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CAVIAR water vapour laboratory FTS measurements

CAVIAR water vapour laboratory FTS measurements. Dr Robert McPheat CAVIAR Meeting Cosners’ House, 15/12/2009. As part of the CAVIAR project we are measuring the mid- and near-IR absorption spectrum of water in the MSF.

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CAVIAR water vapour laboratory FTS measurements

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  1. CAVIAR water vapour laboratory FTS measurements Dr Robert McPheat CAVIAR Meeting Cosners’ House, 15/12/2009

  2. As part of the CAVIAR project we are measuring the mid- and near-IR absorption spectrum of water in the MSF. These measurements are intended to target the weak water vapour continuum. Long path lengths and / or high vapour pressures. 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. CAVIAR Measurements

  3. Long path lengths and / or high vapour pressures. Use Long pathlength absorption cell (LPAC) / Short pathlength absorption cell (SPAC). LPAC 32 to 640 metres in IR, over 1 km in visible Multi-pass reflective optics (White) Room temperature 0 to 1 bar pressure SPAC 1.2 to 20 metres Multi-pass reflective optics (White) Temperature range 300 to 470K 0 to 5 bar pressure LPAC & SPAC

  4. LPAC Measurements • Long path-length absorption cell (LPAC) • 9m long stainless-steel inner vessel, 8 m mirror separation • Mirrors steerable from outside vacuum vessel for optimising alignment and changing pathlength • Fitted with Vaisala HMP234 humidity sensor at each end • 2 fans in centre to ensure even mixing • Temperature measurements by 14 PRTs • 3 MKS 690 (1, 10 & 1000 torr) & 625 (1000 torr) Baratrons

  5. 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 650 to 2000 cm-1 MCT detector / KBr beamsplitter / Globar 1800 to 5500 cm-1 InSb detector / CaF2 beamsplitter / Globar 2000 to 11000 cm-1 InSb detector / CaF2 beamsplitter / Quartz tungsten halogen source LPAC Measurements

  6. Example LPAC Spectra 17hPa H2O + air to 1000hPa

  7. Example LPAC Spectra 17hPa H2O + air to 1000hPa

  8. 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 • Up to 13 hPa pure water • Room temperature

  9. 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 • 486 measurements covering mid & near IR

  10. 5 mm Cell

  11. Mid-IR Spectra Wavenumber /cm-1 1250 2000

  12. Mid-IR Spectra Wavenumber /cm-1

  13. Near-IR Spectra 1600 6000 Wavenumber /cm-1

  14. Near-IR Spectra Wavenumber /cm-1

  15. Water vapour partial pressure limited by vapour pressure at room temperature Increase temperature to observe weak lines & continuum Desired upper temperature for water vapour measurements 470K All parts of the cell & filling lines need to be at same temperature to avoid condensation in cold spots. Need 200ºC, 5 bar pressure gauge Other measurements using SPAC require lower temperatures therefore SPAC redesigned for -200 to 200ºC temperature range Other requirements 5 bar maximum pressure Mirrors adjustable from outside Accurate temperature & humidity measurement Corrosion resistant SPAC redesign

  16. Insulating Vacuum Helium Electrical heaters Liquid Nitrogen Sample Gas & White Optics

  17. SPAC upgrade Optics Inner Cooling Jacket

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

  19. Electrical feedthroughs Electrical feedthroughs for PRTs moved to bottom of the cell produce a much simplified vacuum / pressure system

  20. High temperature SPAC Measurements • Measurements intended from 300 to 470 K • Up to 5000 hPa water vapour (at higher temperatures) • Air broadened measurements to 5000 hPa • 0.05 to 0.3 cm-1 resolution depending on pressure • Argon backgrounds before & after at same pressure as sample for pure vapour spectra. • Air background at same total pressure for foreign broadened spectra

  21. Example Spectra at 160 °C Background 3000 hPa Water vapour 1600 Wavenumber / cm-1 12000

  22. Self Broadened Spectra 100 % 3000 hPa Water vapour at 160 °C, ~ 20 m, 0.2 cm-1 Transmittance 2000 Wavenumber / cm-1 11000

  23. Air Broadened Spectra 100 % 600 mbar water vapour in 5 bar total pressure mixture with air at 160 °C, ~ 20 m, 0.1 cm-1 resolution 2500 Wavenumber / cm-1 10000

  24. Conclusions • NIR out of band continuum larger than predicted by CKD • Further SPAC spectra required • See talk by Igor Ptashnik

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