IN-E-HyFlex-V4_10
Rotronic AG
Bassersdorf, Switzerland
Document code
Unit
Instruction Manual
Document Type
HygroFlex transmitter version 4: instruction
manual
Document title
Page
50
of 61
© 2006; Rotronic AG IN-E-HyFlex-V2_10.doc
Wet bulb temperature
The wet bulb temperature of moist air at pressure Pb, 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]
Specific humidity
The specific humidity (also known as mass concentration or moisture content of moist air) is the
ratio of the mass M
v
of water vapor to the mass (M
v
+ M
a
) of moist air in which the mass of water
vapor M
v
is contained.
Q = M
v
/ (M
v
+ M
a
)
Q = p m
w
/ (p m
w
+ (P
b
– p) m
a
)
m
w
= 18.016 gram
m
a
= 28.966 gram
p = partial pressure of water vapor [Pa]
p
a
= partial pressure of dry air [Pa]
P
b
= total or barometric pressure [Pa]
