Jack Ingle is the BSAC's Technical Diving Adviser. He is a BSAC National Instructor, an IANTD Technical Instructor, a TDI Tri-mix instructor and co-author of NSAC nitrox courses.

I have been diving for a couple of years and am interested in different equipment and techniques. Everywhere I look, magazines, books, dive shops and the Internet, I see the word nitrox. Could you please tell me what are the advantages of diving with nitrox over using air, and should I be using it
Clive Edwards

The answer to this one is YES, YES and YES again. Before listing the advantages, let me explain the differences between air and nitrox. Air contains 79 per cent nitrogen and 21 per cent oxygen. Nitrox is any nitrogen/oxygen mix that contains less nitrogen and more oxygen. For example, nitrox 36 contains 36 per cent oxygen and the balance is mainly nitrogen. Whatever percentage of oxygen the gas contains, that is the name of the mix.
Nitrogen is the gas that saturates a divers tissues and demands short, no-stop diving times or decompression stops before you can surface. Using nitrox to reduce your nitrogen content offers these advantages:
  • Longer no-stop decompression diving when either nitrox decompression tables or computers are used.
  • Shorter deco-stop times required, sometimes described as accelerating tables. For anyone carrying out deco-stop diving, nitrox is a must to enhance this phase of the dive.
  • Some divers use air decompression tables or computers while breathing nitrox to give a greater safety margin against the risk of decompression illness (DCI). Their gas mix contains far less nitrogen than their air table or computer assumes they are breathing.
  • Some ultrasound studies have shown that dives on nitrox produce fewer micro-bubbles than the same dives using air. These micro or silent bubbles are sub-clinical (do not cause DCI symptoms), but many divers report feeling less fatigued after diving with nitrox than with air.
  • Many nitrox divers report that they consume less gas, though this has not been conclusively proven by research.
  • A nitrox diver who suffers a DCI will have loaded the body tissues with proportionately less nitrogen than an air diver. The increased oxygen percentage in the breathing mix should also assist any oxygen-starved or damaged tissues.
    Its only fair that I list the disadvantages as well:
  • Nitrox divers can still suffer from DCI by overstaying dive time, missing stops, carrying out rapid ascents, being unfit, dehydrated, etc.
  • Nitrox is not a gas to be used deep. Divers can suffer from oxygen toxicity even when breathing air, though it becomes a serious problem only at considerable depths. However nitrox, with its higher oxygen content, can cause oxygen toxicity in far shallower water. It all depends on the depth (pressure) and the mix.
  • While air is readily available and easy to compress, nitrox needs to be mixed, so problems of supply and analysis need to be considered.
  • Nitrox use calls for equipment that is in oxygen service to handle and compress the oxygen-enriched gas safely. Certain items of equipment must be dedicated to nitrox use, particularly the cylinder.
I believe the advantages of nitrox far outweigh the disadvantages and that in the future we will teach divers from day one to use nitrox. Air is simply not the best gas mix.
My favourite regs
I am thinking of buying a new regulator and would like your opinion on which are the best.

Graeme Sargeant

This is a difficult one. We all demand different qualities in a regulator and find these in various makes. I dont believe you can buy a bad regulator these days as they all have to undergo rigorous testing.
What I look for in a regulator is ease of breathing. Its important to be able to get gas easily, and even when you are working a little harder the flow rate must be able to cope with your breathing rate.
I also look for ease of maintenance. On expeditions I might be miles from a dive shop and must be able to carry out simple repairs. My main workhorse for years has been a Poseidon Cyklon (below), which breathes very well. I can adjust it and completely strip the second stage with a simple toolkit. Remember, however, that you must be qualified as a service technician to strip down and adjust a regulator.
Recently I have come across a new regulator which is probably the easiest-breathing unit I have used. The Explorer is produced in the USA by Abyss Diving, which also produces decompression software. Its president Chris Parrett asked Poseidon to produce a modified first stage with reduced interstage pressure. He then asked full-face mask manufacturer Kirby Morgan to make the second stage with a flow-adjuster.
This regulator is superb. I can make tiny adjustments to the flow rate at any time, from demand to free-flow. I have used it in varied conditions, including cold water and with deliberately increased breathing rates, and it has simply offered ready gas at all times.

Trimix or heliair
I am using trimix for my technical dives but am interested in using heliair. Do you think it is better, and if so why

J Manley-Dexter

Trimix and heliair are the same insofar as both contain three constituent gases: oxygen, helium and nitrogen. The major difference is in how the gases are obtained. Trimix is blended by taking helium, adding oxygen and topping off with air. In this way you can plan a mix to get the best partial pressure of oxygen (ppO2) and equivalent narcotic depth (END) for the maximum depth of the dive.
Heliair is mixed by putting helium in the cylinder and simply topping off with air. If you plan the ideal ppO2 the END will not be very good; if you plan a good END the PPO2 will be very low, which means longer decompression. So it is not the ideal mix concept you get with trimix. Another downside is that you need a lot of helium, and it is expensive.
The advantage of heliair is that the mixing is easier. You need only a helium source and an air compressor, which can be very useful on expeditions, where transport of equipment is difficult. But for many divers this one advantage will be outweighed by the disadvantages.

Practice vital when using an isolator
I want to start using a twinset. Can I just add a second cylinder of the same size and use two regulators, or should I link them together with a manifold
Peter Maxton

I used twin independent cylinders for years and the benefit is total redundancy in the event of a free-flow.
You need a regulator with a contents gauge on each cylinder, and the main disadvantage is that you have to switch regulators throughout the dive while breathing each cylinder down a little at a time. Many divers use the 50 Bar Rule, breathing each cylinder down 50 bar at a time, to ensure that there is always redundant gas to which to revert in an emergency.

IÂ now use the twin-manifold system with an isolator because I like the idea of not having to switch regulators throughout the dive. The cylinders are manifolded together with an isolator valve between them. You can access gas from both cylinders via either regulator, but in the event of a free-flow would lose gas from both, hence the isolator.
In an emergency you have to close the isolator valve, after which only one cylinder will be losing gas. You then make sure that the regulator from which you are breathing is the functional one and close down the cylinder valve of the free-flowing regulator. This isolates that regulator, and the central isolator can then be re-opened, making gas from both cylinders accessible again.
This shut-down procedure sounds tricky but is quite easy once you are used to it. If you do go this way, practise over and over again until it is a reflex.