Will,
Are you trying to say that if one of us is exerting at 200 feet, there is no
difference in O2 consumption, versus if we are running at a heart rate of 65
bpm???
There is no possible way you can think this---but from your post it looks
like that was what you were saying. Please clarify this.
Dan


-----Original Message-----
From: William M. Smithers <will@tr*.co*>
To: Kevin Connell <kevin@nw*.co*>
Cc: techdiver@aquanaut.com <techdiver@aquanaut.com>
Date: Monday, August 03, 1998 5:31 PM
Subject: Semiclosed Bailout (was Re: Re: Death was a Bigot)



>
>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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