FPH
change note: 2022-04-05 08:23:12.0 [tstevens] update AltLabel (Frost Point Hygrometer);
in scheme: instruments
definition: 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"
type: Concept

Hygrometers
type: Concept