Thunder Scientific 2500 series, Руководство по эксплуатации и обслуживанию

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1-5
It can now be seen by inspection of the relative humidity formula, expressed in its final form
in equation 3, that known relative humidities may be accurately generated, using air, through
measurement and control of pressure and temperature alone.
1.2.4 Pressure Ratio
The term
P
c
P
s
, in equation 2 of section 1.2.2 and equation 3 of section 1.2.3, is simply the
ratio of the chamber pressure to the saturator pressure. This is the "idealistic" portion of the
relative humidity formula which ignores minor temperature differences between the saturator
and chamber. It also assumes that moist air behaves as an ideal gas. This ratio closely
approximates the actual relative humidity, and is often used alone to express the humidity
when ease of calculation outweighs the need for the additional accuracy provided by the
temperature and pressure corrections. The pressures
P
c
and
P
s
are measured directly through
the use of high accuracy absolute pressure transducers.
1.2.5 Effective Degree Of Saturation
The term
e
w
(T
s
)
e
w
(T
c
)
, in equation 2 of section 1.2.2 and equation 3 of section 1.2.3, referred to as
the "effective" degree of saturation, is a temperature correction ratio which accounts for
super-saturation and under-saturation of the gas in the saturator, at temperature T
s
, with
respect to the chamber temperature, T
c
, due to any small temperature differences which may
exist between them. When the chamber and saturation temperatures are identically matched,
the effective degree of saturation is exactly equal to 1.0 and no temperature correction takes
place. To obtain the effective degree of saturation when temperature differences do occur,
the saturation vapor pressures at the saturation and chamber temperatures,
T
s
and
T
c
, must be
calculated. There have been several formulas derived which express the saturation vapor
pressure as a function of temperature. However, the most accurate formulation currently
available for the limited range of 0 to 100 °C is that of Wexler, given as
e
w
(T) = exp
( ) ( )








+ ⋅ +








+
−
=
∑
15 . 273 ln 15 . 273
2
6
0
T D T C
i
i
i
where C
0
= -2.9912729 x 10
3
C
1
= -6.0170128 x 10
3
C
2
= 1.887643854 x 10
1
C
3
= -2.8354721 x 10
-2
C
4
= 1.7838301 x 10
-5
C
5
= -8.4150417 x 10
-10
C
6
= 4.4412543 x 10
-13
D = 2.858487
T = temperature of the gas in °C.
Saturation vapor pressures, e
w
(T) , calculated with this formula are expressed in Pascals.
When taking the ratio of e
w
(T
s
) to e
w
(T
c
) , the units will cancel, leaving the effective degree of
saturation as a dimensionless quantity.