Does "dive your deepest dive first" still apply when using a computer?
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?
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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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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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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.
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