A compressor that runs perfectly can still produce gas that is not fit to breathe. Filters reach the end of their life, an inlet gets sited downwind of an exhaust, or lubricating oil breaks down inside a hot compressor and puts carbon monoxide into the air going down the umbilical. None of that is visible at the panel. At depth, where every contaminant is concentrated in proportion to the ambient pressure, a level that would be tolerable on the surface can incapacitate a diver. This is why breathing gas quality is tested on a schedule rather than checked by eye, and why the duty sits with the diving contractor rather than the equipment supplier.
In summary: BS EN 12021:2014 is the appropriate standard for compressed breathing gases used by divers. HSE Diving Information Sheet No. 9 (rev2), published in January 2018, sets out how that standard applies to diving and provides the composition tables for air, nitrox and heliox. Breathing air must contain oxygen in the range of 21 plus or minus 1 per cent, no more than 500 ppm carbon dioxide, no more than 5 ppm carbon monoxide and no more than 0.5 mg per cubic metre of oil. A competent person should test the gas at least every three months where the source is a compressor system, and more often where quality cannot be assured.
What the law requires, and where the numbers actually come from
Two layers matter here. The legal duty comes from the Control of Substances Hazardous to Health Regulations 2002 (as amended), known as COSHH, and its associated Approved Code of Practice and guidance. COSHH is what obliges an employer to control exposure to hazardous substances, and contaminated breathing gas falls squarely within it. Commercial diving projects are separately governed by the Diving at Work Regulations 1997 and the Approved Codes of Practice beneath them, L103 offshore and L104 inland and inshore.
The numbers, though, come from a standard rather than from the regulations. BS EN 12021:2014, Respiratory equipment: Compressed gases for breathing apparatus, supersedes the previous 1998 version and is the standard HSE identifies as appropriate for divers. HSE then publishes Diving Information Sheet No. 9 (rev2) to interpret it for diving specifically, and that document is where the tables reproduced below are set out.
One point of accuracy worth stating plainly. DVIS9 is guidance, not law. Following it is not compulsory unless specifically stated, and you are free to take other action. But HSE inspectors seek to secure compliance and may refer to it, so if you depart from it you should be able to show how you achieved an equivalent standard.
The composition limits for divers’ breathing air
These are the limits for compressed breathing air, measured at atmospheric pressure.
| Component | Limit |
|---|---|
| Oxygen | In the range of 21 plus or minus 1 per cent |
| Carbon dioxide | No greater than 500 ml per cubic metre (500 ppm by volume) |
| Carbon monoxide | No greater than 5 ml per cubic metre (5 ppm by volume) |
| Oil (droplets or mist) | No greater than 0.5 mg per cubic metre |
| Odour and taste | No significant odour or taste |
| Liquid water | None present |
The 5 ppm carbon monoxide figure is worth pausing on, because an earlier revision of DVIS9 carried a 3 ppm limit derived from the workplace exposure limit calculation under the 1998 version of the standard. Revision 2 aligned the air limit with BS EN 12021:2014 at 5 ppm. If your procedures still cite 3 ppm for air, they are working to a superseded document. As the next section shows, 3 ppm has not disappeared, it now applies to nitrox rather than to air.
Water content, the requirement most often missed
There must be no free liquid water in the breathing gas, and the dew point must be low enough to prevent condensation and freezing. Water is the requirement generic breathing air guidance most often glosses over, because the diving limits vary with supply pressure rather than sitting at a single figure.
High pressure supplies
| Nominal maximum supply pressure | Maximum water content at atmospheric pressure |
|---|---|
| 40 to 200 bar | No greater than 50 mg per cubic metre |
| Greater than 200 bar | No greater than 35 mg per cubic metre |
| Compressors charging cylinders above 200 bar | No greater than 25 mg per cubic metre |
Supplies up to 40 bar
This is the range that covers low pressure compressor supplies for surface supplied diving and for compression chambers. Values are measured at atmospheric pressure and 20 degrees Celsius.
| Supply pressure (bar) | Max water content (mg per cubic metre) |
|---|---|
| 5 | 290 |
| 10 | 160 |
| 15 | 110 |
| 20 | 80 |
| 25 | 65 |
| 30 | 55 |
| 40 | 50 |
On dew point, DVIS9 is specific. Where the apparatus is used and stored at a known temperature, the pressure dew point should be at least 5 degrees Celsius below the likely lowest temperature. Where the conditions of use and storage are not known, the pressure dew point should not exceed minus 11 degrees Celsius. For a spread that mobilises to unpredictable locations, the second figure is the one to design to.
Mixed gas: why nitrox and heliox limits are tighter
Where an operation uses oxygen and nitrogen mixtures or oxygen and helium mixtures, the limits change, and in several respects they tighten considerably. Carbon monoxide is the clearest example. Air is permitted 5 ppm, nitrox 3 ppm and heliox 0.2 ppm. Oil tightens from 0.5 to 0.1 mg per cubic metre for both mixtures, and water drops to a flat 15 mg per cubic metre.
| Component | Nitrox (oxygen and nitrogen) | Heliox (oxygen and helium) |
|---|---|---|
| Oxygen tolerance | Below 20%: stated ±0.5%At or above 20%: stated ±1.0% | At or below 10%: stated ±0.25%Above 10% to 20%: stated ±0.5%At or above 20%: stated ±1.0% |
| Balance gas | Nitrogen, remainder | Helium, remainder |
| Water | No greater than 15 mg/m³ | No greater than 15 mg/m³ |
| Carbon dioxide | No greater than 5 ppm | No greater than 5 ppm |
| Carbon monoxide | No greater than 3 ppm | No greater than 0.2 ppm |
| Oil | No greater than 0.1 mg/m³ | No greater than 0.1 mg/m³ |
| Total volatile hydrocarbons (as methane equivalent) | No greater than 30 ml/m³ | No greater than 30 ml/m³ |
| Hydrogen | Not specified | No greater than 10 ppm |
| Other non-toxic gases | Less than 1% | Less than 0.5% |
Percentages stated are those declared by the supplier, and the tolerance is expressed as a percentage of the total gas mixture. Other non-toxic gases include argon and the other noble gases. These figures apply to mixtures supplied by industrial gas suppliers. Where a diving contractor generates nitrox themselves using membrane systems, decanting or gas booster arrangements, the same composition requirements are the reference point.
Gas recovery systems in saturation diving
Saturation operations using a diver gas recovery, or reclaim, system are treated separately, because recovered heliox can legitimately carry more moisture, nitrogen and carbon dioxide than a freshly supplied mixture. Three points apply.
- Nitrogen: a maximum of 5 per cent nitrogen content in reclaim gases for depths up to 350 metres is acceptable.
- Carbon dioxide: levels within the circulating gas should normally be limited to a maximum partial pressure of 5 mbar at the depth of the diver, corresponding to 5,000 ppm when the diver is on the surface. The make-up gas source should still comply with BS EN 12021:2014.
- Water: higher moisture raises corrosion rates inside pressure vessels and pressurised pipework, so arrangements must be in place for periodic examination, testing and certification of that plant at an appropriate frequency.
Reclaim water vapour limits run from 410 mg per cubic metre at 10 bar down to 110 mg per cubic metre at 40 bar, based on a dew point of zero degrees Celsius. Separately, and worth planning for, divers’ breathing gas requires active heating for dives deeper than 150 metres.
How often you must test, and who can do it
The interval is the question most operators ask, and the answer is more precise than the generic industrial guidance suggests.
A competent person should carry out the tests. The frequency should be based on a risk assessment, but tests should take place at least every three months where the source of the divers’ breathing gas is a compressor system, including a reclaim compressor system unless the original equipment manufacturer has deemed it oil free. Tests should be more frequent where the quality of the breathing gas cannot be assured to the levels above.
Three months is therefore a floor, not a target, and it is tied to the presence of a compressor rather than applied indiscriminately. Anyone working to a six-monthly interval on a compressor-fed diving spread is outside the guidance.
On competence, DVIS9 defines a competent person as someone with a combination of training, knowledge and experience that means they can do the job required in a safe and efficient manner, using the test apparatus provided for the task. It then adds the point that matters commercially: the duty holder decides who that person is. You cannot contract the duty away by buying a test.
One further requirement is easy to miss. Where gas is to be breathed at ambient pressures greater than 10 bar, or for periods longer than 8 hours, the calculations in EH75/2, Occupational exposure limits for hyperbaric conditions, should be applied to account for the increased pressure and duration.
Assuring quality between samples
A quarterly sample tells you the gas was within limits on one day. Contamination does not respect that schedule, so DVIS9 sets out additional methods of assurance.
- Monitor filter life by running hours or by the volume of cylinders filled. Both rely on contamination not exceeding what the manufacturer assumed when setting those figures, so neither is conclusive on its own.
- Continuous in-line gas quality monitoring is described as a more reliable method. Monitoring moisture content at the filter outlet is one technique, since drying elements are usually designed to saturate before other elements deteriorate.
- Carbon monoxide deserves separate treatment. It can be produced inside a compressor by pyrolysis, the breakdown of lubricating oil by heat, which can occur when a system is hot without necessarily overheating. The resulting short-term spikes would not necessarily show up in periodic sampling. A carbon monoxide catalyst in the filter system, online monitoring for carbon monoxide, or both, should be considered.
A hard constraint sits alongside this: do not modify any filtration system or compressor without seeking advice from the compressor or filter manufacturer.
Contaminants beyond those named in the standard need a risk assessment. Check the compressor lubricant safety data sheet and the manufacturer’s operation and maintenance manuals for substances that should be tested for, and check where the compressor inlet is drawing from. If you identify a source such as a ventilation exhaust and cannot relocate the inlet, determine the likely contaminants and test for them, more frequently if there is any doubt.
Checking what is actually in the cylinder
DVIS9 records that experience shows it is possible for a gas mixture to be supplied which does not correspond to the cylinder markings. The response is procedural rather than technical. All diving breathing mixtures should be checked on receipt, and re-checked immediately before being connected to a diving gas supply or a breathing apparatus charging system. Two checks, not one, and the second happens at the point of connection.
How we can help
We supply the equipment that sits behind gas quality rather than the laboratory analysis itself. Our range covers high pressure and low pressure compressors, supplementary filtration including the HYPERFILTER and MEGAFILTER systems, and gas quads and cylinders for stored supplies, alongside the control panels and scuba replacement packages that deliver gas to the diver. Everything is available new or used, and much of it for hire: where a project does not justify a purchase.
Where equipment is bought used, treat the certification and examination history as part of the specification rather than an afterthought. We also offer bespoke engineering to IMCA standards, including witness and built to class systems, and our technician’s service and calibration training supports teams who maintain their own plant.
We understand commercial diving is fast paced and time critical, so we pride ourselves on extensive high quality stock, honest and realistic delivery timeframes, and flexible operating procedures across global time zones. We ship worldwide by sea or air freight, door to door. To talk through a requirement, no matter how big or small, call +44 1253 767 788, email info@isubc.com, or request a quote and we will come back to you.
