Why diving air is a separate class of service
A diver breathes compressed air at increased ambient pressure. At depth, the partial pressure of every contaminant rises with pressure, so traces of carbon monoxide, carbon dioxide, oil vapour or moisture that might go unnoticed on the surface can become dangerous. Water in the air can also freeze in regulators in cold water and corrode cylinders. For these reasons diving air is controlled by breathing-air standards and diving regulations, and the responsibility for it sits with the diving contractor, the dive supervisor and the system supplier.
Commonly referenced standards include EN 12021 for the quality of compressed breathing air, alongside national diving regulations and industry guidance used by commercial diving contractors. The applicable standard and edition for a particular operation must be confirmed by the diving organisation; this page explains the engineering boundaries but does not replace those documents or a dive operation procedure.
Types of diving air supply
| Supply type | Typical pressure | Main features |
|---|---|---|
| Cylinder filling for scuba | Commonly around 200–300 bar (2,900.8–4,351.1 psig) | Multi-stage high-pressure breathing-air compressor with purification, filling panel and cylinder testing regime |
| Surface-supplied diving (low-pressure) | Typically well below cylinder pressure, set by depth and equipment | Compressor or HP bank with regulators, volume tank, filtration and umbilical supply to diver |
| HP storage banks | High pressure, as the bank is rated | Reserve and emergency gas, filled by a breathing-air compressor |
| Hyperbaric chambers | Depends on chamber design | Chamber air supply with its own system design and medical oversight |
Each has different equipment and procedures, so the first step is to define which service is required, the number of divers, depths and durations, and the emergency supply arrangements.
What makes air fit to breathe
Breathing-air quality is a measured property of the air delivered at the point of use, not a label on the compressor. Breathing-air standards typically limit:
- Oxygen content within a narrow band around normal atmospheric oxygen.
- Carbon monoxide to a very low level.
- Carbon dioxide to a low level.
- Oil (mist and vapour) to a very low level.
- Water content, with stricter limits for high-pressure cylinder air.
- Odour and taste, which must be absent.
The exact limits come from the applicable standard. Meeting them depends on intake air quality, compressor condition, purification media, cylinder and pipework cleanliness and correct operation. A factory test certificate for a compressor shows its condition on the test day, not the air your divers will breathe next month.
Intake siting: the most common hidden hazard
A purifier removes oil, water and some contaminants, but most purification systems used for breathing air do not remove carbon monoxide unless specifically fitted with a catalyst stage for it. The intake must therefore draw clean air. Typical hazards on Indian sites, boats and ports include:
- Engine exhaust from the compressor’s own petrol or diesel engine, nearby generators, vehicles or the vessel.
- Paint, solvent and fuel vapour in workshops and boatyards.
- Smoke from cooking, burning or welding.
- Dust and pollen, which load filters quickly in dry, hot months.
- Changing wind direction carrying exhaust towards a previously clean intake.
Intake position, height and direction should be planned for the actual site and checked each time the set-up changes. A remote intake pipe, kept clear and upwind, is common practice.
How a breathing-air compressor package is built
- Intake filter at a clean, controlled location.
- Compressor stages with intercoolers, usually three or four stages for cylinder pressures, designed and lubricated with oils approved by the maker for breathing-air service.
- Separators after each stage to drain condensate and oil, with automatic or regular manual draining.
- Purification using cartridges that typically combine molecular sieve for water, activated carbon for oil vapour and odour, and a catalyst where carbon monoxide removal is required.
- Back-pressure valve to keep purifier pressure high enough for effective drying.
- Final filter, safety valves and filling panel rated for the working pressure.
- Monitoring, such as moisture indication or online gas monitoring, plus periodic laboratory or field testing.
Purifier cartridge life depends on throughput, temperature and humidity. In hot, humid coastal conditions in Andhra Pradesh, cartridges are consumed faster, and cartridge changes must follow the maker’s method, with records kept.
Pressure and capacity: getting the basis right
Breathing-air compressors are rated by free air delivery, usually in litres per minute at the intake, against a final pressure. Filling time for a cylinder is roughly the free-air volume needed divided by the compressor’s free air delivery. Example: a 12-litre cylinder filled from empty to 200 bar holds about 12 × 200 = 2,400 litres of free air (ignoring real-gas effects). A compressor delivering 250 litres per minute would need about 10 minutes. Real-gas behaviour at high pressure, temperature rise and partially full cylinders change this, and cylinders heated by fast filling show a lower pressure once cool.
Higher pressure does not mean a safer supply. Every component must be rated for the working pressure, and safety valves, burst discs and hoses must suit the system. General industrial hoses and fittings are not suitable for breathing-air or high-pressure duties unless rated and approved for them.
Continuity, redundancy and emergency supply
For surface-supplied diving, the diver’s life depends on continuous air. Diving practice normally requires a primary supply, a secondary supply and the diver’s own emergency bail-out gas, each independent enough that one failure does not remove the others. A standby compressor is not the same as an emergency supply: if both compressors draw from the same contaminated intake, share one purifier or rely on one power source, a single problem can affect both. Common-cause failures should be identified and designed out.
Records, testing and change control
- Air-quality test results at the defined interval, with sampling method and laboratory or instrument identified.
- Purifier cartridge change log, with operating hours.
- Compressor service records, oil changes and valve maintenance.
- Safety valve and pressure-gauge checks.
- Cylinder and bank inspection status.
- Retesting after any change of intake position, filter, purifier type, oil or major repair.
Common mistakes and how to avoid them
| Mistake | Why it matters | Better approach |
|---|---|---|
| Using an industrial compressor with an added filter | No breathing-air design, lubricant or purification basis | Use a purpose-built breathing-air package from a specialist |
| Assuming oil-free means breathable | Carbon monoxide, carbon dioxide and moisture are not controlled by being oil-free | Test delivered air against the breathing-air standard |
| Intake near exhaust | Carbon monoxide passes many purifiers | Remote, upwind intake, rechecked each set-up |
| Running cartridges too long | Moisture and oil breakthrough | Change by hours and conditions, keep logs |
| One power source for primary and standby | Single failure stops all supply | Independent supplies and emergency gas |
How Reddy Compressors helps
Reddy Compressors is an independent multi-brand industrial compressor specialist at Tarnaka, Secunderabad, serving industry since 2002. We sell, rent and service industrial air compressors, dryers and filtration, and supply spares across India. Certified breathing-air and diving systems are a specialist field governed by diving regulations, and we do not present ourselves as a diving-air system supplier or test laboratory.
- We help clients separate breathing-air duties from industrial air duties, so plant compressors are not used for life support by mistake.
- We supply, rent and service the industrial compressed-air systems used around ports, shipyards, marine workshops and construction sites in Telangana and Andhra Pradesh, through our team of more than 20 service engineers and technicians.
- Where you need breathing air, we explain the engineering questions to put to a specialist breathing-air supplier and diving contractor, so your enquiry is complete.
What to send us
- A short description of the work: diving, cylinder filling, surface supply or industrial air near the dive site.
- Pressures and flows required, and the site location.
- Details and nameplate photos of any existing compressors.
- Your diving contractor’s or safety adviser’s requirements, if available.
Frequently asked questions
Can I use a normal air compressor to fill scuba tanks?
No. A normal workshop or industrial compressor is not designed, lubricated, purified or tested for breathing air, and usually cannot reach scuba cylinder pressures safely. Scuba cylinders must be filled by a breathing-air compressor with suitable purification, rated components and regular air-quality testing to the applicable breathing-air standard. Contaminated air can cause carbon monoxide poisoning or other serious harm at depth.
Does an oil-free compressor produce breathing air?
Not automatically. Being oil-free removes one contaminant source from the compressor, but breathing air must also meet limits for carbon monoxide, carbon dioxide, oxygen content, moisture and odour. Those depend on a clean intake, correct purification and testing of the delivered air. Many purifiers do not remove carbon monoxide without a specific catalyst stage, so intake siting matters just as much as compressor type.
What standard applies to compressed breathing air for diving?
EN 12021 is widely referenced for the quality of compressed breathing air used with breathing apparatus, including diving. Diving operations must also follow the national regulations and industry guidance that apply to them, which set requirements for supply arrangements, testing intervals and records. The diving contractor or responsible authority should confirm which standard and edition applies to a specific operation.
How often should diving air be tested?
The testing interval is set by the applicable standard, regulation or diving contractor’s procedures, and many operations test at regular fixed intervals plus after any change. Retesting is also good practice after changing the intake position, purifier cartridge type, compressor oil, after major repairs or whenever air quality is in doubt, such as an odd smell or taste. Records of every test should be kept with the system.
Why does carbon monoxide get into compressed breathing air?
Most often because the intake draws in engine exhaust, from the compressor’s own engine, a generator, vehicles or a vessel, especially when wind direction changes. It can also come from overheated compressor oil in a poorly maintained machine. Standard filters and molecular-sieve dryers do not remove carbon monoxide, so a clean, remote intake and, where specified, a catalyst stage and monitoring are needed.
Is a standby compressor enough as an emergency air supply for divers?
Not on its own. A standby compressor that shares the same intake, purifier or power source can fail at the same time as the main unit. Surface-supplied diving normally requires a primary supply, an independent secondary supply and the diver’s own emergency bail-out gas. The diving contractor and supervisor define the arrangement in line with the applicable diving regulations.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.