Nitrox, Mixed Gas and Rebreathers
By Richard Bull ()
Article by Richard Bull, Managing Director – Oceanaire Diving Services Limited
What do we use diving for? Quite simply, we use diving to get to the place where we intend to carry out our work whether it be as a presenter in front of the camera, a cameraman behind the camera, lighting crew or safety assistants. Since the early 1990s equipment and techniques have become available to us that were previously only available to the military and some of these offer the underwater film maker distinct advantages over air and conventional open circuit SCUBA. But you don’t get something for nothing. Read on.
Nitrox is an oxygen/nitrogen mixture just like the air that is all around us but the percentage of oxygen has been increased beyond the 21% that is found in air and there is less nitrogen. When nitrox was first introduced to the diving public in the UK it was regarded with great suspicion. The diving press had a ball. They called it “Snake Oil”, “Devil Gas” and a bunch of other names and early British pioneers whole imported the technology from the USA were often referred to as “The Crazy Gang”. All that has changed and green and yellow nitrox cylinders are familiar to even the most casual British diver.
I believe that nitrox has so many advantages over air that we should use it as a matter of course. The question should be “Is there any reason why we shouldn’t use nitrox?”. Don’t get me wrong, nitrox is not universally applicable and often the answer to this question will be “Well, yes there is”
Nitrox comes in a variety of mixes but the most common are those with 32% oxygen and 36% oxygen. These have a nitrogen content of 68% and 64% respectively. The jargon refers to these as EANx 32 and EANx 36. (EANx means Enriched Air Nitrox)
Let’s have a look at the pros and cons of using a breathing gas with a percentage of oxygen higher than that of air. First of all the advantages. Some of these are quantifiable using simple maths and others have to experienced if you are to be convinced.
At a given depth nitrox will give a longer “No Stop Time” than air - and that means more filming time.
Surface intervals will be shorter – and that means more filming time.
Using nitrox but planning decompression obligation as if you were you breathing air will give a big safety factor. This is a real advantage if you are working at locations remote from emergency help
At a given depth, the diver will be less affected by inert gas narcosis. It’s simple – less nitrogen means less “Narks” – and that means a more efficient, more productive diver.
All the above can be demonstrated by a few simple calculations but this next point cannot. Nevertheless, I think that this is one of the most important advantages and of this, I am totally convinced. A diver breathing nitrox will get less fatigued than a diver breathing air. Now, we can’t come up with a load of calculations to substantiate this but what has been observed and experienced leaves no doubt that this is true. We know why this is so but here is not the place for a long physiological treatise. On a long shoot where divers are in and out of the water every day over a period of weeks it is almost impossible to overestimate this benefit of breathing nitrox.
It can’t be all good news and it’s not so here is the down side of nitrox use.
There is a depth limit to which the various nitrox mixes can be used. This is because of the increased percentages of oxygen mean that the toxic effects of that gas occur at a shallower depth.
Nitrox has to be blended and this can be time consuming. It can also be dangerous if done carelessly by someone who is not trained to do it.
Transporting enough oxygen, for blending nitrox, to remote locations, can be a problem.
Specially prepared diving equipment may have to be used.
In short, if nitrox is available and appropriate for the task in hand, use it. It’s worth a little bit of extra effort.
Mixed gas and Rebreathers
This is not to be confused in any way with nitrox diving. If fact the opposite question must asked. “Why are we using mixed gas and or rebreathers instead of something less demanding and more forgiving?” The answer may be “A rebreather is the only way”. At that point you will have to be absolutely certain that everybody involved is capable of carrying out their part of the operation. Closed circuit self mixing rebreathers are capable of going to great depths but that doesn’t mean that the diver wearing it can. It’s like me buying a Japanese motorbike capable of 180 mph. It can, I can’t.
If you are going to dive deeper than about 50 metres you must realise that the percentage of oxygen in air starts to become toxic at around this depth. Much deeper than this we have to start substituting some of the oxygen and nitrogen in the air for an inert gas like helium. For about a decade from about 1991, the way sport divers tackled this was to carry an array of cylinders each with a gas mix for a particular depth. They breathed from different cylinders as they descended and ascended. This is just as cumbersome as it sounds and the whole process has virtually given way to the Closed Circuit Self Mixing Rebreather. This bit of kit electronically mixes an ever changing gas mixture that is suited to whatever depth the diver is at. And because it recycles the exhaled gas, a very small quantity of gas lasts a very long time.
A closed circuit rebreather used a shallower depths will give the diver much, much longer time underwater without having to do decompression stops on the way up.
Yet another advantage of these things is that, except during ascent, they don’t emit bubbles. The advantages of this are obvious when it comes to wildlife filming.
There is a price to pay for all this and that price is the convenience of throwing a cylinder on your back and jumping into the water. The preparation of a rebreather is lengthy and must be precise if the divers are to return safely. Failure to complete the preparation correctly will create a situation which could lead to a very serious accident. I would like to quote a Royal Navy life support scientist here. He said “Rebreathers are essentially safe, but when they bite, they bite hard”. Keep in mind that this life preserving preparation in taking place when lots other important things are happening like cameras going into housings and presenters making sure that they have got their piece to camera correctly prepared.
Yes, closed circuit self mixing rebreathers can achieve fantastic results because of their depth and duration capabilities, but, be sure of one thing –complacency will kill. Experience is essential before these things are used for work. You don’t learn it one day and use it in anger the following day.
I should mention at this point, the oxygen rebreather. This does not use a mixture of gases but 100% oxygen which immediately puts a very shallow depth limit on its use – around 6 metres in fact. Again this is not the place to go off into the physiology of this but it is accepted science. This apparatus makes no bubbles, it can be very small, and a little bit of oxygen goes a long, long way. What could be better for filming shallow water wildlife - seals for instance – in difficult to get to, remote places?
The recently available diving technologies – nitrox, mixed gas, and rebreathers - are tools which can be of great benefit to the underwater film maker but as with all tools they are only as good as the person using them. And as with most tools they can bite back if that person is insufficiently skilled in and respectful of, the use of these tools.
Managing Director – Richard Bull Oceanaire Diving Services Limited