DECO StOPs

[IndigoBlue]

patmandu once bubbled...


Good question, and I'm not sure I have a good answer for you. 80% certainly isn't a hard and fast rule for deep stops, and no I don't have any 'hard copy' studies of findings that 80% is THE place to start deep stops. Pyle stops at 50% may be the answer....I don't know.

I read the WKP Project discussion recommending 80%:

http://www.wkpp.org/articles/Decompression/first_deep_stops.htm

They postulate it without any discussion of where it comes from. So now we are left to guess where it comes from. An old IANTD deco manual that I have mentions a 1.72 maximum pressure change gradient for helium and 1.61 for air. The reciprocal of 1.72 is 58% and the reciprocal of 1.61 is 62%. Of course, these are old manuals from the 1990s.

80% is half way to 60% (rounded). I am therefore guessing that this is where the 80% comes from. But I do not know either.

The rest of their discussion uses these %s:

"These deep stops are equally divided at all depths up to 65 percent of the profile. At that point you begin lengthening the stops. Between 65% and 45%, the steps slightly lengthen, but max out at 10 minutes. Between 45% and 35%, the max is 20 minutes, between 35 and 25%, the max is 30 minutes, subject to certain parameters."

I have to wonder if RGBM is based on similar "reduced" gradients as well, since RGBM is reduced-gradient?

50% is the reciprocal of 2 which is Haldane. And Haldane is really old thought.

 

[pdoege]

The idea behind the depth percentage seems to be that there is some critical pressure differential where outgassing is fast enough for reasonable offgassing yet not so fast as to cause DCS.

There doesn't seem to be a set number, and the number probably changes a bit depending on the person, depth, temp, etc. The TDI deco manual lists numbers from 2.0 to 1.3, depending on the table/algorithm. 1.3 is around 77%, ie reasonably close to 80%.

I don't think that I would dive based on a set number. I would feel a lot better running a custom table. That being said, 80% seems like a relatively safe rule of thumb.

Peter

edit: I do not have the data in front of me. IIRC, the 2.0 number is from a Navy table. I think the 1.3 was DCIEM. No way to know for sure without checking at home.