Gas standards

Pressurised (hyperbaric) environments

Measuring volatile compounds for the marine sector

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 in the atmosphere of these hyperbaric environments from paint, thinners and cleaning fluids used during routine maintenance and periodic refurbishment work and the toxicity of these contaminants is greatly enhanced under pressure. It is therefore essential that these compounds can be detected and measured.

NPL has more than 15 years’ experience in rapid analysis and reporting the results to check 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 (gas chromatography – field ionisation detection)

All of these compounds are quantified using GC-FID analysis 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).

Class of compound


ethane, ethene, propane, propene, i-butane, n-butane, acetylene, trans-2-butene, 1-butene, cis-2-butene, 2-methyl-butane, n-pentane, 1,3-butadiene, trans-2-pentene, 1-pentene, 2-methyl-entane + 3-methyl-pentane, n-hexane, isoprene, n-heptane, benzene, 2,2,4-trimethyl-pentane, toluene, n-octane, tthyl-benzene, m+p-xylene, o-xylene, 1,3,5-trimethyl-benzene, 1,2,4-trimethyI-benzene, 1,2,3-trimethyl-benzene, *i-butylene, *i-heptanes, *unknown C8, *unknown C9, *C9/C10 aromatics


±α-pinene,  ±β-pinene, limonene


trichloroethylene, tetrachloroethylene

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


TD-GC-FID-MS analysis (thermal desorption - gas chromatography - flame ionisation detection - mass spectrometry) 

All the compounds can be quantified using TD-GC-FID-MS. 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.

Class of compound


propyl benzene, 1-methyl ethylbenzene, 1-ethyl-3-methylbenzene, 1-ethyl-4-methylbenzene, 1-ethyl-2-methylbenzene, 2-methyl-1-hexene

Oxygenated hydrocarbons

acetone, ethanol , 2-methyl-2-propanol, *ethanal (acetaldehyde), methyl acetate, 2-*propenal (acrolein), *2-butenal, *2,3-butanediol, vinyl acetate, ethyl acetate, isopropyl acetate, 1-butanol, butan-2-one (MEK), isopropyl alcohol (IPA), methyl isobutyl ketone (MIBK), 2-butoxyethanol


dichloromethane, chloroform, 1,2-dichloroethane


*hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane

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


GC-TCD/PDID analysis (gas chromatography – thermal conductivity detectors)

Using GC-TCD 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).

Class of compound

Permanent Gases

carbon monoxide, carbon dioxide, methane, helium, nitrogen and oxygen.

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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