Pressurised environments

Measuring volatile compounds

Hyperbaric marine environments 

NPL can analyse a wide variety of volatile compounds (hydrocarbons, oxygenated hydrocarbons, halocarbons, siloxanes and terpenes) in air sampled from both hyperbaric marine diving environments and off-gassing from pressurised marine equipment, such as umbilicals. There is a real danger of contaminants entering the atmosphere of these hyperbaric environments, e.g. from paint, thinners and cleaning fluids used during routine maintenance and periodic refurbishment work. The toxicity of these contaminants is greatly enhanced under pressure. It is therefore essential that these compounds can be detected and measured. 

Pressurised aircraft cabin air 

NPL can analyse samples of aircraft cabin air for a variety of compounds of concern. Examples include CO, CO2 and O3 as well as a range of VOCs associated with an array of sources, including aircraft engine emissions.  

Analysis capabilities 

NPL has more than 15 years of experience in rapid analysis and reporting results to verify compliance with occupational workplace exposure limits (HSE EH40/2005), occupational exposure limits for hyperbaric conditions (HSE EH75/2) and associated ISO guidelines. NPL has developed methods to identify a wide range of compounds and can perform any or all parts of the full analysis of hyperbaric gas samples: 

GC-FID analysis

These compounds are quantified using GC-FID analysis (gas chromatography – field ionisation detection) and NPL reference materials, prepared in house gravimetrically and traceable to national standards. Compounds with an amount fraction greater than 1 nmol/mol will be reported. Typical uncertainties are 2 - 5 % (k = 2). 

_{* Quantified using the response factor of a similar compound contained in the reference gas mixtures.} 

TD-GC-FID-MS analysis

All the compounds can be quantified using TD-GC-FID-MS analysis (thermal desorption - gas chromatography - flame ionisation detection - mass spectrometry). Samples are transferred to suitable sorbent tubes for this analysis. Calibration is achieved using a multi-point standard calibration derived from gravimetric liquid standards prepared by NPL. Typical uncertainties are 10 - 25 % (k = 2). Unknown compounds will also be reported and identified if possible. 

_{* Quantified using the response factor of a similar compound contained in the liquid standards.} 

GC-TCD/PDID analysis 

Using GC-TCD (gas chromatography – thermal conductivity detectors) with a range of carrier gases and detectors, the composition and make up of balance gases in a diving atmosphere, chamber or tank can be analysed.  Presence of other permanent gases that are harmful, yet abundant at lower concentrations, can also be tested for. Uncertainties are typically under 2 % (k = 2). 

Representative sampling 

NPL experts are at hand to provide advice on the best techniques for obtaining representative gas samples and can provide appropriate sample collection cylinders (typically double-ended of 1 litre volume). These are specifically designed for hyperbaric sampling and come with specialist passivation to avoid sample decomposition. Fittings and other connectors can be provided on request. 

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