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CAVIAR water vapour laboratory FTS measurements
Dr Robert McPheatCAVIAR Meeting
Cosners’ House, 15/12/2009
CAVIAR Measurements
• 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.
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– 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 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
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 / CaF2 beamsplitter / Globar– 2000 to 11000 cm-1
• InSb detector / CaF2 beamsplitter / Quartz tungsten halogen source
Example LPAC Spectra
17hPa H2O + air to 1000hPa
Example LPAC Spectra
17hPa H2O + air to 1000hPa
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
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
5 mm Cell
Mid-IR Spectra
Wavenumber /cm-11250 2000
Mid-IR Spectra
Wavenumber /cm-1
Near-IR Spectra
Wavenumber /cm-11600 6000
Near-IR Spectra
Wavenumber /cm-1
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 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
Insulating Vacuum
Helium
Electrical heaters
Liquid Nitrogen
Sample Gas & White Optics
SPAC upgrade
Optics Inner Cooling Jacket
High temperature pressure gauge & filling line
Temperature & pressure logging
Temperature control
Outer vacuum jacket
Transfer optics vacuum tank
Detector box
Electrical feedthroughs for PRTs moved to bottom of the cell produce a much simplified vacuum / pressure system
Electrical feedthroughs
• 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
High temperature SPAC Measurements
Example Spectra at 160 °C
Background3000 hPa Water vapour
1600 Wavenumber / cm-1 12000
Self Broadened Spectra
3000 hPa Water vapour at 160 °C, ~ 20 m, 0.2 cm-1
2000 Wavenumber / cm-1 11000
100 %
Tra
nsm
ittan
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600 mbar water vapour in 5 bar total pressure mixture with air at 160 °C, ~ 20 m, 0.1 cm-1 resolution
Air Broadened Spectra
2500 Wavenumber / cm-1 10000
100 %
Conclusions
• NIR out of band continuum larger than predicted by CKD
• Further SPAC spectra required• See talk by Igor Ptashnik