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A dive marshal advised a RIB-load of divers that because the second dive was to be 20m deep they should try to get beyond 20m on the first dive in order that the second dive was shallower and therefore "safer". Discuss.
 

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TTFN
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It has been discussed many times I believe... and is lobbox...
+1

Trying to go deeper on the first dive to make it safer is one of the more laughable things I have heard.



IIRC This whole myth arose decades ago because if you do a deep dive then a shallow dive the no stop times are 'longer' than doing a shallow dive then a deep dive - one of the agencies (I'm not going to say which) then misinterpreted this as it is safer to do deeper first. The myth continued (and obviously still does in some corners) until a few years ago.
 

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Perhaps more intesresting but OT (although nearly related) - what do folks think about having the deepest point at the end of the dive?
 

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Dive tart, just can't say no :-)
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it's historical based on having only one cylinder and doing 2 dives on it, do the deepest first, the use the remaining air for the shallower one. Nowadays and with 2 cylinders it has no basis in science whatsover, as the others have said it's bollox.
 

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It's historical. The old RNPL table basically did not allow for repeat dives. So they came up with the idea that you could do a shallower one after a few hours but the deco time was punative, so punative that a deeper dive was out of the question. Typically, this went into the mythology of diving and when diving computer became popular, the mythology stayed.
There's still a lot of mythology being regurgitated by instructors who were taught by instructors who were... etc.
 

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All we wanted was a home... Manics
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it's true. BSAC books say so :D
 
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The worlds slowest sailor.
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Perhaps more intesresting but OT (although nearly related) - what do folks think about having the deepest point at the end of the dive?
do the deco first..?
 

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do the deco first..?
I often do my 15 mins at 6m at the start of the dive. That way it clears you for a free ascent at the end of a deco dive :D
 

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Perhaps more intesresting but OT (although nearly related) - what do folks think about having the deepest point at the end of the dive?

Dont think it makes any difference regarding N2, but does make a difference on
a typical UK boat dive. Do a standard slack + drift and you are fresh on the first
and if you havnt swapped out or decanted can use what's left on a drift and cut
it short if low on gas.

Do a reverse and if you cant swap out or you need to decant, then you
have to be pretty tight with calcs on the gas used on the first. That could see
divers close to reserve at the end of the second dive, when liable to
fatique/cramp/cold etc.

Mind dont call eg: 20m to 25m as a deeper second, needs to be a pretty hefty
difference, say 15m first and 45m second. Rather be at 15m before the ascent
on the second than 45m.

As for making divers go deeper so the second is shallower, well that's just
bo**ocks.
 

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Bristol
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As for making divers go deeper so the second is shallower, well that's just
bo**ocks.
Had that on our last live-aboard in Egypt. On the first dive of the trip the guide said to us "the second dive today is 30m so we need to make sure we get to atleast 30m on your checkout dive".
 

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A Moderate from 04/01/07-24/12/12
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Mark Powell does an interesting talk on this explaining the derivation as being Navy Divers, IIRC, were advised to do their working dive first then their "safety" dive second and some how that became enshrined in a *you must do*! There was a study which concluded there is no scientific evidence to prove one way or other but the fact that *most* people believe it and practice it, means there could be some benefit and that to disprove that we would have to advocate the reverse and that would not happen.

Additionally the study wimped a bit on the conclusion and added in an arbitrary depth variance between the two dives - again it appears there is no scientific evidence to support this variance.

Me? I think that doing appropriate decompression from every dive is the wisest thing .... and putting aside enough gas for you and you buddy to do that deco (even minimum deco dives) is a good idea too!

Mal

p.s. I have also been on boats where it was positively recommended to bounce to over 24m because the 2nd dive would be to at least 21m ... I laughed.
 
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