Hi Mark,

> Firstly I would just like to thank you guys for decoplan - got my copy
this
> week and I'm really pleased with it.  The key factors for me are
simplicity,
> ease of change of variables and simple range planning.  Keep up the good
> work!
>
> 1)    Decoplan
>
> Best of all I can now dive with an understanding of how I have set the
> safety factor - which leads to my first question.  The first decision is
> what gradient to commence using these tables on.  I was thinking about
> mailing the list about this when your mail came through viz:

Let me give you some background first so you know where I'm coming from.
I've been using Decoplan for almost 2 years now from some of the earlier
versions up to the current one.  In that time, I, along with some others
have used various low and high gradient factors over a range of dives.
We've dived it in the UK on plans from shallow dives to 100m (330ft) dives.

When I started, I was using a low GF of about 20-30% and a high GF of
80-85%.  I'd read Eric's articles and understood the theory behind the
model, but I didn't understand how my body would react to it.  As I dived
these schedules, I became more comfortable with the plans and discussing it
with the others, we decided to push the high GF's a little, going up to 90%
and we now use 100%.  The low GF has pretty much stayed at 20% for the more
extreme stuff and maybe 30% for shallow dives.

Why have we chosen these settings?  Well, we've found that they work well
for us and have been tested over a range of dives spanning 2 years.  This is
not to say that you should use the same settings.  As with all deco
software, it is a model (which we believe to be a good fit), but we all know
too well how models can diverge from reality.  I would suggest that you get
to know your own body and with experience you will learn what you can do.
Start with more conservative GF's and see how you feel after the dives.
Only you can tell how it works for you.

A simple rule of thumb to remember about the GF's is that the low GF
determines where your first deep stop starts - so on more extreme dives, it
would be prudent to start your stops deeper.  The high GF determines how
much time you spend on your last stop, so a more conservative value will
give you longer at that stop.  The intermediate stops are then interpolated
on the GF low-high line.

> 2)    Gas choices - 330 for 25
>
> Would also like (anybody's) views on gas choices for a wreck dive I am
> planning next year - Planned ABT is 25 mins at 100m (330fsw)
>
> My initial thoughts are
>
> Bottom mix    11/65
> 60m                20/30
> 30m                36%
> 12m                70%
> 6m                   02
>
> Initial logistics thought is to stage O2 on the deco station but to carry
> the rest.  In the case of a problem (ie surfacing away from the station)
> this allows comfortable completion of deco on the 70% (if the surface
> support does not get to me with 02 - unlikely but not impossible).  As
> background this is 20 miles offshore in the North Atlantic with a tight
> window in strong tidal streams.

It is always prudent to carry all gas you need to get out of the water
safely when diving ocean but in dives such as that suggested, carrying all
those bottles may not seem desirable.  If you're going to stage the oxygen
bottle, then you need to have absolute confidence in your support people
that whatever happens they can get the bottle to you - or more specifically,
if you can't make it back to the upline and hence deco station, the
contingencies will work.

Rather than use 70%, I'd suggest changing to 50%.  This has been shown to be
a more efficient gas when combined with oxygen at 6m (20ft).  The backgas
choice is fine, so that leaves an intermediate travel/deco gas.  From a deco
point of view a 35% mix is more efficient, however as this is a bounce dive
you could substitute a deeper travel gas such as 21/35 or 18/40.  This will
open up the oxygen window for you deeper.  We did a similar dive at the
weekend (shorter BT) using 11/65, 21/35, 50% and 100% with no problems at
all.

Another advantage is that you've 1 less bottle to carry - 3 stages are
easier to manage than 4.  You can clip the oxygen bottle to your left waist
d-ring and then when you're on your 9m stop you can swap the 21/35 bottle
with the oxygen bottle.

Best,
John.

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