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When doing my PADI open water course, including all the stuff around pressure groups and calculating maximum depth times by hand, I was told that I should always do my deepest dive first, and go shallower in subsequent dives in the day. When doing things with the pressure tables, that seemed to make sense.

Last week, while diving with a computer on a liveaboard, the PADI qualified instructor who was also the dive guide repeated the same thing.

When diving with a computer, does the "do your deepest dive first" rule still apply, and if so why? Or can I just rely on the dive computer to calculate how much time I can safely spend at each depth on each dive, and go to whatever depth on each subsequent dive makes most sense for the conditions?

  • I'm not qualified enough to completely answer this, but it doesn't matter if you use a computer or tables, you dive deep first. It has do with the nitrogen building up in your tissues as you dive, and of course deeper dives cause it to happen much more rapidly. Putting shallower dives after deep ones act effectively somewhat like safety stops, so that you don't get nitrogen narcosis or worse, some type of decompression illness (or in extreme cases, death). Just curious, did they not cover the reason for this in your course? – manoftheson Jan 5 '15 at 0:02
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    In short, it's much safer practice to do this. Theoretically, you could dive deeper after a shallower dive if absolutely necessary, since your computer will calculate the estimated nitrogen saturation and appropriate bottom time for you. I personally would not make a habit of doing this because you're pushing your margins of safety. – manoftheson Jan 5 '15 at 0:11
  • The ban against reverse profiles is old school thinking. Latest thinking is that it's fine. However, deco thinking isn't an exact science so consider the sources and make your own decisions. – Christopher Painter Jan 9 '15 at 12:01
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Reverse profile dives are not taboo as they were once thought:

Deepest Dive First? Not Anymore

Plan your dives and dive your plans. Going shallow and then deep should be fine as long as you don't push the limits.

That said, there are times when deep first can be better logistically. For example I can do a deep dive on double 80s and use half my gas with half my gas for emergency. Then on a second shallow dive I can dive for as long as my gas will allow. I wouldn't have nearly as much reserve gas for my deep dive if I did it second.

  • I hope this is not technical dives you are referring too :) – AquaAlex Jan 9 '15 at 9:38
  • No. The findings apply to recreational diving. BTW just because I have a twinset on doesn't mean I'm doing technical diving. It just means I'm diving with gas redundancy in case of unplanned deco and other emergencies. Most divers are told to consider having redundancy for deep dives and some are actually taught to do so in deep and solo classes. – Christopher Painter Jan 9 '15 at 11:54
  • lol I know, but a nice AL40 sling /stage would give you a lot of redundancy and allow you to dive normal recreational setup. In an emergency you can also hand off the sling / stage cylinder to another diver if he needs it. I have no issue with people diving with twins however as long as it is setup correctly and they get their trim and tilt right in the pool first. :) – AquaAlex Jan 9 '15 at 12:49
  • Interesting, the finding in the article still leaves some rules as to depth difference; "The attendees found no reason for diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 meters (130 feet) and depth differentials less than 12 meters (40 feet)." – AquaAlex Jan 9 '15 at 12:53
  • I have an AL40 also that I sometimes use when diving single tank. The "rules" were basically to frame the discussion to recreational diving aka <130 NoD. FWIW the pictures of the single tank diver with the slung AL40 on this page are of me. dailymail.co.uk/news/article-2697346/… – Christopher Painter Jan 9 '15 at 13:56
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There are many discussions going on around this. Just remember the decompression models are theoretical mathematical models, backed by practical dives/research. We implement some rules for safety, like slower ascend rates, deep stops, safety stops, deep dives first, etc.

Remember many factors determine your ability to on and off gas from day to day. The big problem is that we are still collecting data to compare theory with practical, and as more data is analysed and more research is done we understand decompression better.

So basically doing a deeper dive after a shallower dive or doing two deep dives on same day is not a problem. But it might be if coupled with other factors like for example, if you are dehydrated, ascend rates where very fast, you dived to the limits, short surface intervals, lots of body fat, etc.

My advice, do not always push the limits. :) And do not stress if you once in a while do deeper dives after shallow dives. Computers work the same for everyone but not everyone is the same. that is why on a dive 10 people may have no symptoms of DCS and the 11th person might.

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    In other words decompression theory isn't an exact science, theres always some unknown risk but reverse profiles don't appear to have any known additional risk at this time. :) – Christopher Painter Jan 9 '15 at 11:58
  • @ChristopherPainter Exactly :) It is mathematical calculations, certain aspects have been tested ad infinitum over long periods and others have not. So always err towards safety/conservatism. – AquaAlex Jan 15 '15 at 9:01
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The simple answer is yes. Dive your deepest dive first is a safety mechanism that pairs with other safety mechanisms (like a dive table). Using a computer (or any technology) does not obviate the need for all other safety mechanisms. The computer replaces the dive table, not the safety rules.

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    simple answers are not always correct or based in the latest research data :) – AquaAlex Jan 4 '17 at 7:08

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