Computer or Not

[rjack321]

In your earlier post, where you said "However, there were references to 'recreational dive profiles' which would be different than 'decompression obligation incurred' dives - which are indicated when saturation is reached in a compartment. " <emphasis added>, it would be more correct to say that a decompression obligation is incurred when the maximim allowable value, aka M-value, is reach in a compartment.

I would say "a decompression obligation is incurred if a direct ascent to the surface exceeds the maximim allowable value, aka M-value, in a compartment"

Otherwise, Charlie99 you have quite eloquently described the situation. None of the last 8 pages is required reading for someone considering buying a computer.

 

[Charlie99]

Does the Erik Baker article use the same algorithm as the computer models I've discussed? If not, then it doesn't really apply to my observations, since I'm referring to that model's definition of '100% saturation'.

Those computer models don't have the suffix letter "C" in the documentation I've examined.

Actually, I could have done the math for Oceanic computer using DSAT values and the numbers would have come out pretty close to the same.

The Understanding M-Values article is a general review of dissolved gas models and and has the compartment info for a variety of models, including Workman 1965 (US Navy Tables); DSAT (PADI RDP and wheel); DCAP MF11F6; both the 12 compartment Buhlmann ZH-L12; and the A, B, and C verssion of the 16 compartment ZH-L16. The A version was the original one, with the various limits for the different compartments being derived mathmatically. The B version reduced the limits a bit for the middle speed and slow compartments and it was was used for tables. C is just a smidgen more conservative. The algorithms for Workman, Buhlman, DCAP, and DSAT models are all the same, except that you have to plug in the different values for the compartment halftimes and limits. Indeed, even the "Suunto RGBM" model uses the same basic dissolved gas math, except that for repetitive dives, the M-values are modified according to any "misbehavior" noted in the previous dive --- too fast ascent, skipping safety stop, too short of SI, reverse profile, etc.

In practice, instead of using the standard values of the models upon which they are based, many dive computers add some conservatism by uisng a fraction of the limit, such as 0.9/90% of the various compartment limits.

 

[Charlie99]

I would say "a decompression obligation is incurred if a direct ascent to the surface exceeds the maximim allowable value, aka M-value, in a compartment".

Actually, when at depth, most if not all computers ignore future ascent time in the NDL calculations and just simply show decompression obligation/NDL time remaining based upon whether or not the current compartment pressures exceed the surfacing value M-value limit, aka M0. This is why it is pretty common to have a couple minutes of deco stop called for when at depth, but then to have the deco stop go away during the ascent. OTOH, if one does tooooo slow of an ascent, the deco obligation will increase.

The oft-repeated statement of "computers don't give credit for deep stops" is not entirely true.

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Another common red herring is the argument that somehow a computer forces one to race up to a safety stop. Computers DON'T set your profile. While there are some minor problems, such as some computers wanting to see 3 minutes of hang under 20', in general you should be able to dive any profile you want, and if it is a proper one, then the computer will be happy. Computers most definitely do NOT interfere with doing deep stops.

IMO, smart computer use includes doing a proper ascent and stops.

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As to the original topic of this thread, I look at computers as a useful tool. I pretty much know how much air I have at any given time, and know what my SPG should read when I look at it. I don't throw it away though. Another analogy would be that I both rely upon my senses and upon a speedometer when driving.


My attitude towards a computer is similar. I have a pretty good idea of what I'll see when I look at it, but I don't throw it away.

One can argue whether your brain should be the primary decompression tracker, with a computer as a backup; or whether my method of using the computer to track decompression status and using my brain as a general validity check is optimum. But in either case, using both is better than just relying solely on either the computer, or solely upon one's ability to track depth and time.
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As I said in an earlier post, there are some situations where computers don't add much value (square profile dives, shallow dives, dives with significant deco where prior gas planning and adherence to the plan is important). They are very useful tools, however, in many diving situations --- most signficantly multilevel, repetitive dives without signficant decompression obligation.

 

[TSandM]

BTW, I would like to recommend quite strongly the source I used for the data I provided earlier. I am working my way through the new Lippmann book and finding it fascinating -- very readable, and very thorough in its discussions of the various decompression models. I've read both of Erik Baker's papers which are on line, as well as other pieces about decompression, but this book is making concepts fall together in a beautifully coherent way.

 

[bobv]

I'm not entirely sure what "RDP" means. If you could help me out with that, I'd appreciate it.


"RDP" = Recreational Dive Planner - Commonly refered to as dive tables.

 

[tribaltim]

A computer is just a really neat toy to have and it saves you a lot of brain cells in between dives.

 

[espenskogen]

I think Buehlman originally had 16 compartments. Oceanic and derivatives use a 12 compartment Haldane model.

"Suunto RGBM" is actually pretty close to a Buehlman model, its apparently not the same as Wienke's full blown RGB model. The only computer I've heard of using the latter is the VR3 with a very expensive upgrade. I have only heard negative comments about how long it stretches out deco.

Maybe someone here is using a $1500-1800 VR3 for recreational profiles. I'd be interested in seeing how those NDLs compare.

The Suunto's use the Buhlman model with a wrapper of conservatism based on the Wienke RGBM model. They call it the Suunto RGBM 100 on the new computers and the old ones were called Suunto RGBM 50 (I believe).

The VR3 uses RGBM by default, but you can get an upgrade to VPM for it.