Are some computers unsafe?

[RonR]

I saw a comment from Liquivision over on The Deco Stop that they had a full implementation of RGBM for the Xeo in beta test.

Yes, they will be offering it, and have offered VPM, another fully iterative bubble model, for some time.
Ron

 

[boulderjohn]

It's not really a debate, more of a marketing based misconception. There are two broad implementations of RGBM. "Folded" RGBM is what Suunto and every other RGBM computer on the market with the (partial) exception of the Atomic Cobalt use. This refers to a Haldanian calculation that is modified with various factors to emulate the results that the fully iterative RGBM algorithm would give. It's a way of getting about the same results without having to run the extremely computationally intensive "fully iterative" RGBM calculations, and it is more appropriate for recreational depths. It's not because there is or was any problem getting RGBM to work- except that the "full" version requires a lot of computational horsepower and some clever coding to be efficient enough to run on anything that is also diveable. The "Suunto RGBM" label is, just like "Mares RGBM", etc., only a marketing indication that the implementation is to some extent custom to that manufacturer.

Ron, that is a great explanation that clears things up quite a bit. We need to find a way to make this available every couple of months, whenever this question arises.

 

[Mr Carcharodon]

The slope of the line defining saturation would be much steeper, even though the two computers might not be the same, in terms of minutes of no-deco time they would seem much more alike. With shallow dives, even though the variation in times that the two computers gave you seems large in terms of minutes, in terms of tissue calculations it is probably very slight. So in real terms, the risk differential between them is probably negligible.
Ron

A twenty or more minute of difference is negligible? If the DSAT computers are calling for twenty minutes less deco and thousands of divers do those dives uneventfully eventually you have to conclude that one computer is right and one is not. A twenty minute difference on a 60 minute dive does not seem slight or negligible but definitions vary.

 

[RonR]

A twenty or more minute of difference is negligible? If the DSAT computers are calling for twenty minutes less deco and thousands of divers do those dives uneventfully eventually you have to conclude that one computer is right and one is not. A twenty minute difference on a 60 minute dive does not seem slight or negligible but definitions vary.

The actual calculated saturation difference between the two computers may well be insignificant, but the fact that each has to choose one finite point to switch from no-deco to deco (an artificial distinction) can make the difference expressed in minutes considerable- when dives are shallow. In dives that are deeper, where saturation builds up more rapidly (a steeper slope on the graph) the difference in minutes will be much less, even if the difference in conservatism is the same. A computer that is 1% more conservative might show a 20 minute difference in NDL's on shallow dives, while that same 1% might mean less than a minute difference on a 150' dive. Divers tend to see the large difference in minutes of no deco time on shallow dives and assume that translates into a large difference in conservatism, when in fact it may not. It should not be a reason for concern about the safety of any computer.

The original poster compared going into deco to falling off a cliff- and lots of divers see it that way. But it's a lot more like hikers descending a very gradually increasing slope, and deciding at what % grade to turn back. If one person decides to turn back at 30% and another at 32% grade, and they both head straight downhill, they will turn around at almost the same time. If they traverse the slope at a very shallow angle, the 30% hiker may turn around a long time before the 32% hiker. His turnaround point is no more conservative, his risk no greater, it just took relatively longer to get there.



I need to come up with a graphic for this, it's a common misunderstanding. But yes, two computers could be calculating almost identically and show considerable differences in no deco time on shallow dives. The risk differential could be negligible, because the "in deco" one is perhaps barely in deco and requiring a very short stop, while the "not in deco" one may be requiring a safety stop. A 20 minute difference in a series of shallow dives may not mean much (unless, of course, you have a DM or boat who treats going into deco as if it were a mortal sin); a 20 minute schedule difference in a 130' dive would be a lot more significant.


I hesitate to call any computer "right" or "wrong". They aren't measuring anything in the diver, they are providing broad statistical correlations with dive profiles. Differences between algorithms probably matter less than differences between divers, hydration, physical condition, etc.

Ron