Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Progress Meeting Cosners’ House, 14/12/2009
Previous MSF Measurements Measurements in the mid- and near-IR. These measurements were intended to target the weak water vapour continuum. –Long path lengths. Measurements carried out at moderate (0.01cm -1 ) & high resolution (0.002cm -1 ) using the Bruker IFS125 HR. Initial measurements revealed a need for better line data too.
Water line parameter measurements Although CAVIAR is targeted at improving the water vapour continuum, errors in line parameters contribute to the measured continuum First measurements with upgraded Bruker IFS120 HR –Upgraded electronics / moving mirror to same specification as IFS125 High resolution measurements cm -1 –Long integration times (~ 12 hours) in mid-IR
5 mm Cell 5 mm path length Electropolished stainless steel for low surface adsorption Spherical reservoir volume to increase volume to surface ratio All metal vacuum system Parylene coated KBr windows attached with low vapour pressure epoxy resin
5 mm Cell
Water line parameter measurements 486 measurements covering mid & near IR Up to 13 hPa pure water Room temperature Spectra, temperature & pressure measurements delivered to Reading for analysis
Mid-IR Spectra Wavenumber /cm
Mid-IR Spectra Wavenumber /cm -1
Near-IR Spectra Wavenumber /cm
Near-IR Spectra Wavenumber /cm -1
SPAC upgrade OpticsInnerCooling Jacket
High temperature pressure gauge & filling line Temperature & pressure logging Temperature control Outer vacuum jacket Transfer optics vacuum tank Detector box Electrical feedthroughs
SPAC upgrade modifications Additional base heaters added Inner modified to avoid problems with thermal expansion
Example Spectra Wavenumber /cm – 450 K Up to 1 bar water vapour Up to 5 bar total pressure
SPAC upgrade problems Problems with water condensation on bottom mirror Degradation of mirror surfaces Move feedthrough to bottom of cell Will also solve wiring problems Replace thin film heaters Kapton wires More PRTs Remove humidity sensors Replace O ring Simplify connections
Electrical feedthroughs for PRTs moved to bottom of the cell produce a much simplified vacuum / pressure system Electrical feedthroughs
Example Spectra at 160 ° C Background 3000 hPa Water vapour 1600Wavenumber / cm
Self Broadened Spectra 3000 hPa Water vapour at 160 °C 2000Wavenumber / cm % Transmittance
Example Spectra Air Broadened Spectra 600 mbar water vapour in 5 bar total pressure mixture with air at 160 deg. C 100 % 2500Wavenumber / cm
Further work Further NIR SPAC spectra Extend SPAC measurements towards visible MIR SPAC measurements up to 1.5 bar MIR LPAC measurements