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Richard Pyle writes:
> 
:
> There are two factors involved: partial pressure of inspired nitrogen
> (PPN2) which depends both on the mixture breathed and the depth; and ambient
> pressure (which, of course, dependends only on the depth).  The rate of
> nitrogen uptake into your system depends only on the inspired PPN2.  By

Actually, this is a little more complicated.  The rate of N2
absorption is determined by the *ratio* of inspired PPN2 to dissolved
PPN2.  That means that ongassing slows down as your dissolved PPN2
level approaches that of the inspired gas, forming the traditional
"exponential" curves.

>Decompression "Ceiling" DEPTHS are determined by the AMBIENT PRESSURE required
> to prevent excess nitrogen in the blood & tissues from coming out in the
> form of bubbles, and thus are not affected by what the diver is breathing
> - only by the amount of excess dissolved nitrogen. 

A simpler way to state this is to state it as a ratio: the model
limits for Haldanean models (which includes Buehlmann models) are
simply the supersaturation ratios for various compartments.  That
means that you have a ratio, say 5:1, below which you will not form
bubbles.

Put another way, if your dissolved nitrogen PPN2 is less than 5 ATM
absolute (ATA), you can safely return to 1 ATA (the surface.)  If it
is more than 5:1, you cannot safely surface: you can only ascend to a
depth at which the ratio is 5:1 or less.

For example, if your dissolved PPN2 is 10 ATA, you can ascend to 2 ATA
(10:2 == 5:1) without forming bubbles -- in theory, at least.

By doing some examples, you can see that your ceiling may vary.  If
you then add one more complication -- multiple compartments, each with
a different supersaturation ratio -- then you are pretty close to
understanding the basics of Haldanean theory.

Cheers,

David Story                        NAUI AI Z9588, PADI DM 43922, EMT
story@be*.wp*.sg*.co*		   Oxygen is a drug in California.

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