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Hygrometers measure water. Some measure water vapor content,
some measure liquid water. The NOAA Lyman-alpha Total Water
Hygrometer measures the amount of all water that enters the
sensor, be it in any form-- solid, liquid or gas. The instrument
ionizes the water molecules themselves as they pass
through. Using a high intensity direct current discharge lamp,
light at 121.6nm (called Lyman-alpha light) photodissociates
water molecules producing excited OH radicals. Fluorescence is
produced as the OH radical emits photons at 309nm. Making use of
a phototube sensitive to this wavelength and a counter, the
amount of OH can be calculated. The ambient air in the sample
diminishes this signal by an amount proportional to the mixing
ratio, so knowing the ambient pressure and temperature yields
the mixing ratio from which the total water is determined. The
instrument is periodically calibrated in flight by injecting a
known amount of water vapor directly into the sample.
Additional information available at
"http://ghrc.msfc.nasa.gov/uso/readme/c4enlh.html"
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