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3.4.2.2
molbloc-S OPERATION
The molbox RFM gases available for use with molbloc-S at the time of this
manual printing are listed in Table 10.
Table 10. Available molbloc-S Gases
In addition to dry air, molbox RFM supports measurement of ambient (humid) air
flow with molbloc-S. When Air is selected as the molbloc-S test gas, the user is
prompted to enter a value of the humidity ratio (also known as the absolute
humidity or water ratio) of the ambient air. The humidity ratio, W, is defined as
the ratio of water mass to gas mass in the flowing air. It is different from the
relative humidity value, which is usually expressed as a percentage. Typical values
of W are between zero and 0.06. molbox RFM does not accept an entry for W
greater than 0.1.
Typically, humidity measuring instruments report relative humidity, which is
dependent on the ambient pressure and temperature. Users who do not have
the W value available can use DHI's free Unit of Measure Converter software
utility or COMPASS for molbox calibration software to calculate W from
measured
pressure,
www.dhinstruments.com, or see your DHI sales representative for a copy of the
Unit of Measure Converter software utility. Air relative humidity, pressure and
temperature are converted to the humidity ratio, W, following Dalton's Rule and
thermodynamic principals using water saturation properties:
W
P
is the water saturation pressure, which can be calculated as:
g
=
P
g
If dry air will be measured, then the user should enter a W value of zero when
prompted. Zero is the default W value.
When a non-zero W value is entered, molbox applies a correction to its air flow
measurement for the change in air density due to humidity. If a correction for W
is being applied to air flow measurements, a <W> is placed to the right of the
<Air> gas indication in the MAIN run screen.
GAS
<1N2> Nitrogen
<2Air> Air (dry and humid)
temperature,
and
relative
⎛
RH
⋅
⎜
P
g
⎝
100
=
⋅
.
62188
⎛
−
⋅
⎜
P
P
amb
g
⎝
+
+
3
2
C
T
C
T
C
0
amb
1
amb
2
where
:
=
C
. 0
0649289
0
=
−
C
53
.
0528
1
=
C
14509
9 .
2
=
−
C
1327760
3
Page 33
© 1998-2007 DH Instruments, a Fluke Company
3. OPERATION
humidity.
⎞
⎟
⎠
⎞
RH
⎟
⎠
100
+
T
C
amb
3
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