You want to keep the dissolved gas pressure less than a experimentally determined limit. That looks like this where the pressure is on the left of the inequality and the limit is on the right:
Po+(Pi-Po)(1-2^(-t/ht))<Mo+dM*D
where:
Po, is the starting inert gas pressure for a given half time compartment
Pi, is the inspired inert gas partial pressure
t, is the exposure time for a given depth
ht, is the half time in minutes (it has already been suggested that these are 40 and 120 minutes)
Mo, is constant part of the maximum allowable pressure for a given depth. Mo can apply for 1 atmosphere, or zero pressure (which are approximately 56, and 47 fsw for the 40 and 120 minute compartments respectively)
dM, is the variable part of M and lets you calculate a different M for each depth (which are approximately 1.18 and 1.08 for the 40 and 120 minute compartments respectively).
D, is the depth, which is really pressure.
The values for the M’s in feet of sea water, but these are units of pressure and not depth. In fact it is all about pressure and time.
So how does this relate back to pressure groups? Well I don’t know what was done with all the different tables but as a start it would be worth trying dividing the saturation gas pressure by the number of letter groups. Anyway that should be enough of a word problem.
