ROTRONIC HygroLab - Manual (Page 34)

IN-E-HyLab-V4_11
Rotronic AG
Bassersdorf, Switzerland
Document code Unit
Instruction Manual
Document Type
HygroLab bench top indicator version 4:
instruction manual
Document title
Page 34 of 51
© 2006; Rotronic AG IN-E-HyLab-V4_11
Water activity is the same as relative humidity except for the fact that it is expressed in p.u. instead
of %RH: 100.0 %RH = 1.000 Aw.
Dew point / Frost point temperature
The dew point temperature of moist air at the temperature T, pressure P
b
and mixing ratio r is the
temperature to which air must be cooled in order to be saturated with respect to water (liquid).
The frost point temperature of moist air at temperature T, pressure P
b
and mixing ratio r is the
temperature to which air must be cooled in order to be saturated with respect to ice.
Wet bulb temperature
The wet bulb temperature of moist air at pressure P
b
, temperature T and mixing ratio r is the
temperature which the air assumes when water is introduced gradually by infinitesimal amounts at
the current temperature and evaporated into the air by an adiabatic process at constant pressure
until saturation is reached.
Vapor concentration
The vapor concentration (density of water vapor in a mixture) - or absolute humidity - is defined as
the ratio of the mass of water vapor M
v
to the volume V occupied by the mixture.
D
v
= M
v
/ V , expressed in grams/m3 or in grains/cu ft
This can be derived as follows from the equation PV = nRT:
a) M
v
= n x m
w
, where :
n = number of moles of water vapor present in the volume V
m
w
= molecular mass of water
b) D
v
= M
v
/.V = n x m
w
/ V = m
w
x p / RT, where:
m
w
= 18.016 gram
p = partial pressure of water vapor [Pa]
R = 8.31436 Pa x m3 / °K x mole
T = temperature of the gas mixture in °K
D
v
[g / m3] = p / 0.4615 x T
1 gr (grain) = 0.0648 g (gram)
1 cu ft = 0.0283168 m3
D
v
[gr / cu ft] = 0.437 x Dv [g / m3]