Redundancy Required for Decompression Diving?

[scubadada]

I suppose by these definitions you could argue that the PADI deep diver specialty teaches deco without requiring redundant gas? I have not taken it myself (nor do I plan to), but I believe they introduce the concept of "mandatory safety stops". I'd venture to say that fits John's definition closer than what's being discussed in this thread.

No, not really. Light deco does not generally refer to optional or "mandatory" safety stops. It is exceeding the NDL of your computer (or table), resulting in a modest duration of decompression time.

Obviously, the specific computer and decompression algorithm used has tremendous implications regarding the nature of the light deco. Exceeding the NDL of a Cressi computer is vastly different than exceeding the NDL of a Cochran. Exceeding ZH-L 100/100 is quite different than 30/70.

 

[AdivingBel]

You have to know the years of history behind this.

mmm...not really. I haven't been on the board that long and IMHO he was pretty obviously making a run at you. I generally enjoy reading DD's posts. They read like a big fish story. Whether 100% factual or not, they are very entertaining. It's too bad when he runs down rabbit holes like this. Oh well...nobody's perfect...not even me. Safe diving.

 

[Remy B.]

I see no real need to dive much past 200 feet and I am not comfortable with a lot of decompression and have a bad back which would make diving large double tanks impractical, so I don't see myself pursuing technical training in the near future - primarily because I doubt it would change the way I dive.
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Have you consider sidemount, and/or Carbon tanks, ( I don't know if they are legal in the US and they are not cheap, and you will need to find a place that fills 4500psi to get the benefit out light weight and big volume ) or two AL-7L tanks, that makes for 14L tank, redundant and compact, and not as heavy as two AL-11L / AL-80cf.

My dad did a lot of crazy diving in his days by the today standards, on air, single tank, deep and ascent slower than his small bubbles, that were his deco profiles.

It is fine if it works for you, but think that a little more safety is not bad thing, and redundancy is #1 step.

 

[sea_otter]

Light deco does not generally refer to optional or "mandatory" safety stops.

It comes down to semantics, but that was the point I was trying to make. Let's say you're planning a dive to 130' for 10 minutes. By PADI's tables, you have a 3 minute "mandatory safety stop" at 15'. By a Perdix, depending upon conservatism settings, you have a deco stop for 2-5 minutes at 20'. It's a grey area. That grey area is the kind of thing the original post was referencing.

By contrast, if you were to spend 15 minutes at that depth, I suspect most would agree that falls into the category of "light deco". I sincerely doubt any respected tech instructor would advocate doing this on a single tank, and as far as I know all agencies which teach that (whether they are called rec or tech classes) require proper redundant gear (and instruct how to use it).

 

[Rhone Man]

I have not taken any technical dive training, so I am curious about this topic. Recently, a technical instructor has indicated that he was "OK" with exceeding the no-deco limits and continuing a dive causing the accrual of a (moderate) decompression celing without the benefit of redundancy.

Is this consistent with current technical training? Is reliance upon a buddy's gas supply considered sufficient redundancy in this sort of situation? Based on training guidelines, would this be OK with a buddy and maybe not OK while solo diving?

Do the technical dive courses discuss this sort of thing or is it left to the discretion of the instructor or the diver?

Sorry, didn't read the whole thread, but I'll chip in anyhow.

When I did the TDI Advanced Wreck course (which was a few years ago) I was a little bit astonished that we were being trained to leave the deco gases at the point of ingress. The logic was simple - wrecks are usually too tight to carry around multiple deco cylinders inside. But what I thought then (and still think now) is if sh** goes sideways and you have to make your egress from a different point to your ingress, you will have no deco gas, and depending on how big the wreck is or why sh** went sideways, it may be no simple matter to relocate them quickly.

Things may have changed since then of course.

 

[WarrenZ]

I was taught that that was one option however if there is the chance of exiting in a different location I wouldn't leave my Deco gas. I haven't had the opportunity to stage my Deco tank.

 

[scubadada]

It comes down to semantics, but that was the point I was trying to make. Let's say you're planning a dive to 130' for 10 minutes. By PADI's tables, you have a 3 minute "mandatory safety stop" at 15'. By a Perdix, depending upon conservatism settings, you have a deco stop for 2-5 minutes at 20'. It's a grey area. That grey area is the kind of thing the original post was referencing.

By contrast, if you were to spend 15 minutes at that depth, I suspect most would agree that falls into the category of "light deco". I sincerely doubt any respected tech instructor would advocate doing this on a single tank, and as far as I know all agencies which teach that (whether they are called rec or tech classes) require proper redundant gear (and instruct how to use it).

Thank you, see my post #61 regarding influence of decompression algorithm.

You do a dive to 60 feet on air and have two computers, an Oceanic computer running DSAT and a Perdix running the medium conservation rec setting of 40/85. You start your ascent at 56 minutes when your Oceanic computer has 1 minute of NDL remaining. The Perdix gives you 7 minutes of deco at 10 feet. Is this a light deco dive or do you just blow off the Perdix deco, knowing that it is considerably more conservative than the Oceanic? This is the kind of argument that went on in the light deco thread.

Despite the fact that the Perdix would not lock me out, I would consider this a light deco dive and clear the obligation. Obviously, other conservative computers would lock you out and you would lose them for backup. If I didn't want to do the deco of the more conservative computer, I would set it at a point where it would more closely match the liberal computer, in this case that would require a GF hi of 95-100. Alternatively, I would dive another Oceanic computer running the same algorithm.

 

[Centrals]

Thank you, see my post #61 regarding influence of decompression algorithm.

You do a dive to 60 feet on air and have two computers, an Oceanic computer running DSAT and a Perdix running the medium conservation rec setting of 40/85. You start your ascent at 56 minutes when your Oceanic computer has 1 minute of NDL remaining. The Perdix gives you 7 minutes of deco at 10 feet. Is this a light deco dive or do you just blow off the Perdix deco, knowing that it is considerably more conservative than the Oceanic? This is the kind of argument that went on in the light deco thread.

Despite the fact that the Perdix would not lock me out, I would consider this a light deco dive and clear the obligation. Obviously, other conservative computers would lock you out and you would lose them for backup. If I didn't want to do the deco of the more conservative computer, I would set it at a point where it would more closely match the liberal computer, in this case that would require a GF hi of 95-100. Alternatively, I would dive another Oceanic computer running the same algorithm.

What is the point of using two computers if you chose to ignore the info given by one of them? In this case it is the Perdix. You should have started your ascent when the ndl of the Perdix is down to 1 min.
I also dive with two different computers, Uwatec Aladin and Oceanic Veo180, and I follow the conservative one ie. Aladin.

 

[stuartv]

What's the distinction between training and diving?

If you do the technical (or whatever) training... and then don't APPLY that training... then you're not doing squat. You wasted money on training.... You bought a card... and nothing less.

You don't see any difference between learning and knowing a set of rules and then choosing when and how to break them versus taking the same action with no knowledge of what "rules" you're breaking?

A tech-trained diver who dives a single tank with no backup, runs a tech computer with GF set to 30/70, racks up one deco stop for 3 minutes, and then blows off the deco stop and goes straight to the surface, just bought a card and wasted his or her money? An OW diver (thus, trained to never exceed their NDL) dives a single tank with no backup, stays down until they one deco stop, for 2 minutes, knows their SAC and has calculated to verify that they have ample gas, ascends and does the mandatory deco plus another 3 minutes of "safety stop", then gets out, also wasted their money and just bought a card?

It comes down to semantics, but that was the point I was trying to make. Let's say you're planning a dive to 130' for 10 minutes. By PADI's tables, you have a 3 minute "mandatory safety stop" at 15'

There's no "just semantics" in my mind. If a dive has a mandatory stop then, by definition, it is not a No Deco or No Stop or whatever you want to call it dive. It is not a recreational sport dive. It is a technical dive. And, in my opinion, any person or agency who attempts to suborn the definition of recreational diving to include dives with MANDATORY stops needs to be taken out behind the woodshed and re-educated.

I am only aware of dive safety statistics peripherally, via reading posts here on SB. So, my opinion may be very skewed from reality. But, I have the impression that recreational diving (as generally taught and practiced here in the U.S., anyway) has a good track record of safety when it comes to DCI. People rarely get bent. People rarely get AGE. It may be considered by some to be a bad thing that people have it so instilled into them to NEVER exceed their NDL. That kind of training may result in the occasional ignorant behaviors (as already described). But, overall, I suspect that that training is a major contributor to why incidence of DCS is so low. Teaching recreational divers that it's okay to do dives with mandatory stops IS teaching people that it's okay to do deco dives without further training. If you teach someone that they can do a dive with a 3 minute mandatory stop, how can you possibly expect them to take you seriously when you try to tell them that a 5 minute mandatory stop (according to their computer) is not okay? Or a 7 minute stop is not okay? And so on...

 

[stuartv]

What is the point of using two computers if you chose to ignore the info given by one of them? In this case it is the Perdix. You should have started your ascent when the ndl of the Perdix is down to 1 min.

In the example, the Perdix is telling him that, if he leaves the bottom then and stops for 7 minutes at 10', he will hit the surface with a GF99 of 85. If he understands what it is telling him and decides that he's okay with getting to the surface with a GF99 that is higher than 85, what is the problem? How does that equate to ignoring the information it is giving him?

If nothing else, the point of diving with two computers is so that you have a backup if one dies.