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Jason,

Compartment halftimes vary from 2.65 minutes (fastest in Dr. Buhlmann's
model for nitrogen) up to 600+ minutes for nitrogen.  

Some models have used halftimes as long as 1000 minutes or more, but
these are not very common in the usual models used for operational
dive tables.

Halftimes for different gases vary based on the inverse of the square root
of the molecular wt. (again, as used by Buhlmann), thus lighter molecules
have shorter halftimes for the same compartments (ie, if a compartment has
a 26.5 minute halftime with nitrogen, that same compartment has a 10 minute
halftime for helium - as helium diffuses 2.65x faster than nitrogen for the
same gradient).  This is an extremely simplistic explaination for a very
complex (and still not fully understood) concept.

Hope this helps 

John

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