To finish the thought (and accept my final judgement)
The new depth would present a whole new set of stops using standard tables/protocols for sea level. These times would not scale, they would be assigned to new shallower (but corresponding) depths in the altitude dive.
It appears to me that if this is not correct, then gas diffusion into tissue is highly dependent on absolute pressure and relative concentrations rather than relative concentrations alone.
Honestly, you could check if Buhlmann and VPM compensate for altitude in a similar way. You could create a dive profile at sea level, compensate it for altitude (like 30m@SL and 24m@2000m altitude) and validate it that way yourself. It wouldn't be perfect, but a very similar result would give credence to the concept.
Remember that decompression models are based on pressure ratios, rather than on absolute pressures. In determining how your body rids itself of excess inert gas, decompression models rely upon the ratios of the pressures you experience at depth, and the surfacing atmospheric pressure you experience after the dive. The key to not forming inert gas bubbles in your body -and thereby avoiding DCI- is to keep those pressure ratios within tolerable limits.
So if you're at altitude using a classic dissolved gas Buhlmann Model and ascending to the leading tissue compartment's M-value with the highest allowable gradient between dissolved inert gas tensions and ambient pressure at a particular deco stop depth -and maximizing your offgas rate ideally without bubbling- then you would have to compensate a sea level deco table for a deeper theoretical ocean depth with corresponding time in order to convert an actual given depth at altitude (as well as depth in ffw versus fsw if indicated), because of the greater decompression stress brought on by driving a steep pressure gradient, and subsequent tissue surfacing supersaturation tensions post-dive at a lesser ambient atmospheric pressure at altitude.
A simple NDL compensated sea level dive table example:
For a given altitude
A, the atmospheric pressure
Pa (in atm) at that altitude is
Pa = (1 atm) * exp(5.255876 * ln(1 – (C * A))).
where
C = 0.
0000068756 per 1 foot; or
C = 0.
000022558 per
1 meter, depending on whether the altitude is given in feet above sea level or in meters above sea level.
With a calculated Pa (Pressure at Altitude determined from the above equation) and given Da (actual depth at Altitude in ffw), we have the general equation below yielding a Theoretical Ocean Depth (TOD), and with these compensated depths we can use dive tables that are based upon sea level diving in the ocean:
TOD = Da * (1 atm / Pa) * (33 fsw / 34 ffw);
or TOD = Da * (1 atm / Pa) * (10 msw / 10.3 mfw).
So given a dive to 60ffw at 4600ft altitude:
Pa = (1 atm) * exp(5.255876 * ln(1 – (
C *
A))).
Where C= 6.8756E-6, and Given A=4600 ft.
Evaluating for Pa:
Pa = 0.844 atm;
So a dive to Da = 60 ffw with Pa = 0.844 atm:
TOD = Da * (1 atm/Pa)* (33fsw/34ffw) = 68.9 fsw (approx 70fsw).
Therefore a 60 ffw at 4600ft altitude, is equivalent to a 70 fsw sea level referenced dive table with a conservative NDL time of 40 min (see table
http://scuba-training.net/scuba/images/hugi.pdf).
Some Equivalent TOD's for actual depths given in ffw at 4600ft altitude are:
50 ffw => 57.5 fsw
60 ffw => 68.9 fsw
70 ffw => 80.4 fsw
80 ffw => 91.9 fsw
90 ffw => 103.4 fsw
100 ffw => 114.9 fsw
110 ffw => 126.4 fsw
17 ffw => Safety Stop 20 fsw
9 ffw => Safety Stop 10 fsw.
