Switch Air to Nitrox (underwater)

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Messages
2
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Location
Kuwait
# of dives
100 - 199
Hello Divers...
I was chatting with an experienced diver and he told me that underwater, I can change from Air to Nitrox to shorten the time of my deep or safety stop, can you please explain this to me? and is that possible with D9? this technique is diving with side mount an Air tank and another Nitrox32 tank
 
I'm no where a guru on things but here is my take. This is what sounds like a basic nitrox question where you pick you benefits of using nitrox. Longer dive times OR shorter SI but not both. The rest is DECO theory. If you use nitrox you should have shorter stops than using air. Using nitrox you should have longer NDL's which translates to shorter stops. The higher the O2 the shorter the stops are. The things that do not make sense in this statement is that you have no mandatory stops till you exceed NDL. Once exceeding NDL it doesn't matter if you did it on nitrox or air. it isnow purly deco and yes nitrox should shorten the stop times. Is he suggesting that you use air till it is out and then change to nitrox? Perhaps he is saying that you push air to ndl and then ascent on nitrox. I'm not sure there is much benefit to using 30 or 40% as a deco gas.
 
The short answer is yes, the longer answer is.....yeeeeeeesssss but...

If you use a higher O2 gas (32% vs air) at the safety stop, then you would offgas quicker. However, the safety stop is a fixed length in recreational diving so you wouldn't get out of the water faster but you would get out with less dissolved gas so a little safer. However, that benefit extends throughout the dive so it would be MUCH more useful to use the nitrox throughout the dive ie have 2 tanks of 32% and just use them.

IF you can't for some reason (eg the dive is too deep for 32 at the bottom), then you are diving with a tank of unbreathable gas (at least at some points) and that would no longer be recreational diving.

The reason tech divers use this gas switch approach is because they use gases that have serious depth limits (ie pure oxygen is limited to 6m, 50% to 21m etc)

Getting the switches wrong and going onto the wrong gas is still one of the leading causes of death for tech divers.

Back to your point, while it is technically possible and what he said is true, it is almost never going to be of any use for you. Dive the highest O2 suitable for the depth you are diving at, within your limits, and don't bother about switches.
 
In addition, if you don't have a computer that can switch between the two gasses, you will have no way to quantify any benefits you might get from the evolution.


Bob
 
Thank you all for the replies... @Bob DBF @RainPilot @KWS ...
I'm not planning to switching gas in the same dive, I was only wondering how possible is this technique. The person who told me on this said this was used when reaching 50m or deeper, he uses nitrox on ascend to shorten safety & deep stops ... it seems too complicated, I would not do it anyways...

Thank you all for the replies, that was really helpful
 
He's referring to Accelerated decompression, check PADI Tec40/45/50 or TDI ADP or any of the other various agency courses.

It's not something I would recommend you do without instruction, mainly for the reasons already stated here.
 
Thank you all for the replies... @Bob DBF @RainPilot @KWS ...
I'm not planning to switching gas in the same dive, I was only wondering how possible is this technique. The person who told me on this said this was used when reaching 50m or deeper, he uses nitrox on ascend to shorten safety & deep stops ... it seems too complicated, I would not do it anyways...

Thank you all for the replies, that was really helpful
Afwan Firas.

What he was referring to is "Technical Diving", in which the optional "stops" that you know (deep and/or safety) become replaced by compulsory stops, due to enough on-gassing that a straight ascent to the surface would most likely lead to decompression sickness. In that environment there are several methods of reducing those stops, mostly involving using higher and higher percentages of oxygen as you get shallower.

Just for clarity, whether you need to have a decompression stop is a function of time and depth, so it is VERY easy to go into that environment a lot shallower than 50m. For example, a 30m dive on air would have you needing a stop after 20 min or so, depending on your computer settings etc. At 50m you would have almost no time at all on the bottom before you would have compulsory stops.

Again, this is a type of diving that requires specialized training, skills, equipment and techniques to carry out safely. PLEASE don't go on these types of dives with someone who says "trust me, it will be fine".

It almost never is.

Happy diving sadeqy!
 
A basic nitrox course doesn't cover gas switching with/and/or multiple gas mixes on a single dive.

Whilst a nitrox diver is qualified to dive using gasses 21-40% O2, they are only qualified to use a single gas per dive.

There are dangers inherent with diving multiple gasses and also with switching gasses... and they deserve some simple training for the sake of risk mitigation.

Messed up gas switches and/or using the wrong gasses account for a disproportionate amount of technical diving fatalities. It seems simple in comparison crept, but even higher trained divers can mess it up (with fatal consequences).
 

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