Dive and Fly - - -

[tlj3071]

There are no-D limits for 30 feet, the generally accepted saturation depth that permits return to the surface at 30 fpm is 24 fsw, not 30 fsw.

24fsw (relative to surface) = 0.727273 bar (at a constant temperature)
15fsw (relative to 8893.118' MSL) = 0.727273 bar

Thus, 24fsw (relative to surface) < 15fsw (relative to 8000' MSL)

I'm afraid Mr Carcharodon's point is still valid.

Tom

 

[Thalassamania]

From 24 FSW to the surface there is a drop of about 42%.

From 15 FSW to 8,000 feet is a drop of about 50%.

It is, I believe, the percent change, not the absolute change that is the critical factor.

 

[tlj3071]

From 24 FSW to the surface there is a drop of about 42%. From 15 FSW to 8,000 feet is a drop of about 50%. It is, I believe, the percent change, not the absolute change that is the critical factor.

In a saturation model (e.g. Haldane), the absolute pressure gradient controls the physics. Perhaps this would not be true in a perfusion/diffusion model (e.g. RGBM).

Also, perhaps my math is suspect, but I get a 40.3092% change in pressure from 24 fsw to the surface: 24fsw = 100*(1 - [1.0832501/(1.0832501 + 24/32.80839895)]) - note that 1 atm surface pressure = 1.0832501 bar on a "standard" day. Originally, I used approximations (e.g. 1 + 24/33, etc).

To replicate a 40.3092% delta pressure gradient using an initial (instantaneous) ascent from 15fsw to the surface would require an instantaneous further ascent to 3701.544' MSL. To replicate the 42% pressure gradient would require a further instantaneous ascent to 4264.481' MSL

For reference, from the 1976 U.S. Standard Atmosphere: pressure in the Troposphere (below 36089'):
p = p0(1+ah/T0)^5.2561.
Rearranging for h:
h = T0/a * (exp[ln(p/p0)/5.2561]-1)
Where: p = pressure at altitude, p0 = Sea level pressure (in same units as result), a = temperature lapse rate in the troposphere (-0.003566 degF/ft), T0 = standard temperature at sea level (518.67 Rankin), h is geometric height (in ft) [note: formula is somewhat different if geopotential height is used].

Tom

 

[Thalassamania]

I appreciate your detail, but we are getting into that realm that Billy Bob Hamilton identifies as measuring with a micrometer, marking with chalk and cutting with an ax. Approximations are more than good enough.

While ultimately the driving force for any model is the partial pressure of a gas, a function of the absolute pressure. However, for the comparison that you are trying to make (e.g., looking for a way to compare an ascent to the surface from 24 feet and an ascent from 15 feet to 8,000), I believe that percentages are the correct way to look at it.

Also keep in mind that we are analogizing SATURATION at 24 FSW then going to the surface at 30 FSW/M and SATURATION at 15 FSW then going to the surface at 30 FSW/M.

 

[Mr Carcharodon]

It is, I believe, the percent change, not the absolute change that is the critical factor.

Why would you say that?

Haldanian models just use pressure gradients. In the end this is just Newton's first law of motion which says things do not move if there is no force acting on them. The pressure gradient is the force. And setting surface tension effects aside the bubble models appear to be the same (except for the added book keeping of tracking two phases).

 

[Thalassamania]

Actually Haldanian models are based on ratios (2:1, basically) and then are modified by practical experience. Percent change is a good way to look at this because were are comparing two situations not calculating the absolute effect of a single situation.
If you doubt using percentage change to see if two situations are similar, give some thought to how you do END calculations.

Doc, or any of the other deco experts here ... am I off base?

 

[theduckguru]

I keep reading all the 8000 ft comparisons. If you fly heavy iron in a presurized cabin, the pressurization is 8000 ft +/-. Loss that pressurization at 35,000 ft, you are at 35,000 ft. I doubt that many on this board are flying in a piston single engine or piston multi engine aircraft below 8000 ft traveling home from diving.

 

[tlj3071]

I doubt that many on this board are flying in a piston single engine or piston multi engine aircraft below 8000 ft traveling home from diving.

Maybe not, but I do. And, I prefer not to fly my C172 much above 8000' MSL anyway - makes it difficult to enjoy the scenery.

Tom

 

[Mr Carcharodon]

Actually Haldanian models are based on ratios (2:1, basically)

Thal,

Of course you are right. J. S. Haldane did use ratios circa 1908. That has not been the prevalent approach since the 1960’s however. See for example the work of Workman or Buhlmann where the m-values, and hence the ratios, vary with ambient pressure. But you likely are viewing this from a longer historical perspective, which is good to hear. Pressures, or ratios of pressures, is nit picking in any case.

A.

 

[theduckguru]

Maybe not, but I do. And, I prefer not to fly my C172 much above 8000' MSL anyway - makes it difficult to enjoy the scenery.

Tom

Flying a 182RG on this end. Even with 235 hp and a constant speed prop not much flying above 8 unless the wind gods are smiling.

Flying in a tubine or jet at low altitude is not fuel efficient and you are going to be considerably higher than 8000 even jumping pretty short distances.