosphere can not be held in equilibrium in its gaseous form only. It will characterized by the emergence of a liquid phas.
The dew point value is the image of the water vapor concentration.
Do not mistake dew point and wet bulb temperature which is linked with the use of a psychrometer.
ABSOLUTE HUMIDITY is the water vapor concentration in the air. This concentration value is either linked with the volume (in kg/m3, or g/m3 or grains/ft3) or linked with the mass (in kg/kg or g/kg or lb/lb). In the latter case, it will be named the MIXING RATIO.
Very often, the method used to express humidity will be choosen as function of the application. This choice will induce the type of sensor to be used whenever. Two of the three parameters (Relative Humidity, Dew Point, Ambient Temperature) are known, the third one can be calculated by using the partial laws.
Knowing the Dew point and the Ambient Temperature, the Relative Humidity can be calculated.
Knowing the Relative Humidity and the Ambient Temperature, the Dew Point can be calculated.
Knowing the Relative Humidity and the Dew point, the Ambient temperature can be calculated, allowing for determine a safety margin versus condensation. However, a temperature sensor is more appropriate for this application.
The Concentration in water vapor value can be calculated by using the partial pressure equations.
The Absolute Humidity in Volume can be calculated when the concentration in water vapor is known.
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