Campbell Scientific KH20 Krypton Hygrometer - Manual (Page 10)

KH20 Krypton Hygrometer
5. Datalogger Programming
The KH20 sensor outputs 0 to 5 Vdc analog signal. These signals can be
measured using the VoltDiff instruction on the CRBasic dataloggers or the
Volt (Diff) (P2) instruction on the traditional Edlog dataloggers.
5.1 KH20 Calibration
Each KH20 is calibrated over a vapor range of approximately 2 to 19 g/m 3 .
The calibration is performed twice under the following two conditions:
window clean, and scaled. The water vapor absorption coefficient for three
different vapor ranges are calculated from the collected calibration data: full
range, dry range, and wet range. Table 2 shows a sample of the KH20 vapor
ranges over which three different water vapor absorption coefficients are
calculated. See Appendix A for more information on KH20 calibration.
TABLE 2. KH20 Calibration Ranges
Ranges Vapor Density (g/m 3 )
Full Vapor Range 2 – 19
Dry Vapor Range 2 – 9.5
Wet Vapor Range 8.25 – 19
Before the water vapor absorption coefficient, k
w
, is entered into the datalogger
program for the KH20, the following decisions must be made:
• Will the windows be allowed to scale?
• What vapor range is appropriate for the site?
Once the decision is made, the appropriate k
w
can be chosen from the
calibrations sheet. The calibration sheet also contains the path length, x, for a
specific KH20. Using the water vapor absorption coefficient for either the dry
or the wet vapor range will produce more accurate measurements than using
that for the full range. If the vapor range of the site is unknown, or if the vapor
range is on the border line between the dry and the wet vapor ranges, the full
range should be used.
6