Campbell Scientific KH20 Krypton Hygrometer - Manual (Page 14)

KH20 Krypton Hygrometer
should see an immediate decrease in the voltage reading, and it should go close
to zero. No noticeable change in the voltage output, when the optical path is
completely blocked, indicates a problem in the detector tube. If the decrease in
the voltage reading takes place but the reading remains below 50 mV, when
the paper is removed from the optical path, the source tube may be at fault.
Obtain an RMA from Campbell Scientific, Inc. and send the unit in for repair.
6.4 Managing the Scaling of KH20
The KH20 cannot be used to measure an absolute concentration of water vapor,
because of scaling on the source tube windows caused by disassociation of
atmospheric continuants by the ultra violet photons (Campbell and Tanner,
1985 and Buck, 1976). The rate of scaling is a function of the atmospheric
humidity. In a high humidity environment, scaling can occur within a few
hours. That scaling attenuates the signal and can cause shifts in the calibration
curve. However, the scaling over a typical flux averaging period is small.
Thus, water vapor fluctuation measurements can still be made with the
hygrometer.
To see if the source tube window has been scaled, get a clean, dry cotton swab
and slide it across the source tube window. The scale is not visible to the
naked eye, but if the window is scaled, you will feel a slight but noticeable
resistance while you slide the swab across the window. There will be little
resistance if the window is not scaled. If you determine the window is scaled,
you can clean it with a wet cotton swab.
Use distilled water and a clean cotton swab to clean the scaled window. After
cleaning the window, slide a clean, dry swab across the window to confirm the
scale has been removed.
You can use the water vapor absorption coefficient for scaled
window from the calibration sheet if the window will be allowed
to scale during measurements. NOTE
6.5 Older KH20
The older version of the KH20 used to suffer permanent damage when exposed
to water, due to corrosion and loss of vacuum within the tubes. The new
version of the KH20 has employed a new sealing around the window to
prevent this type of damage.
6.6 Calibration
For quality assurance of the measured data, Campbell Scientific, Inc.
recommends the KH20 be recalibrated every two years. Please contact
Campbell Scientific, Inc. to obtain an RMA number for recalibration.
For more information on the calibration process, refer to Appendix A of this
manual.
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