| Dalton’s
Law (Pm)
John
Dalton was the first to surmise that the total pressure (Pm) exerted
by a mixture of gases or vapors is the sum of the pressures of each
gas if it were to occupy the same volume individually. The pressure
which each gas component of a multiple constituent gas (such as
air) exerts is called its partial pressure. If Px, Py, and Pz represent
the respective partial pressure of gases X, Y, and Z in a mixture,
Dalton’s Law states:
Pm
= Px + Py + Pz + ...
Elementary
as it may seem, the concept of Dalton’s Law is often overlooked
when considering problems in humidity, because one forgets that
the “water vapor” in a gas is actually a gas itself
and must therefore be treated in accordance with the gas laws. Air
must be considered a mixture of gases: oxygen, nitrogen, and water
vapor (neglecting the minor constituents). All discussions of humidity
can then be reduced to discussions of water vapor pressure, and
all definitions encountered in humidity can be expressed in terms
of vapor pressure.
Dew
Point (DP)
Dew
point is that unique temperature to which the air (or any gas) must
be cooled in order that it shall be saturated with respect to water.
Frost
Point (FP)
Frost
point is that unique temperature to which the air (or any gas) must
be cooled in order that it shall be saturated with respect to ice.
The
dew point or frost point DEFINES the partial pressure of the water
vapor in the gas, from the Smithsonian Meteorological Tables.
Relative
Humidity (RH)
Relative
Humidity (RH) is the ratio of the actual vapor pressure in the mixture
to the saturation vapor pressure, with respect to water, at the
prevailing dry bulb temperature.
NOTE:
RH is arbitrarily defined with respect to water even though it seems
that it should be with respect to ice at -40°C (-40°F).
PPM
by Volume
Parts
per million (PPM) by volume is the ratio of the partial pressure
of the water vapor to the partial pressure of the dry gas.
PPM
by Weight
Parts
per million (PPM) by weight of dry gas is identical to PPM by volume
except that the weight ratio changes with the molecular weight of
the carrier gas.
 For
the complete datasheet with examples and
conversion
charts please click here. |
