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> I wrote:
>> These things are expensive
>>and definitely more dangerous to use than open circuit.
>
>Raimo wrote:
>Why is it (the Atlantis I) more dangerous than open circuit?????
This is a fair question. I haven't dove the Atlantis I and hence can't comment
on it specifically. However I have built a unit of the Atlantis I type and
hence have an understanding of the principles involved. The true risk of
the Atlantis I can only be born out with experience. Anyway here are a few
of the added risks which I believe exist:
1. The inspired FO2 depends upon your metabolic rate. If you work harder
the FO2 decreases. If you work less the FO2 increases. This leads to the
danger of hypoxia, ox tox or the bends.
2. The gas addition is dependent upon a very small orifice. These have
been known to plug ( I know of at least 2 cases). If this happens your
inspired FO2 will drop. This can lead to hypoxia or the bends.
3. The gas addition is dependent upon the constant intermediate stage preasure
of a regulator. If this varies the inspired FO2 will change and you risk
as above in 1.
4. The system chemically scrubs CO2 from the loop. Scrubbers can fail in
a number of ways including:
a. Bad absorbent.
b. Cold.
c. Bad pack.
d. Flooding.
e. Depth.
f. Desication.
If the scrubber fails you will experience hypercapnia.
5. Caustic cocktail. If the scrubber is wetted then a very alkaline solution
will develope. The danger exists that this material will flow into
the mouth piece and possibly into your respiratory system causing burns.
This could also happen if the scrubber material produced dust do to too
much handling.
6. Breathing loop rupture. In the event of a catastrophic flood of the
breathing loop you will instantly be about 10 Kg ( ruff number )
heavier and unable to breath. Even a faulty dump valve could
suddenly leave you negatively bouyant.
7. Biological contamination. The breathing loop is an extention of your
respirator system and may infect you if not kept clean.
8. Since the volume of gas caried with these types of units is much smaller
than on open circuit the amount of backup or bail out is reduced in an
emergency. A free flow with a small cylinder may be much more critical
than a free flow with a large cylinder.
Now I don't claim that these are insurmountable problems. I have a semiclosed
constant mass flow rebreather and I do dive it. However I do claim that they
are risks which either do not exist with OC or are increased with a semiclosed
constant mass flow rebreather. Some of the above risks are specific to
the semiclosed constant mass flow and some are more rebreather generic.
The above risks must be weighed against whatever benefit the rebreather
will bring to your mission and also your ability to deal with these risks.
It's not clear to me what mission the Atlantis I type rebreathers are good
for.
You can't dive deep ( at least not with this model ). They don't increase
deco efficiency ( quite the opposite ). I doubt they are going to save
on gas costs. Some people may like the weight to duration benefit but I
wouldn't
use one for that reason. Some people may like the quiet although I wonder how
quiet they are to a fish or marine mammal.
Barrie Kovish
Vancouver, Canada
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