|Advantages of a constant ppO2|
The advantages of nitrox are well-known. By increasing the level of oxygen in the gas we breathe, the nitrogen in the mix is reduced, so we absorb less of it under water. This means longer no-stop or shorter deco-stop times.
However, with any particular nitrox mix there is a maximum operating depth (MOD) caused by the toxic effects of oxygen under pressure. It is generally thought that a partial pressure of oxygen (ppO2) in excess of 1.6 bar can cause harmful effects to the nervous system that can lead to epileptic-like fits and, under water, to death by drowning.
Most nitrox training agencies suggest an MOD based on a ppO2 of 1.4 bar.
The Buddy Inspiration rebreather makes nitrox by mixing air (O2 and N2) with pure O2. Sensors constantly monitor the O2 levels being breathed and keep the ppO2 constant at (normally) 1.3 bar by adding O2.
Oxygen is added to the breathing gas when the level is reduced by metabolism or by ascending to a shallower depth. By choosing a set point of 1.3 bar, minor fluctuations can be accepted without breaching 1.4 bar. Some divers will inevitably choose a higher setting. The mixes delivered at depth are as follows:
The effect is to allow the diver to decompress during an ascent on a progressively stronger mix of nitrox.
The Inspiration rebreather can also be safely used deeper than 52m, but with a diluent other than ordinary air, such as heliox. Trimix divers can carry a separate cylinder of bottom gas which they can plug into the diluent feed valve when they need it.
|Where we did the test|
Ellaidhoo is a tiny island in Ari Atoll, about 50 miles from Male, the capital island of the Maldives, in the Indian Ocean. It is reached by Twin-Otter float-plane or boat.
Ellaidhoo, with enough space for about 50 simple bungalows for a total of 100 guests, restaurant, bar and tourist shop, is a diver's island.
You can walk around it in less than 15 minutes but it has a deep-water reef immediately at its shore giving easy access and the possibility of diving as much as your body can stand! Dive-boat trips are scheduled twice a day, but you are free to shore dive whenever you want.
The dive centre can provide nitrox 32 and 36 from de-nitrogenised air, as well as pure O2 for other uses.
We had made special arrangements with Emirates Airlines to ship out the four weighty 3-litre cylinders, not knowing that the centre could have provided identical O2-clean cylinders from stock.
Ellaidhoo already offers semi-closed rebreather facilities and training with the Draeger Atlantis and now has one of the first dive centres to be equipped with the closed-circuit Buddy Inspiration.
One imagines that in future Inspiration-equipped dive centres will be more common worldide, and travelling divers will not have the hassle of taking the heavy components on the plane.