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

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