Like most scuba gear, breathing air compressors for filling scuba tanks come in a range of different shapes and sizes with plenty of options for added extras to boot.
There comes a point when owning your own breathing air compressor is more cost-effective (and less logistically challenging) than taking your tanks to be filled elsewhere, but it’s still a big up-front investment – even bigger if you buy the wrong one – so choosing the right compressor for your needs comes with a few caveats.
Whether you’re thinking of setting up shop or just wanting to fill your own tanks at weekends, here are a few tips to get you started in making the right choice.
- Types, sizes, and capacities of cylinders to be filled
- Number and frequency of cylinder refills
- Power requirements and available supply
- Other key features
- A note about nitrox
Type, size and capacity of cylinders
Most compressors are rated to either 232 bar (3,365 psi) or 300 bar (4,350 psi).
The standard 12l/80cuft aluminium tanks which are used around the world – and virtually exclusively in tropical diving destinations – have a working pressure of between 200-230 bar (2,901 – 3,336 psi).
If that is your only market, or you plan to use only recreational aluminium tanks for personal use or with a small group of buddies, then the 232 bar compressor will be more than sufficient.
Stronger steel cylinders, however – which are the mainstay of UK diving, for example – have a working capacity of 300 bar (4,350 psi), so a higher-pressure model of compressor is required to get the biggest possible fills.
If you’re planning to go into business anywhere that steel tanks are in regular use, it makes sense to invest in a higher-pressure model so that all your customers can get their preferred fills – even if you don’t offer the use of steel tanks yourself.
Be aware that the different pressure ratings require different fittings for the tank-filling hoses (also known as ‘whips’). 232 bar compressor whips usually have INT fittings – the same as the standard A-clamp regulators – but 300-bar compressors are DIN only, which – like DIN regulators – screw into the tank valve, not over it.
Neither fitting is a major issue, you just need to have a supply of proper adapters to make sure you can fill both types of tank.
Number of fills, frequency and flow/charge rate
If you’re looking to buy a compressor for personal use, diving at weekends or some weekday evenings, then a smaller compressor such as the Coltri Icon provides a relatively inexpensive solution, typically taking around 20-25 minutes to fill a standard 12l tank.
For dive centres or clubs, however, you would need a compressor designed for commercial use, such as Coltri’s Smart, Ergo or Mark III ranges, which are capable of filling two tanks at a time, and will fill a tank from empty in 10-15 minutes, depending on whether or not you’re filling to 200 or 300 bar, or using the 210 or 315 litre/min model.
The amount of air that a compressor can pump is known as its flow rate – or charge rate – measured in litres/min (cuft/min or cfm in the US). The bigger the flow rate, the faster your fills.
To put some tank-fill timing into context: filling twenty 12l aluminium tanks from empty to 200 bar would probably take around 3-4 hours using a 210 l/m compressor, and between 2-3 hours for refills after a diving day, depending on how much air your divers chomp through. Add at least another hour to each for 300 bar fills and 15 litre tanks.
Double the number of tanks means double the fill time, so a business using 40 tanks per diving day on a mid-range compressor would need 6-8 hours just for refills, in which case you either need to look at a compressor with a faster flow rate, such as the 315 l/m charge rate of the Coltri Ergo 315 or Mark III 315 ( and invest in a 3-phase power supply to run it) – or be prepared to clock up some serious overtime.
If the scale of your operation requires more than 40 tanks per diving day, then dual compressor systems, air banks (compressed air storage facilities) or industrial-scale compressors are on the cards, and you need expert advice.
Power supply considerations
Electric breathing air compressors are often preferred for filling scuba tanks as they are quieter and require less maintenance than petrol or diesel compressors; plus, there’s no risk of exhaust fumes being pumped into the tanks.
On the downside, they are obviously not very portable, and if you only have a single-phase electricity supply – which is the global standard for most homes and businesses – then you will be limited to a smaller compressor and slower tank fills.
Larger compressors with bigger flow/charge rates require a three-phase power supply, which is usually only found in commercial premises, and which in the UK can cost anywhere between £3,000-£15,000 to fit, depending on your location.
Petrol (and sometimes diesel) compressors for scuba tank fills are a much better alternative in terms of portability, especially for boats and in places where power supplies are less-than-reliable.
They are noisier than electric compressors, and the engines will require regular care and maintenance, but basic servicing is simpler than it is for their electrical counterparts.
One of the most important considerations for petrol engines is their exhaust fumes – especially carbon monoxide (CO), which is deadly if breathed under pressure. Proper compressor filtration systems, replaced at regular intervals, and an exhaust routed as far away from the air intake as possible are essential.
Additional air-quality monitoring devices are also a good idea, especially in locations where there is heavy road traffic or other industrial discharge into the atmosphere.
Other key features to consider
Filtration system
Breathing air compressors for scuba tank filling require chemical filters to remove moisture, oil residue and other contaminants from the air intake to prevent divers from breathing them and to reduce the risk of damage and corrosion inside the tank.
Filters are usually purchased as cartridges for easy removal and replacement from the compressor, and are often specific to the model of compressor in use, and the type of power the compressor is using.
All breathing air compressor filters require a ‘molecular sieve’ which removes moisture from the air, and activated carbon, which removes oil and unpleasant odours. It is highly recommended that filters for petrol and diesel compressors also contain a CO-catalyst to remove carbon dioxide.
Automatic condensate drainage
Moisture is a by-product of compressing atmospheric air and is dangerous if it makes its way into divers’ tanks. The condensate created during the filling process needs to be regularly drained from the compressor’s separator units when filling tanks – every 5 to 10 minutes, depending on local humidity.
The draining is usually done by a hand-operated valve, and although it is not difficult, it does require that you pay attention to draining the system during the filling process to make sure the condensate is removed from the compressor.
Automatic condensate draining is often available as standard on some top-end commercial models of compressor, and an optional extra on smaller models.
It’s a luxury rather than a necessity, but it does make the arduous task of tank filling and compressor maintenance that little bit easier.
Air quality monitoring
Moisture, carbon monoxide (CO) and carbon dioxide (CO2) are all potentially serious – even fatal – problems for scuba divers, should they be mistakenly pumped into a tank.
Air quality measuring devices which can monitor all three can be an important consideration, especially in places with high humidity, inner-city areas and locations where there is heavy road traffic and other potential air quality issues.
A note about Nitrox
Nitrox blending – creating gas mixtures with a different oxygen content than that of regular air – requires an article all to itself. It can be mixed using a standard breathing air compressor, but doing so requires a significant investment in other equipment.
The term ‘nitrox’ refers to any mixture of oxygen and nitrogen, but the ‘enriched air nitrox’ for recreational divers is a gas mixture containing more oxygen than the 21% found in regular air, up to a maximum of 40%, but most commonly 32% or 36%.
There are three main methods for nitrox blending:
- Partial pressure blending: This is essential for the precise nitrox mixtures, and mixtures containing helium (eg trimix) used by technical divers. Partial pressure blending typically involves putting pure oxygen into a tank and topping it up with air from a regular breathing air compressor passed through an additional filtration system to ensure the air is ‘oxygen clean’.
- Continuous blending: made famous by the ‘Nitrox Stik’, continuous blending mixes medical-grade oxygen from a separate tank with air from a regular breathing air compressor to create nitrox mixtures up to 40%.
- Membrane system: Air from a regular compressor is ‘filtered’ through a membrane to remove nitrogen and create mixtures up to 40%
All three are possible using a standard, unmodified breathing air compressor, but will require a significant investment in additional equipment, and may be affected by your location.
In many countries, local regulations stipulate that anything up to 40% enriched air nitrox can be treated like regular air, and does not require any specialist equipment or filling technique.
In the UK and Europe, however, any mixture containing more than the 21% oxygen content of regular air requires that tanks and filling equipment are ‘oxygen clean’ and compatible for use with pure O2, making the process a lot more complicated and a lot more expensive.
Specialist nitrox compressors are also available, which function using a membrane system to deliver nitrox in a self-contained system – but they come with a heavy price tag.
Regardless of what type of gases you’re planning on mixing, if it’s not normal air, you need specialist advice!
Buying the right compressor
There are plenty of things to consider when purchasing a compressor – and many dive professionals will have had to deal with the impact of working for dive centres that didn’t plan their system properly, or take into account future growth of the business or the different requirements of their customers.
While this article is a good guide to get you started, it’s important to discuss your plans with professionals who can give their sound advice before you part with a significant quantity of your hard-earned cash.
For more information or to investigate a potential purchase, it’s worth contacting a company such as CompressorShop UK, specialists in the world-leading Coltri brand of high-quality breathing air compressors for scuba diving.
