Instruction manual

SECTION 2. STATION INSTALLATION
2-3
FIGURE 2.2-2 CA27, KH20, HMP35C, and Wind Sentry Set On a CM6 Tripod
2.3 KH20 CALIBRATIONS
Each KH20 is calibrated over three different
vapor ranges. The vapor ranges are
summarized in Table 2.3-1. This calibration
may have been done under the following
conditions: windows scaled and clean, and at
sea level or 4500 ft (Logan, UT, 1372 m).
TABLE 2.3-1. KH20 Vapor Ranges
Range g m
-3
Full 2 - 19
Dry 2 - 9.5
Wet 8.25 - 19
Before the KH20 is deployed in the field the
following decisions must be made:
• Which calibrated elevation is most
appropriate for the site?
• Will the windows be allowed to scale?
• What vapor range is appropriate for the site?
Once those decisions are made, then the
appropriate -k
w
can be chosen from the KH20
calibration sheet. The path length (x) of the
KH20 is also given on each calibration sheet.
The Wet and Dry ranges will provide higher
resolution of vapor density fluctuations than the
Full Range. However, If the vapor range is
unknown or the vapor density is on the border
between Wet and Dry, then the Full range
should be used.
2.4 FINDING WATER VAPOR DENSITY
The ambient air temperature and relative humidity
are needed to calculate the vapor density. The
vapor density can then be used to determine the
correct vapor range to use on the KH20.
From the ambient air temperature, the
saturation vapor pressure can be found using
the following sixth order polynomial:
(
e a aT aT aT aT
s
=× + + + +
01
01 2
2
3
3
4
4
.
)
++
aT aT
5
5
6
6
(4)
where e
s
is the saturation vapor pressure (kPa),
T is the ambient air temperature (K) (Lowe,
1977). The coefficients for Eq. (4) are given in
Table 2.4-1.