ROTRONIC HygroLab - Manual (Page 35)

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 35 of 51
© 2006; Rotronic AG IN-E-HyLab-V4_11
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]
Q [g / kg] = 1000 p / (1.6078 P
b
– 0.6078 p)
1 gr (grain) = 0.0648 g (gram)
1 lb = 0.4535923 kg
Q [gr / lb] = 7 x Q [g / kg]
Mixing ratio by weight
The mixing ratio r of moist air is the ratio of the mass M
v
of water vapor to the mass M
a
of dry air
with which the water vapor is associated:
r = M
v
/ M
a
M
v
= n x m
w
= m
w
x p V / RT
M
a
= n x m
a
= m
a
x p
a
V / RT = m
a
x (P
b
– p) / RT, where:
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]
R = 8.31436 Pa x m3 / °K x mole
T = temperature of the gas mixture in °K
r = m
w
p / m
a
(P
b
– p)
r = 621.97 x p / (P
b
– p) [g / kg]
1 gr (grain) = 0.0648 g (gram)
1 lb = 0.4535923 kg
r [gr / lb] = 7 x r [g / kg]