Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Annual Meeting Imperial College, 16/12/2008
Mar 28, 2015
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 / CaF2 beamsplitter / Globar– 2000 to 11000 cm-1
• InSb detector / CaF2 beamsplitter / Quartz tungsten halogen source
Example LPAC Spectra
17hPa H2O + 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
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
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.
Example Spectra
Wavenumber /cm-1
Tra
nsm
issi
on