Progress in water vapour spectroscopy at RAL

Dr Robert McPheat

CAVIAR Progress Meeting

University of Leicester, 24/09/2009

Previous MSF Measurements

• Measurements in the mid- and near-IR.

• These measurements were intended to target the weak water vapour continuum.– Long path lengths.

• 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.

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

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

5 mm Cell

5 mm Cell

Water line parameter measurements

• 486 measurements covering mid & near IR

• Up to 13 hPa pure water• Room temperature• Spectra delivered to Reading• Ongoing analysis of pressure

measurements to be completed early next week

– Leak rate– Outgassing

Mid-IR Spectra

Wavenumber /cm-1

Mid-IR Spectra

Wavenumber /cm-1

Near-IR Spectra

Wavenumber /cm-1

Near-IR Spectra

Wavenumber /cm-1

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

SPAC upgrade modifications

• Additional base heaters added• Inner modified to avoid problems with thermal expansion

Example Spectra

Wavenumber /cm-1

420 – 450 KUp to 1 bar water vapourUp to 5 bar total pressure

SPAC upgrade problems

• Problems with water condensation on bottom mirror • Largely solved with shield & higher

temperatures• Degradation of mirror surfaces• Move feedthrough to bottom of cell• Will also solve wiring problems• Kapton wires

• More PRTs• Remove humidity sensors• Replace O ring• Simplify connections

SPAC upgrade modificationsElectrical feedthroughs

LPAC Measurements

• Previous 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

• Planned measurements in 8 – 10 cm-1 region for other work.– Will extend to higher wavenumber if window materials allow

Schedule

• Complete analysis of 5 mm cell measurements– Next week

• Complete analysis of existing SPAC measurements & deliver data

– Early next month

• Finish modifications to SPAC & start new measurements– November