News 1996

Munich, January 1996

Drastic Weight Reduction - Portable Compressors

Does this sound familiar? You want to do a series of dives and have to haul numerous cylinders to an inaccessible dive location. This could be a lot simpler employing a compact and man-portable compressor. "Unterwasser" looks into the practicality of these mobile units.


Tractor · BAUER compressors are "mobilized" by a set of wheels. The CAPITANO is the right choice for diving clubs and sports divers with a high air demand. The compressor has a free air delivery of about 140 litres per minute and can be operated using a four-stroke diesel or petrol engine. This semi-professional compressor can be set to 200 or 330 bar filling pressure.

Do you remember the pictures of vintage divers that get their air supply via a hand pump and pipe feed? Sports divers have trouble imagining how difficult air supply really was. The development of small, high performance compressors is possible in virtually every location. Choosing an air supplier was easier in the old days; will Bob or Bill do the pumping? Nowadays the "right" choice of drive is a little more complex. Internal combustion and electric units are available. A combination is also available. After all, who'd expect an electric socket at your local quarry?

Petrol engines ensure total mobility. To fill your cylinders in your garage, the electric drive might be a better choice.

The units are available for 220 and 380 volt. For greater mobility, the 220-volt-compressor should to be given preference.

In terms of filling pressure, two configurations are possible: sports divers generally use 200-bar cylinders; commercial divers and fire services generally opt for 300-bar air supply.

Portable compressors are either three- or four-stage high pressure piston compressors. From the end user's point of view, the specific design of the cylinders is of minor importance.

Operation · How is the final pressure generated in a three-stage compressor? The first stage builds up a pressure of around 6 bar. Since compression heats up the air to around 120 °C, the air is cooled before flowing into the second stage. The second stage compresses the air to approximately 45 bar. Again, the air heats up and has to be cooled. At that stage, the moisture vapours contained within the air, start to condense and can be separated out. Final pressure of 225 bar is attained in the third stage. Before entering the cylinder, the air is cleaned by a special filter system. Water vapour is condensed in a cyclonic separator; an activated carbon filter absorbs other gaseous contaminants. Finally, the breathing air is passed through a special drying agent, which prevents corrosion inside the air tanks or the regulator from seizing up.

Operation of a Compressor

  1. Intake hose with paper filter
  2. 1st stage (ca. 6 bar)
  3. Inter cooler
  4. 2nd stage (ca. 45 bar)
  5. Inter cooler
  6. Water separator
  7. 3rd stage (ca. 225 bar)
  8. After cooler
  9. Multilayer filter
  10. Filling hose with pressure gauge

Important criteria when purchasing a mobile compressor are weight and free air delivery. The lighter the compressor, the better. That 10 kg more or less make a big difference will become painfully evident when you carry the compressor to an inaccessible dive site. If you disregard the air delivery, you will also have a nasty surprise when you start filling. The higher the air delivery, the sooner the bottles will be filled. Around 2000 litres of air have to be squeezed into a standard 10-litre cylinder. If you count on supplying an entire dive club, the fill will have been completed in time for a night dive only.

Mobile compressor units usually stick to the basics, which may at times be detrimental to filling convenience. Condensate has to be drained mechanically. Generally the engine does not cut off automatically once final pressure has been reached. The safety valve blows off excess air; the motor keeps on running until switched off manually. Furthermore, the compressor's operating hours must be recorded manually.

For ease of operation optional components can be purchased: an automatic condensate drain operates autonomously, but needs regular maintenance; a pressure switch cuts the engine once a certain pressure is reached; a operating hour counter helps determine the maintenance intervals. However, these add-on components will quickly raise the unit's price.

In Germany, compressors in private use do not need an operating licence from your local authority. If unit is used commercially, wholly or partially, a certificate of approval is required. Amongst other aspects, operators will have to demonstrate that they have been trained to use the unit safely. Whether club compressors are to be considered private or commercial is still a matter of legal debate.

Please take the following recommendations to heart when installing a compressor unit: the place of installation has to be level, absolutely dry (especially for electric motors) and well away from potential pollutants (car fumes, camp fires or the compressor's engine itself). Regular servicing is essential. For all breathing air compressors, only special oils, which have been approved by the compressor manufacturer, may be used. Other lubricants may contaminate the breathing air. After an oil change, oil can be returned to your local garage. Standard maintenance work can be done without much compressor experience. Please avoid toying around with safety devices. The compressor stages are protected by special safety valves. These safety valves are totally off limits for end users.

When filling diving cylinders, impurities have to be removed. Breathing air compressors achieve this through various mechanisms: the intake filter traps dust, pollen and other particles. During compression moisture and chemical impurities (such as fat and oil vapours) are removed by a series of separators and filters. Before streaming into the cylinder, residual moisture is removed by a drying agent. These filters are contained in several filter housings or a combined filter module. Because of easy maintenance, the multilayer filters have gained popularity. Albeit the complex purification system, it's a worthwhile precaution to check the freshly filled bottle for suspicious odours or to check for contaminants with a white tissue. Beware if you determine any odours or colour changes! Do not dive with such a bottle! A brief word on environmental protection and cooperative behaviour: do the first filling at home, not at the dive site, where this will disturb fellow divers.

Filter SystemSimply the Best - Exclusively for Divers
Breathing air must be absolutely dry and clean. Neither fatty nor oily substances ought to be anywhere near the compressor's intake hose. This is especially important for high pressure compressors, since high temperatures build up inside the compressor stages. Fat and oil vapours may spontaneously combust - much like a diesel engine. Also the intervals for filter changes have to be followed meticulously.

Verdict: A compressor of your own is suitable for committed divers that prefer derelict dive sites, far away from filling stations. Otherwise a commercial filling, usually charged at DM 5, will be the more economical choice. We'd recommend that you attend a compressor seminar -ideally with a manufacturer of your compressor- so that you learn to master the high pressure unit. Your prime consideration should be the compressor's weight. Only a lightweight compressor is truly mobile.

Report: "Unterwasser" Divers' Magazine, Jörg Neisser, issue 1/1996, page 94

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