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Date: Mon, 3 Aug 1998 15:10:25 -0400 (EDT)
From: "William M. Smithers" <will@tr*.co*>
Subject: Semiclosed Bailout (was Re: Re: Death was a Bigot)
To: Kevin Connell <kevin@nw*.co*>
Cc: techdiver@aquanaut.com

On Mon, 3 Aug 1998, Kevin Connell wrote:

> Will, again, I have little knowledge of rebreathers, but I thought fully
> closed units have one dilutent gas (helium or nitrogen) and oxygen.  
> 
> If so, how can you possibly use two seperate non-breathable gasses without
> a working PO2 sensor?
> 
> If not, then the dilutent must be breathable, but it would have to be like
> a really low O2 heliox or nitrox, and I can't see that as "very efficient"

First of all, I'm no longer subscribed to techdiver - so
I won't get any follow-up posts unless I'mm CC'd on them.

OK, on the efficeint semi-closed bit, check back in the
rebreather archives a month or two (subject was
"depth modified SC" or something like that).  I
listed the math for how to calculate the optimal
breathe-purge ratio for semi-closed operation.

First of all, the method only uses diluent up
to 20ft, where you switch to 100% O2.
The basic concept is that a ratio of 5 breaths
to 1 exhale-through-the-nose-and-add-gas method,
which is the classic semi-closed ratio, is 
massively wasteful.  Why? Well, it typically
takes something like 10-20 minutes to breathe
down a 1.2 loop at depth to .4 or so.  That's
alot of breaths - depending on depth and mix,
you can actually get in excess of 200 breaths
before dumping a breath.  As you get shallower,
and the PO2 of your gas drops, you get less
breaths per purge, BUT, you are inhaling 
less gas per breath, so in terms of volume
of gas used, it's about equal for both
deep and shallow.   Once you get shallow,
that ratio may drop to 20:1 or so if you
are breathing your bottom mix. 

One of the key elements here is that the
amount of metabolized o2 is roughly the
same per breath - regardless of how fast
you are breathing.   In other words, your
PO2 per minute consumption rate stretches
proportionately as your breathing rate lowers.
For this method to work, you must know your 
maximum working breath rate and how fast you 
metabolize O2 at that rate.  A table is constructed that
shows the number of breaths you can
take for a given depth, as you ascend.

Here's a sample table where you have 
small quantites of heliox10 and
EANx40 available.  To keep it short,
I'm only showing periodic samplings.  Note
that this is using MY personal values
on MY rebreather, and will be different
for each user.  Also, this assumes
you have planned a set of bailout
tables that allows "sawtoothing"
PO2's from ambient to .4

Depth  Mix  Breaths before dump, workload independant.
-----  ---  ------------------------------------------
0      40%  30
30     40%  110
60     40   220
100    40   360
110    10   10
120    10   20
160    10   55
200    10   90
250    10   135
300    10   180
400    10   273
 
As you can see, you can get quite a bit more distance
out of your SC gas than is commonly thought.  The same
basic idea was applied in designing the Halcyon, but
since they (justifiably) don't want the Po2 varying
very much while diving, they tend to inject gas
much more often.  But for bailout purposes, it's
acceptable to calculate for varying PO2 in order
to get maximum time out of your available gas.

Hope this helps clarify it,

Regards,

-Will


> At 01:05 PM 8/3/98 -0400, <will@tr*.co*> wrote:
> >That's why the concept of redundancy is used in electronic rebreathers.
> >And again, even if all the electronics fail, you can then
> >fly it in a very efficient semi-closed mode.
> >
> 
> 
> -----------------------------
> Kevin Connell <kevin@nw*.co*>
> 
> NW Labor Systems 
> http://www.nwls.com
> -----------------------------
> 
> 
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