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Peter 

Thanks for the references on PEEP.

>> Interesting Drager seems to have no problem with exceeding the MOD of the
>> supply gas for both their military and recreation breathers.  I have US Navy
>> documentation recommending both exceeding the MOD of the supply gas and not
>> exceeding the MOD of the supply gas. The more conservative limit is the
>> newest.
>
>They either wised up to the fact that we are "at peace" or had some bad
>experiences. I don't think you can predict accuarately without a pPO2
>meter.

There are probably several issues at work here.  What is the "safe" PO2 level,
what is your lowest metabolic rate, if you operate the bypass how long
will you remain at elevated PO2 etc.  I think with the USN Mark 15 and 16 
they expect temporary PO2s of 1.8 during descent.

>> The cool thing is if you have a Bridge II and are running 8 l/m and 50%
>> you can use the Bridge idiot mode for both O2 and deco calculations.
>
>Sounds way cool - how does this work?

Assume the following

l  = supply flow rate = 8 l/m
r  = metabolic rate = 0..3
fs = supply gas O2 fraction = 0.5
fi = inspired FO2

then from the counter lung equation

    fi = (fs*l - r)/(l-r)

plugging in the numbers

    (8*0.5-3)/8-3 <= fi <= 0.5

which gives

    1/5 <= fi <= 0.5

which gives 

    0.2 <= <= 0.5

Now the Bridge II has a default mode where the FO2 is assumed to be 0.5
for oxtox calculations and the FO2 = .21 for decompression calculations.
Hence it will work perfectly for the above semiclosed circuit dive.  For
other semiclosed dives where the Fs is lower than 0.5 you could use the 
Bridge II for Oxtox and an air computer for decompression.  Note I haven't
actually followed the above proceedure its more of a theory at this point!

Barrie

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