TAKE YOUR BREATH AWAY

In preparation for the much awaited retail launch of the Phibian rebreather, Oceanic has launched training and familiarisation courses at its Honiton, Devon, headquaters. The course is presented by Stuart Clough of Undersea Technologies who developed the electronics package for the Phibian for Oceanic and uses pre-production protypes.

Most divers have heard of the benefits of rebreathers - longer bottom times and shorter decompression penalties. But many are put off by the perceived complexities and the cost.

The seminar explains the history and the concept of rebreather technology and describes the benefits of reclaiming and recycling the oxygen in the gas mix and adding fresh oxygen as required. It is at this stage that the course participants really begin to sit up and take notice, whether their interest in technical diving, cave diving, or marine biology, as the comparative bottom times, gas consumption and decompression requirements are unravelled before you.

For example a standard air dive to 40m for a theoretical 60 minutes would mean 118 minutes of decompression. The same dive using the Phibian rebreather would mean 46mins decompression time.

The Phibian system can be specified for use up to 50m depth in which the diluent gas will be compressed air, or for deeper 'technical' diving in which the diluent gas will be Heliox or another suitable inert gas mixture. In both specifications none of the expired gas mixture is lost from the system but is recycled, scrubbed and enriched with oxygen as necessary.

There follows a detailed description of the way that the hardware and electronics software functions. This is all displayed using computer generated graphics and animated diagrams which are very easy to follow no matter how rusty your recollection of diving physics and physiology.

There were two pre-production prototype units available for trial at the time of my visit - a closed circuit, pure oxygen re-breather and the Phibian CCS50 designed for air range diving using oxygen at a constant partial pressure with compressed air as the diluent gas. Monitoring, mixing and delivery of the breathing gas in the CCS50 is controlled by the electronics package which has an integral dive computer, while the oxygen rebreather is an entirely manual unit which provided an interesting contrast.

The mixed gas model, the CCS100, is similar to the CCS50 with the exception of breathing gas and computation requirements. All of the Phibian's hardware is contained in a moulded casing with a waistcoat BC attached. Buoyancy compensation and bale out are provided by the diluent gas cylinder using a standard demand valve and low pressure inflator assembly.

The first step is to accustom yourself to the breathing characteristics of the unit on the surface and to be reminded that the mouthpiece valve must be closed if it is removed from your mouth for any reason.

Initially you are required to trim the counter-lungs to your breathing rate and lung volume on the surface. This is achieved by bleeding off or injecting diluent gas into the system until the inhalation and exhalation effort is comfortable. This is followed by a slow descent down a shot line in the centre of the tank to its maximum depth of 10m. As you descend the increase in water pressure progressively collapses the counter-lung and you must manually increase the volume of diluent gas in the lung until breathing is comfortable.

The first impression is, of course, the total lack of bubbles and almost silent operation, except for the reassuring click of the solenoid on the oxygen injection valve. Unlike a conventional demand system there is no positive pressure delivery as you are breathing the gas at ambient pressure. Breathing in and out has no effect on your buoyancy, for the same reasons ,and it is not until you begin to ascend that your buoyancy changes as the volume of the gas in the counter-lung expands. On the current prototypes this expanding gas is vented either by breathing out through your nose or the corner of your mouth , however production models will be fitted with pressure relief valves to eliminate this requirement. Ascending and descending a few times soon makes you comfortable with the basic operation of the system and lets you concentrate more on this strange and exciting experience.

One or two drawbacks of the current prototypes were apparent. The bulk of the enclosed unit would make swimming difficult in current or a shallow swell and similarly the size and buoyancy of the hoses restrict head movement (although we were informed that production units will be trimmed down in both these areas). Also, because the counter lung is on your back, if you roll over you find a noticeable increase in breathing resistance due to the increase in hydrostatic pressure. The Phibian will cost between £5-10,000 dependant on specification.

Oceanic suggests that interested technical divers should start with the familiarisation day and then perhaps take a more comprehensive course, tailored to their intended use, with further extended and open water practical sessions. Course Fee: Mon-Fri £65, Sat/Sun £75. Contact Oceanic on 01404 891819