UVHygrometer

Measurements Provided: The fundamental measurement is the water vapor number density, determined by UV (Lyman-alpha) absorption. Typically this is converted to a mixing ratio or and/or dew point.
Manufacturer or Builder of the Instrument: RAF

Operational Status: Requestable, ready for deployment
General Description:
- The UV Hygrometer measures the absorption by water vapor of vacuum-ultraviolet light at the Lyman-alpha wavelength of atomic hydrogen (121.6 nm). This light is generated by an RF-excited lamp from Resonance, Ltd. of Barrie, Canada. The detector is a vacuum photodiode with a solar-blind diamond photocathode from Hamamatsu (PN R6800U-26). The power supply and amplifier electronics are RAF-designed and built. Air from the outside is brought to the absorption cell by a forward facing open-ended cone inlet (SCAI inlet) and exhausted through a scarf tube. Pressure and temperature at the absorption cell are recorded but not normally used. The shortest possible absorption path-length, 2.4 mm, is used for high water vapor mixing ratio conditions, 2-15 g/kg.
Measurement Characteristics:
- Overall estimate of uncertainty: 0.5 °C for mixing ratios of 2-15 g/kg.
- Response time: 35 Hz (-3dB analog bandwidth).
- Precision: Noise in mixing ratio is about 0.5% for mixing ratios of 2-15 g/kg.
- Other measurement characteristics (comments on signal/noise, bias limits, etc):

Description of Software, Data Handling, and Data Flow: Data acquisition is on three analog channels: detector signal, pressure, and temperature. Pressure and temperature are converted to engineering units by factory calibration. Detector signal is converted to approximate mixing ratio or dew-point by a curve-fit for real-time display purposes only. Post flight processing calculates the mixing ratio or dew-point using either the loose-coupling algorithm, or with the calibration constants and curve fitting to the reference mixing ratio.
- Software Components (data acquisition, display, processing, and analysis): Nimbus for loose-coupling, linear least-squares algorithms in Matlab, Mathematica, etc. for curve-fitting.
- Data Processing Chain (from flight to archival): Three analog voltages are digitized and recorded, then a loose-coupling algorithm and a calibration is used to generate a mixing ratio for the data files. Data processing follows the standard data path for RAF measurements: instrument -> NIDAS -> nimbus -> netCDF archives. See this page for more information.
QA Procedures and Needs: Examine data for drift and possible hysteresis, especially at start of flight and at large altitude changes.

Lead Contacts

Documentation

Internal users: See http://wiki.eol.ucar.edu/rafscience/UVHygrometer (last edited 2012-12-10)