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Progress in water vapour spectroscopy at RAL

Progress in water vapour spectroscopy at RAL. Dr Robert McPheat CAVIAR Progress Meeting University of Leicester, 24/09/2009. Measurements in the mid- and near-IR. These measurements were intended to target the weak water vapour continuum. Long path lengths.

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Progress in water vapour spectroscopy at RAL

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  1. Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Progress Meeting University of Leicester, 24/09/2009

  2. 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. Previous MSF Measurements

  3. 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 0.002 cm-1 • Long integration times (~ 12 hours) in mid-IR

  4. 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. 5 mm Cell

  6. 5 mm Cell

  7. Water line parameter measurements • 486 measurements covering mid & near IR • Up to 13 hPa pure water • Room temperature • Spectra delivered to Reading • Ongoing analysis of pressure measurements to be completed early next week • Leak rate • Outgassing

  8. Mid-IR Spectra Wavenumber /cm-1

  9. Mid-IR Spectra Wavenumber /cm-1

  10. Near-IR Spectra Wavenumber /cm-1

  11. Near-IR Spectra Wavenumber /cm-1

  12. SPAC upgrade Optics Inner Cooling Jacket

  13. High temperature pressure gauge & filling line Electrical feedthroughs Temperature & pressure logging Temperature control Outer vacuum jacket Detector box Transfer optics vacuum tank

  14. SPAC upgrade modifications • Additional base heaters added • Inner modified to avoid problems with thermal expansion

  15. Example Spectra 420 – 450 K Up to 1 bar water vapour Up to 5 bar total pressure Wavenumber /cm-1

  16. SPAC upgrade problems • Problems with water condensation on bottom mirror • Largely solved with shield & higher temperatures • Degradation of mirror surfaces • Move feedthrough to bottom of cell • Will also solve wiring problems • Kapton wires • More PRTs • Remove humidity sensors • Replace O ring • Simplify connections

  17. SPAC upgrade modifications Electrical feedthroughs

  18. Previous Measurement conditions Room temperature, ~ 290 K Water pressures up to 0.75 SVP (< 20 hPa) Resolution up to 0.002cm-1 but mainly at 0.01cm-1 Water/air mixtures at total pressures of 150, 500 & 1000 hPa Pathlengths of 32, 128 & 512m Made measurements in 3 spectral ranges covering mid/near-IR Planned measurements in 8 – 10 cm-1 region for other work. Will extend to higher wavenumber if window materials allow LPAC Measurements

  19. Complete analysis of 5 mm cell measurements Next week Complete analysis of existing SPAC measurements & deliver data Early next month Finish modifications to SPAC & start new measurements November Schedule

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