Technical q&a Forum

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Posted : January 26, 2007
Hello, I'm completely new to technical diving and have limited experience in general diving, but i'm very interested to learn more about the technical side as I think it will appeal to me. In my reading I've come across some stuff that I don't understand and was hoping you could explain it in terms that a complete novice will understand!

1) What does an isolator valve on the manifold of a twin-set do?

2) What does 'Oxygen Service' mean?

3) What is a stage cylinder used for?

4) Why do trimix divers use helium as a diving gas?

I hope none of these questions seem too stupid, as I said I'm quite new to diving anyway, and know practically nothing about technical diving!

Posted : January 30, 2007
Well done for taking the first steps - and good for asking. The key things you should adopt as a technical diver is awareness, discipline and knowledge (amongst others like skills, attitude - but not arrogance, reverence etc).

A strange way to open an answer - but not so. Start on the right approach and outlook/attitude and you will safely develop and grow.

[1] Boyles Law shows the deeper you go the more gas that would occupy a given volume. eg your lungs typically take 2L to 4L of volume each breath on the surface. The deeper you go this broadly remains the same, however the pressure at that depth makes the taken up volume equal to the Pressure at that depth x the lung inspired volume. So you end up needing a lot more gas the deeper you go. Technical Diver's came up with a solution, make a bigger cylinder out of two tanks by joining them together. So two 12L cylinders become a 24L tank. The next challange is that we techie's plan for problems and look to duplicate systems and components were such redundancy is not overkill, but totally necessary because if such a failure can cause your life to be threatened then duplication may be a necessary option (or eliminate the risk another way). This isolator valve effectively provides a controlling device in this link between the two cylinders. Turn it on and the two tanks level out their contents irrespective which cylinder valve is breathed from. Turn it off and you will only breath from the cylinder valve that side. So back to your question - what is the isolator for. It is to prevent a valve failure or very unlikely the neck O ring failure - so you can switch off this isolator valve and save effectively half of your (remaining)gas. I've used this twice in 3000 dives and both times it saved my life.

[2] Oxygen Service means that a valve, cylinder, regulator or system component is both Oxygen Clean and Oxygen Compatible. This 'check/service' is often done on an annual basis. Oxygen Compatible is the removing of components that would provide fuel in a fire situation with oxygen and replacing these components with safer aleternatives - this includes special lubricants too, Oxygen Clean is the process by which contaminants are removed through flushing with a special solvent or cleaning fluid that will remove grease and oil (and other fuel sources) and not leave any residue itself.

[3] a stage cylinder is for 'that stage in the dive' that its contents have so been chosen to assist in the decompression of the diver. EG 80% Nitrox would be used for the last 10m until returning to the surface and it would be the majority of the decompression of the diver.

[4] Helium is a great gas - pitty the world is running out of it (it is extracted mainly from the ground). Back to Boyles law... Deeper you go the density in the gas goes up proportionally to depth. Also occuring is Nitrogen Narcosis in greater effect proportional to depth - from around 24m down, after 50m this is getting dangerous (like alcohol you can condition yourself to 'tollerate' this narcosis). The other problem is that this air is getting so dense that you are working much harder to breath, this is producing dangerous levels of CO2 which can creep up and bite you (often in part the cause as to why panic may suddenly occur in skilled divers). Air can be safely used to 66m, according to the toxicity of O2, however using it beyond 45m is questionable due to the narcosis effects. By using a less narcotic and lighter gas this abates the effects of both work of breathing and also narcosis. The price of doing this is generally slightly more decompression to do (depends how rich the % of Helium is added). The clearer the head also the more able you are to do skills and hence survive/enjoy the dive. Hence why Helium is also used on shallower diver than the limits of air.

None of these were stupid questions at all, don't allow peer pressure or embarrasment to ask questions. Feel free to ask more as full answers to these key issues is the only way to proceed...
Answer provided by ADM Diving

Andrew Dawson-Maddocks
The views expressed by the author are their own and the publishers accept no liability for the advice and views expressed by the author, or other users, which are provided as a general service to divers. Users are warned that secondary posts are the views of other users and may not be technically correct.
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