Presentation on theme: "Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Annual Meeting Imperial College, 16/12/2008."— Presentation transcript:
Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Annual Meeting Imperial College, 16/12/2008
MSF Measurements Measurements so far in the mid- and near-IR. 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.
LPAC & SPAC 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 to 190K –0 to 1 bar pressure SPAC –1.2 to 20 metres –Multi-pass reflective optics (White) –Existing temperature range 200 to 350K –0 to 5 bar pressure
LPAC Measurements 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 / CaF 2 beamsplitter / Globar –2000 to 11000 cm -1 InSb detector / CaF 2 beamsplitter / Quartz tungsten halogen source
Example LPAC Spectra 17hPa H 2 O + air to 1000hPa
SPAC redesign 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 450K –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
Insulating Vacuum Helium Electrical heaters Liquid Nitrogen Sample Gas & White Optics
High temperature pressure gauge & filling line Temperature & pressure logging Temperature control Outer vacuum jacket Transfer optics vacuum tank Detector box
SPAC upgrade modifications Additional base heaters added Problems with thermal expansion –Can be used for hot measurements without cooling jacket but modifications required
SPAC upgrade problems Heating capacity limited by rating of spectrometer room ring main. –Use several extension leads from elsewhere Original window seals (Indium) melted. –Initially replaced with gold wire which failed on temperature cycling. –PTFE seals have been installed successfully. Long delays getting a working pressure gauge suitable for 5 bar & 200 °C Ready to begin measurements beginning of new year
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
Water line parameter measurements 5 cm cell to avoid saturation of strong lines Required sample pressures <0.5 hPa Absorption of water on cell walls causes difficulty in maintaining stable sample pressures Large variation of sample pressure with room temperature Filling line all stainless steel and as short as possible Measurements completed 1000 to 2000 cm -1, 2000 cm -1 to 9000 cm -1 next.