Reverse Profile? Can it be done?

Getting ichy to get back into the water again, so Stoney is the obvious
choice for a first dive of the year.

Even tho its only been 4 months since I was last in the water a nice easy
dive would be preferable (esp as my buddy is diving with new kit) which
would suggest keeping to the upper stoney shelf.

However as we all know the tables require that the deepest dive should be
done first, and subsequent dives shallower. This would mean that if a dive
was made to a max depth of the sub (7m) for the first dive (to get into the
water, test kit out, check configuration) the next dive would need to be
shallower (which is hard at stoney!)

Therefore the first dive would need to be the deep one (something like the
stangarth 22m) - but straight in and down is not idea for a first dive esp
with new kit setup to consider. Do all tables use deepest first? I suspect
this is because it is this usage that has the most testing and anything else
is theoretical rather than tested.

As I understand (but correct me if I am wrong) a reverse dive increases the
risks of DCS, but can it still me made safely if enough conservatism is
added to the dive profile? Is so, I feel it would be safer than making a
deep dive first.

I have put the info info XS for Windows (is anything better?), and the
planned dive is allowed, in addition both divers would be diving on
different computers (aladin/oceanic etc). With this combined with the XS
profile, the most conservative plan can be dived?

i.e

If a dive was planned to be:

Dive 1= Depth 7m (maybe short time at 12m) for 40mins, 1.5 hours surface,
and dive 2 = Depth 22m for 25mins

if this is put into XS as

Dive 1-Depth 16m for 40, 1.0 hr surface, Dive 2=Depth 26m for 25mins

This would be allowed without deco. If we add in a 3m stop for 5mins on both
ascents we are adding in further conservatism (in calculating the dive to be
4m deeper than actual, being a square profile where the average depth would
be shallower, shorter surface interval, and the addition of two stops)

Am I on the right lines? Or should a reverse profile always wrong?

> However as we all know the tables require that the deepest dive should be
> done first, and subsequent dives shallower. This would mean that if a dive
> was made to a max depth of the sub (7m) for the first dive (to get into
the
> water, test kit out, check configuration) the next dive would need to be
> shallower (which is hard at stoney!)

For Stoney-type dives I can't imagine you'll have any problems at all.
You're usually so far from deco that a bit of reverse-profiling shouldn't be
a problem. Instructing there on a regualr basis often involves us as
instructors to reverse profile quiet regularly, just because if you're doing
say Ocean Diver 1 and 2, you should do them in order and need to have the
depth progression with new divers. But like I said, with the depths and
times involved it has never been an issue in all the years we've been doing
it.

If you were planning something deeper or longer, say a 40min dive to 20m
then a 30min dive to the 35m section, then I would be more careful (not that
i'd want to go to 35m in Stoney again - murky and miserably - but thats off
the point!).

If you've both got computers, then as long as you aren't going close to deco
(which you won't with 2 dives in Stoney) don't worry about it...

That said, I am not a hyperbaric specialist, so take any of my advice at
your own, ermmm, peril! :O\

David

> Use a table, computerized or otherwise, or use a dive computer. Don't mix
> the two. Within the limits of whatever tool you chose, you can safely do
> the shallower dive first.

I agree that tables/computers should not be mixed half way, but if the dive
is made to the most conservative model, then it is also within the limits of
the more agressive model. If two buddys dive together, on different
computers and dive a profile that is allowed by both computers, then they
are automatically diving the most conservative model (i.e one buddy computer
might be down to 5 min NDL, and the other computer would allow say 10mins,
by both divers ascending it would make the profile more conservation) - I am
assume that both divers follow the same profile, and the difference is in
computer algorithyms.

As for the reverse profile, I would not like to do two deep dives backwares
(say 25m - 40m), but a shallow 7m dive is unlikely to cause problems, and as
other post has mention is quite common with instructors at Stoney, as
trainees start of shallow and work deeper (usually with two training dives
made a day).

Just keep in the limits of the computers, ascend slowley, and make a long
safety stop and you will be well within the limits.

Dont know about the Oceanic computer, but the Aladin will automatically
penalise you for diving in Stoney! (i.e cold), and get upset if you ascent
too quickly (think it is variable and something like 6m/min toward the
surface when PADI allow 18m/min) - dont forget your safety stop (5min@5m?)
and you will be well within any set table you can find.

Mark Cross wrote:
>> Use a table, computerized or otherwise, or use a dive computer.
>> Don't mix the two. Within the limits of whatever tool you chose,
>> you can safely do the shallower dive first.
>
> I agree that tables/computers should not be mixed half way . . .

Or for planning a single dive or a series of dives. You use one or the
other, but not both.

> but if
> the dive is made to the most conservative model, then it is also
> within the limits of the more agressive model.

In which case, you've chosen one or the other.

> If two buddys dive
> together, on different computers and dive a profile that is allowed
> by both computers, then they are automatically diving the most
> conservative model (i.e one buddy computer might be down to 5 min
> NDL, and the other computer would allow say 10mins, by both divers
> ascending it would make the profile more conservation) - I am assume
> that both divers follow the same profile, and the difference is in
> computer algorithyms.

So?

> As for the reverse profile, I would not like to do two deep dives
> backwares (say 25m - 40m), but a shallow 7m dive is unlikely to cause
> problems, and as other post has mention is quite common with
> instructors at Stoney, as trainees start of shallow and work deeper
> (usually with two training dives made a day).

OK, you would not like to do two deep dives in what you consider reverse
order. You have a right to personal choice. The question was, is it
alright to do so and, unless you've got some recent evidence that there's a
problem, the answer is, yes, it's all right. This was a major topic a year
or so ago. Those that should know, diving physiologists, people with the
letters MD after their names, examined the origins of the deepest dive first
"rule" and all available information on the subject and could find no
physiological or other risk basis for it.

Lee

In article <AjO2c.10998$Ix3.163108980@news-text.cableinet.net>,
James <james@hotspam.com> wrote:
>Getting ichy to get back into the water again, so Stoney is the obvious
>choice for a first dive of the year.
>
>...
>
>As I understand (but correct me if I am wrong) a reverse dive increases the
>risks of DCS, but can it still me made safely if enough conservatism is
>added to the dive profile? Is so, I feel it would be safer than making a
>deep dive first.

The one thing to remember is the word that usually comes *after*
Decompression. Theory.

No-one yet knows fully how it all works. Various people have put forward
various theories and produced mathematical models and for the most part,
they seem to work.

Lets think about what might happen. You go down to 6m, fart about for
some time, meanwhile your body is on-gassing, then you start to ascend
Microbubbles form, but most get filtered out and you are fine. You hang
about on the surface, the microbubbles are still whizzing round your
system for some hours, and you kit-up and jump back in.

As you descend the microbubbles on-gas and get bigger! So by the time you
are at depth, they have the ability to on-gas more (more surface area),
and get even bigger when you ascend.

Thats just one theory.

Another theory is that during the first shallow dive, the microbubbles
will actually be eliminated and your subsequent deeper dive will be
safer as your body starts with less microbubbles.

As you see, there are plenty of theories...

But don't let this put you off. I'd do the dives personally. Good for
you for recognising the need to do an easy check-dive before embarking
on something a little more challenging.

Although what I might be tempted to do is go down to 6m, do all my checks,
go over buddies kit, and vice-versa, spend a few moments just getting
usd to it all again, then if I were comfortable, drop over the edge for
a bit of navigation practice...

Gordon

"Gordon Henderson" wrote

> As you descend the microbubbles on-gas and get bigger! So by the time you
> are at depth, they have the ability to on-gas more (more surface area),
> and get even bigger when you ascend.

I don't think that's a very plausable theory... Bubbles always get smaller
under pressure.

>
> Another theory is that during the first shallow dive, the microbubbles
> will actually be eliminated and your subsequent deeper dive will be
> safer as your body starts with less microbubbles.
>

I don't think that's a very plausable theory either. After 4 months surface
interval, the number of microbubbles will be lower than after a shallow
dive. Good examples of what 'theory' means though

The 'problem' with the tables, in my limited understanding, is that they are
unable to take the effects of microbubbles into account, and only really
account for accumulated nitrogen loading, not how it comes out of solution.

For the example given, I think the risk is very small - the nitrogen loading
from the first dive will be small, the excess pressure might be too low to
allow microbubbles in the first place, and the surface interval is
significant compared with the depth/duration of the first dive. On balance,
it might be safer than just doing the 2nd dive in isolation, given that the
practice from the 1st dive might make the 2nd dive better!

Sean

"Gordon Henderson" <gordon@unicorn.drogon.net> wrote in message
news:c2hdr1$7ev$1@unicorn.drogon.net...

> As you descend the microbubbles on-gas and get bigger! So by the time you
> are at depth, they have the ability to on-gas more (more surface area),
> and get even bigger when you ascend.

Ummm, no. The bubbles, in fact, contract as you descend and, to the best of
my knowledge, gas bubbles don't load. They are believed to form the nucleus
for larger bubbles that grow as you ascend. Of course, the same thing
happens if you do your deep dive first, just not as much. There is,
however, a difference in available dive time. If you load the most nitrogen
during the first dive, some tissues are likely to be unloading during the
shallower second dive. The reverse is true when the shallow dive is first.
All tissues will continue to load.

Lee

> > As you descend the microbubbles on-gas and get bigger! So
> > by the time you are at depth, they have the ability to on-
> > gas more (more surface area), and get even bigger when you
> > ascend.
>
> I don't think that's a very plausable theory... Bubbles
> always get smaller under pressure.

IF the number of gas molecules present in that bubble remains the same.
Which it won't, because of the diffusion of gas into bubble due to the
increased partial pressure of the gas.

> > Another theory is that during the first shallow dive, the
> > microbubbles will actually be eliminated and your
> > subsequent deeper dive will be safer as your body starts
> > with less microbubbles.
>
> I don't think that's a very plausable theory either. After 4
> months surface interval, the number of microbubbles will be
> lower than after a shallow dive. Good examples of what
> 'theory' means though

And that's a wonderful example of something called "assertion". Please cite
your references, because I'm fairly sure that Gordon will be able to cite
his.

> The 'problem' with the tables, in my limited understanding,
> is that they are unable to take the effects of microbubbles
> into account, and only really account for accumulated
> nitrogen loading, not how it comes out of solution.

Depends which tables you are talking about. Some of the more recent
tables/dive planning software use algorithms specifically designed to model
the behaviour of microbubbles.

Iain

Mark Cross wrote:
>> Depends which tables you are talking about. Some of the more recent
>> tables/dive planning software use algorithms specifically designed
>> to model the behaviour of microbubbles.
>
> IIRC correctly the new aladin use a "microbubble feature"
>
> Is there any PC software that used micobubble behaviour?

Yes to both. Neither a dive computer, nor a PC program are the tables
referred to.

Lee

<snip>
>This was a major topic a year
> or so ago. Those that should know, diving physiologists, people with the
> letters MD after their names, examined the origins of the deepest dive first
> "rule" and all available information on the subject and could find no
> physiological or other risk basis for it.
>
> Lee

I remember reading an artile in the DAN magazine about this a yera or
two ago - the outcome of a conference was that there seem to be no
physiological reasons not to reverse profiles, but the tables are
based on experimental data where the 'deepest dive first' rule was
used. Therefore, the tables (such as PADI RDP tables) cannot be
guaranteed to be accurate for repetitive dives.

That said, I've done exactly as you plan to in the past - do a quick
check out dive round the bus stop and do a trip to the Stanegarth
later on. Generally you are so far inside the NDLs that there should
be very little risk.