Mailing List Archive Mailing List Archive
Hi there 'DeepTek'
*Someone else* playing around with computers on an Easter Monday ...!
My excuse is the atrocious holiday weather that we now come to expect in
England. Gale force winds blowing across the country & persistant rain stops
even the most hardened diver...unless you are fond of diving inland flooded
quarries & lakes (or even 'deep' rivers...well, not *that* deep, but some of
them have large 3 to 4 ft. 'strokable' salmon in them at the right time of
year :-) !!!
Anyway, thanks for the quick reply.
On Mon, 17 Apr 1995 12:16:01 -0400 DeepTek wrote:
>
> It's really not necessary to fill your O2 deco cylinders all the way because
> you are only using them at the 20ft (6m) and 10ft (3m) stops where your air
> consumption is very near to that of the surface. (ppO2 at 20ft (6 m) is 1.6
> ATA.)
>
> A 300 cu ft. (8500 liter) O2 supply cylinder pressurized to 2250psi (153 bar)
> will fill an empty 80cu.ft (2265 liter) deco cylinder to 1775 psi (120 bar)
> or 47 cu. ft (1330 liters). More if the deco cylinder is not completely empty
> to begin with.
This is fine & is pretty much along the lines I had thought of...without
going through the figures in detail.
I have to convert your trans-Atlantic cylinder sizes as we always quote
cylinder sizes in metric terms of litre capacity at 1 bar atmospheric pressure.
Pretty well all of our cylinders have a 232 bar working pressure, although the
high pressure (& much heavier !) 300 bar cylinders are making an appearance and
some older cylinders working at 207 bar are still around.
From memory what you call a '100 cu.ft.' cylinder we would call a '12
litre @ 232' bar cylinder. So my manifolded/independent twin 10 litre main
cylinders work out at about 80 or 85 cu.ft each and my 4 litre pony/deco
cylinder is about 33 cu.ft.
This means that in your example, the 80 cu.ft. deco O2 cylinder is a
'full-size' cylinder working at approx 1/2 normal working pressure. Is this
what people on your side of the Atlantic are normally using for O2 deco ??
This is fine if the cylinder is waiting on a surface bouyed line ....
assuming you make it back to the line *every* time, but a pain to lug aroung on
the whole dive in addition to the main twinset if you don't want to risk
missing
the deco line.
I had planned to use a much smaller deco cylinder (4 litre/33cu.ft.),
hence the concern about getting a full 232 bar fill.
We normally send up a delayed bouy deco line from the bottom or on the
way up. That way we can do our stops 'hanging free' of a fixed line. It means
that the surface boat has to follow all the dive-pairs separate bouys, but with
our often strong currents the advantage is a stress-free deco stop rather than
all 'flapping like flags together' on a fixed line. There are many other ways
of working this out...but this is our most common practice, so we need to have
all of our deco gas with us on the dive.
Anyway to follow your example, having paid for the hire of the whole 300
cu.ft. storage cylinder and purchased the contents 'full' of O2 at approx 150
bar, the first cylinder gets filled to 120 bar. If it is only 'half-filled'
like this, it's a pretty sure bet that it *will* be nearly emptied when it
comes
back (especially if it isn't as large as the 10 litre/80 cu.ft. example size).
So.....what about the next fill, or your buddies fill ??? This will only be to
about 95 to 100 bar. The 3rd fill about 80 bar (1/3 'full'). The 4th fill will
be 65-70 bar..and so on. I haven't worked these figures out precisely, but
they
are probably not too far out.
Is this *really* what you are all doing...or is a Haskel booster pump
the only way to get an efficient usage out of an O2 cylinder ? Extra costs !!
Alternatively we need to get a bank of 3 or 4 O2 storage cylinders.
More costs !
Any comments ??
>
> An average diver with surface air consumption of around .5 cu. ft. per minute
> will use O2 at the 20 ft. (6 m) stop at the rate of .8 cu. ft. (22.6 liter)
> per minute, and .65 cu. ft. (18.4 liter) per minute at the 10 ft. (3 m) stop.
> This is enough gas to last 20 minutes at the 20 ft (6 m) stop plus 40 minutes
> at the 10 ft. (3 m) stop with a margin of about 5 cu. ft. (140 liters).
>
> This amount of gas is typically more than enough for even a heavy exposure
> dive. Remember to specify pure O2 at the 20 ft. (6 m) and 10 ft. (3 m) stops
> in your dive software when calculating your dive tables.
OK... but using a 4 litre/33cu.ft. 'pony' deco cylinder, it may well not
be enough if it's only '1/2 full' or less.
>
> I typically find it useful to deco on Nitrox 40 from 100 ft. (30 m) to 30 ft.
> (10 m), and then switch to O2 for the last two stops. This gets me out of the
> water fairly quickly. Don't forget to watch the OTU's also.
How many cylinders do you normally take with you on a 50m to 60m dive &
how large are they all ?
>
> As for using Nitrox 50 as an emergency bail-out gas, this is definitely NOT a
> good idea if you are below 72 ft. (22 m). The dynamic physiology of oxygen
> toxicity makes breathing any gas with a ppO2 greater than 1.6 ATA very
> dangerous, even for a very short period. Even maintaining a maximum ppO2 of
> less than 1.6 ATA is no guarantee that you will not take an O2 hit. A better
> approach in this emergency scenario, in my opinion, is to share air, buddy
> breathe, or perform a controlled, emergency swimming ascent until you reach a
> depth appropriate for the Nitrox. An ESA won't work if you are in an overhead
> environment though--obviously.
Yes. I accept this. With redundant twin cylinders, I can't think of a
probable scenario where we would need an extra 'bail-out' bottle.
As a matter of 'academic interest' however, ...if you had a for example
a Nitrox50 deco set and went onto it at say 50m in order to 'bolt for the
surface' (say you had a manifolded twin set, had a catastophic free flow & you
were unable to shut off the central isolation valve & hence lost all of your
bottom gas), the nitrox50 deco bottle would give a max ppO2 of 3 bar, which
would quickly reduce to 2 bar at 30m and finally to a 'safe' 1.5 bar at 20m.
The max ppO2 is nearly 2 times the accepted 'safe' limit (although there seems
to be differing opinion on just how much is really safe) Any idea of how long
you would get (seconds/minutes ??) before an O2 hit 'got' you ?? In other
words, if all else failed would going onto the Nitrox50 set be totally suicidal
or would you stand a fair chance of making it from 50m ?
I would have thought you could get from 50m to 20m in a minute or two if
you really had to, but on the other hand you would probably get a couple of
minutes from a free-flowing cylinder before it completely emptied.
Any thoughts ?
>
> I hope the math wasn't too detailed. It should be clearer after your advanced
> Nitrox class this weekend.
No. It's pretty clear. I've 'read the book'..now its time to put it
into action....safely (!)
Regards, Steve M.
**************************************************************************
* * *
* Dr. S. G. Millard, * E-Mail : ec96@li*.ac*.uk* *
* Senior Lecturer, * *
* Department of Civil Engineering * Tel : 0151 794 5224 (UK) *
* University of Liverpool, * 44 151 794 5224 *
* PO Box 147, * (International) *
* Liverpool L69 3BX, * *
* UK. * Fax : 0151 794 5218 (UK) *
* * 44 151 794 5218 *
* * (International) *
* * *
**************************************************************************
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]