RGBM conservatism

[Diver0001]

I have a question.

I hear a lot about RGBM based computers being more conservative than their Bhulman (sp?) based counterparts. However, I've recently been reading up about decompression practices (with in the back of my mind that I might buy a new computer as a backup to tables for a course I plan on taking next year) and I discovered that RGBM apparently produces significantly faster deco schedules than dissolved gas models.

Now I'm confused.

Why, if RGBM gives you faster deco, is it considered more conservative?

and on a related note:

If Haldane gives you longer deco then isn't this the better model for decompression? I'm speaking only in terms of off gassing, not the practicalities of comfort/boredom etc.

Anybody?

R..

 

[Charlie99]

Diver0001 once bubbled...
I hear a lot about RGBM based computers being more conservative than their Bhulman (sp?) based counterparts. However, I've recently been reading up about decompression practices (with in the back of my mind that I might buy a new computer as a backup to tables for a course I plan on taking next year) and I discovered that RGBM apparently produces significantly faster deco schedules than dissolved gas models.
and on a related note:

If Haldane gives you longer deco then isn't this the better model for decompression? I'm speaking only in terms of off gassing, not the practicalities of comfort/boredom etc.

What you are hearing probably is people that equate RGBM with Suunto, since those Suuntos are by far the most common "rgbm-like" computers. The Suunto RGBM though, is really just a dissolved gas (like Bulhmann) model with the limits being adjusted a bit to try and track the RGBM results. Suunto has chosen rather conservative limits, and if you dive a bad profile (fast ascents, reverse profiles, repeated ups and downs during a dive) the Suunto-RGBM model notes those transgressions and penalizes you significantly by reducing NDLs on later dives.

There are true RGBM computers now being introduced. Hydrospace, Mares, and a few others; as well as some deco programs such as GAP.


As to Haldane vs. RGBM; longer isn't necessarily better, if the mandated stops aren't at the right depths. In profiles that have the same amount of deco time, RGBM will generally spend more of that time in deeper stops, while models that track only dissolved gas spend more time at the last, shallowest stop.

In the NDL or short deco areas, RGBM profiles/calculations aren't all that different from Haldane type calculations. The differences really show up in long deco from deep dives.


BRW, aka "Mr. RGBM" will probably add some more theoretical comments to my observations.

Charlie Allen

 

[Dr Deco]

Hello Scuba Board Readers:

Conservatism

This topic can be approached from several angles; here is mine.

The tables with RGBM have a bit longer surface intervals for the same series of dives than non-RGBM computers for recreational-type diving.

For technical diving, the RGBM algorithm will give a shorter decompression. This is attributed to the fact that micronuclei below the Laplace limit (i.e., subcritical radius) will not be enlarged by Boyle’s law expansion (during the ascent) and automatically become larger than the Laplace limit. When this occurs, they will continue to grow as they experience an inward diffusion of dissolved helium. The secret is to keep them small in the first place.

The Haldane model did not allow for the presence of a bubble phase (that is, humans are really a gas-in-water emulsion for purposes of diving) and thus did not worry about its growth. For several decades, scientists such as Brian Hills, PhD, have been talking about Haldane tables as “treatment tables” for a deep-developed free gas phase.

Safer?

If you are diving a profile with a “decompression tail” such as a Buhlmann table, this extension is needed to fix the problem of bubbles generated in the first big ascent to the surface. The table is not being more conservative since it is simply repairing damage created at the initial ascent.

This is true, however, only if you actually did create decompression bubbles. Most divers do not create bubbles during their decompression. The extra decompression in this case would probably be of benefit. You would not need this benefit if you avoided all of Dr Ds “bubble generators.” These are climbing ladders with all of your gear, lifting bottles and equipment during the surface interval, sleeping during the surface interval, and avoiding strenuous sports during the SI. :nono:

Dr Deco :doctor:

 

[paulwlee]

Dr Deco once bubbled...
Dr Ds “bubble generators.” These are climbing ladders with all of your gear, lifting bottles and equipment during the surface interval, sleeping during the surface interval, and avoiding strenuous sports during the SI. :nono:

Could you clarify this point? Is sleeping to be avoided, or is it
good? (I'm guessing that avoiding strenuous sports is good, so looks like the 'to-avoids' got switched over to 'to-do' mid-sentence.)

 

[BRW]

Yes, Charlie 99 and Dr D,

Thanks, and right on. Plus this:

1) -- for a given deco dive with given
bottom time, RGBM will give deep
stops always, PLUS shorter overall
deco and hang time in the shallow
zone 99% of the time;

2) -- recreational RGBM algorithms for
air, nitrox, and trimix will
track very short SIs (say less than
an 90 min or so), reverse profiles
(say greater than 30 fsw or so), altitude
(say greater 3,000 ft elevation), and
multiday diving (say continuous repets
over 18 hr intervals) and ADJUST
NDLs and associated diver staging
according to dissolved phase plus
bubble dynamics folded over recreational
diving data.

Happy Turkey Day, and don't dive too stuffed
afterward.

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
NAUI BOD Chairman Technical Diving

 

[DeepScuba]

Here's what I've recently read, so don't shoot me if what I read is incorrect, cause I don't know Jack.

RGBM vs Haldane.........Haldane uses the "shallow as possible" and then "sit" on the bubbles, then shallower, then sit, then shallower, type of decompression. Basically just ascend until just before bubbles (if that's what you want to call it), and then "treat" the bubbles with stops. (Bend and treat??)

RGBM incorporates deeper stops, eliminating <most> bubbles from reaching the size that, theoretically, Haldanes bubbles has.

The article that I've read then goes on to say, that basically, within recreational limits, there's not a whole lot of difference that one would see, although RGBM "seems" to have a good theoretical basis to follow it over Haldane.

Then it gives a very good point for all you "must have the latest" dive computer types. The current computers on the market, for Rec purposes have an EXCELLENT track record, as does the dive tables..all of this is without RGBM.

It continues..........RGBM, although of minimal use (but favoured) in Rec use, it really shines in Deco diving, where it can, in fact, get you out of the water quicker.

Like I've said, I'm just writing form the hip, from what I got out of it. The wording might not be perfect, but I believe the general "jist" is correct.

I, for one, although I hate doing long deco, would be just as happy to continue my method of doing all mandatory deco, PLUS use deep stops, PLUS a "buffer" to ensure added safety.

When something tells me I "can" come up 5 minutes early, I'd say thanks, but I'm hanging around just the same

It ain't gonna kill me.

Comments Doc?

(keep it layman).

 

[Genesis]

When something tells me I "can" come up 5 minutes early, I'd say thanks, but I'm hanging around just the same

It ain't gonna kill me.

If that 5 minutes extra is in the shallow parts of the curve, sure, its fine and adds in extra conservatism.

Doing it at the deep parts of the deco would be an extraordinarily bad idea!

I suspect you meant that, but clarification is probably a good thing.....

 

[BRW]

DeepScuba,

Don't what "article" you are reading, but please
know that Mares, Dacor, Suunto, plus others can't
keep up with the demand for RGBM computers. As
posted above, rec RGBM computers are NOT the same as
the "old stuff" that's been around for 20 yrs.

Only for single, no-deco dives (first dive) is there overlap
with the Haldane "models". After that, because of bubble
dynamics, such overlap flies out the door with RPs, short SIs,
altitude, and multiday diving.

Article you are reading sounds like PADI one, and has
mucho misfact in it. Education takes time.

Cheers,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
NAUI BOD Vice Chairman Technical Diving

 

[DeepScuba]

I guess it's a good thing I don't care to waste much time with it.

Basically I'm a "what works, works" type of guy. Although knowing some things give me great pleasure in its understanding, the simple fact that, even though DCS is "real", the true math of it all will forever be unknown with all its human factors etc.

Basically I don't give a crap! I dive my dives with mucho conservatism, and whatever happens, happens.

Simple, and not much learning involved. There have been millions upon millions of dives, both rec and tec, with over 99.99% of them without DCS incident while diving within their perspective limits........RGBM may be XXXX, but it's mostly background noise to those that dive within the prescribed limits to begin with.

I wish for the life of me I could put a finger on the article so I could at least give it it's due.


Regards

 

[Genesis]

Don't what "article" you are reading, but please
know that Mares, Dacor, Suunto, plus others can't
keep up with the demand for RGBM computers.

Suunto, in particular, certainly does not produce what I think of as an "RGBM" computer.

Take your Vytec (or Vyper, or Cobra, or whatever) which is claimed to be an "RGBM" computer into deco.

You will not get deep stops and a nice computed curve to the surface. Instead, what you will get is a Haldane-style ceiling, above which you must not ascend, and a "minimum time to ascend" which reflects going right there to that ceiling and sitting until the timer expires.

If you go way into deco the ceiling will come down, but again, that's the REQUIRED ceiling, not an optimum curve.

In terms of recreational dive computers that can be used as "decompression meters", I've yet to find one of these "real RGBM" models. A buddy of mine has a Mares M1, and his does not give deep stops either; he dives his into deco and "runs by its recommendations", although I've pointed out that he would be well-advised to learn a bit more about the shape of a proper decompression curve lest he some day get a "surprise."

None of them give "recommended" ascent profiles on recreational NDL dives that differ from the "stop at 15' for 3 minutes" M.O. either. None will give you a profile ascent; all figure "heh, get out of the water if you'd like", although all will complain about overly fast ascents, and most will penalize you on repets if you do ascend quickly.

Suunto lights up a "microbubble" warning in many cases, but their meters do this a good part of the time even with an absolutely textbook, with deep stops, ascent from a NDL dive. Basically, if you dive over 50' or so with a Suunto, there's a good chance you'll get that microbubble warning - although occasionally I've managed NOT to light it on deep dives, but for the life of me I cannot determine what I did differently, even examining the profiles in DETAIL in Dive Manager later on.

So where ARE these computers that people can't build fast enough to satisfy demand, yet which produce real RGBM dive and ascent profiles?