Deco cleared "on the go"?

[doctormike]

No disrespect, you are a fine poster, but you should try to be more humble about the nature of your understanding of how all this works. You have an imagined definition of NDL which may not be correct.

Thanks! You are too...

But I think that we are overthinking this. And for the new diver, or non-technically trained recreational diver, implying that a ceiling is anything other than an indication that you have overstayed depth, time or both, is confusing and potentially dangerous. That's all I'm saying.

Of COURSE the ceiling is a mathematical coding construct that may not reflect physiology perfectly. But I don't think that's relevant to this discussion.

I'm not reasoning from first principals at all. I'm making the point that a new diver should be situationally aware and NOT exceed NDLs, and NOT let their dive computer generate a ceiling, no matter HOW that ceiling is calculated. Because if a non-technically trained diver gets a mandatory deco stop on the computer that they choose to use (with whatever algorithm or conservatism), that's a major failure of situational awareness. Implying otherwise is, in my opinion, normalization of deviance. And if we, as experienced divers, imply here on this board that it's OK if it's just a little bit of deco and if it clears on the way up, that's not helping.

 

[doctormike]

@doctormike, I think we are having a terminological difference only regarding the meaning of "ceiling."

I totally agree that we should not encourage recreational divers to casually incur stops, hoping they will burn off during ascent. But, that's different than a ceiling. Where I disagree is that a ceiling "means you no longer have the option of a standard ascent...." I think that is the definition of a stop, or at least a slower-than-normal ascent in lieu of stop, not a ceiling.

A ceiling is simply the depth, at any given moment of the dive, at which your leading compartment would bump up against the limit of what is permitted in whatever model you are diving.\

I think that you are right, we are using similar but different definitions of ceilings. Yes, you could exceed GFhi by surfacing instantly in a situation when you wouldn't exceed it by surfacing normally. But from a diving operations point of view, a dive computer won't give you a ceiling the instant the calculated ceiling in the graph below drops below the surface. For example, if you look at the Shearwater manual, they use the term ceiling and stop interchangeably, and point out that the stop is a rounded up depth of the actual ceiling:

"We have now entered decompression. Our first stop, or ceiling, is at 20 feet and we will need to remain there for up to one minute. Although stops are shown in minutes, the computer willl calculate and change the ceiling in real time and the stop may be less than a minute
...
So a gradient of 40 may reflect a ceiling of 15 feet, but the computer will show a rounded-up 20 foot stop."

But again, getting back to the OP, I think that it's reasonable to tell a non-technically trained diver that when their dive computer tells them that they have a ceiling, that's a failure of situational awareness, and not just something that can be dismissed as irrelevant.

 

[dmaziuk]

I don't think you should quote shearwater's manual to "non-technically trained" divers: they are not shearwater's target user base. Their target is people on planned decompression dives who will be in deco and therefore their ceiling will generate a stop. So it's understandable when they use "stop" and "ceiling" interchangeably.

But when you quote that to us vacay divers who don't get a stop, all that does is confuse those of us who don't understand how the numbers work: :gasp: "I was in deco".

-- Of course you were, "every dive is a decompression dive". Did you get a mandatory stop on your way up?

 

[doctormike]

I don't think you should quote shearwater's manual to "non-technically trained" divers: they are not shearwater's target user base. Their target is people on planned decompression dives who will be in deco and therefore their ceiling will generate a stop. So it's understandable when they use "stop" and "ceiling" interchangeably.

But when you quote that to us vacay divers who don't get a stop, all that does is confuse those of us who don't understand how the numbers work: :gasp: "I was in deco".

-- Of course you were, "every dive is a decompression dive". Did you get a mandatory stop on your way up?

Actually, Shearwater is probably selling a lot more units to recreational divers (hate that term!) than to technical divers, but that's not really relevant.

I'm not sure what is confusing, but I agree that the term "in deco" can be used to mean different things by different people.

In the common vernacular, being "in deco" means that you can no longer do a standard (30 FPM) ascent to the surface without your algorithm requiring a mandatory stop. I think that for the purposes of "vacay" divers, we should put "stops that clear on ascent before you get to the stop depth" in the same category of "being in deco", that is, something to avoid.

Because if you aren't trained or carrying enough gas to do a staged decompression ascent, and you get to the point that you have a stop that will clear on ascent, what happens if there is any further delay? What if your buddy needs gas or other assistance? What if you have some sort of wing failure or entanglement issue or anything that will cause that stop to get deeper or of longer duration?

I want "vacay" divers to think two things:

(1) I should be situationally aware enough so that my NDL does not reach zero, and that my computer does not give me either a ceiling or a stop.

(2) If my computer gives me a stop, I should not panic, I should just ascend and do the stop if it is still required when I reach that depth. Hopefully I will have enough gas, and if I don't and can't get any, I should ascend when gas dictates, no matter what the computer is saying. Then I should go back and think about #1 again.

 

[dmaziuk]

I think that for the purposes of "vacay" divers, we should put "stops that clear on ascent before you get to the stop depth" in the same category of "being in deco", that is, something to avoid.

It won't work: you'll get "in deco" on any dive to 90-ish fsw, ops in FL won't let you in the water for exceeding "NDL" (or is it "your training"?), and nobody's gonna buy it for being way too conservative.

(1) I should be situationally aware enough so that my NDL does not reach zero, and that my computer does not give me either a ceiling or a stop.

I prefer to carry a computer that will, when my NDL reaches zero, give me a 3-5 minute stop at 3 to 6 metres. For which I do carry enough gas because I am planning to make that stop regardless. Whichever transient "ceiling" may or may not exist inside its tiny licktronyx brain at some point in a dive is of purely academic concern to me. (Which, granted, I'll happily rant about on the Internet: I work in academentia, "academic" is in my job description.)

 

[doctormike]

It won't work: you'll get "in deco" on any dive to 90-ish fsw, ops in FL won't let you in the water for exceeding "NDL" (or is it "your training"?), and nobody's gonna buy it for being way too conservative.

Scare quotes aside, I'm not sure what you are saying.

You won't go "into deco", or get a stop, or a ceiling, or go past NDL=0 on ANY dive to 90 FSW, only if you stay long enough, right? So the trick is to not stay that long. Not to keep the dive operator happy, but to be safe.

Hey, if people aren't going to buy it because they insist on getting more dive time in than their training allows, then I guess they will vote with their wallets and patronized dive shops that don't care about that stuff.

I prefer to carry a computer that will, when my NDL reaches zero, give me a 3-5 minute stop at 3 to 6 metres.

So do you (or does your computer) consider that stop a mandatory stop?

 

[dmaziuk]

Scare quotes aside, I'm not sure what you are saying.

You won't go "into deco", or get a stop, or a ceiling, or go past NDL=0 on ANY dive to 90 FSW, only if you stay long enough, right? So the trick is to not stay that long. Not to keep the dive operator happy, but to be safe.

Define "long enough". On the profile I posted I get a ceiling after 7-ish minutes at 90-ish fsw, that would be the "long enough" by your definition. By PADI RDP definition, it's 21 minutes before the "optional means mandatory" grey boxes, and 25 to NDL=0. That's ~300% difference.

So do you (or does your computer) consider that stop a mandatory stop?

Does RDP's "optional safety stop required" mean it's a mandatory stop?

If my computer thinks it's mandatory and I blow past it, it'll lock itself out in omitted deco violation. If it thinks it's not mandatory and I blow past it, it won't. (I think it will come back to haunt me on the next dive, but that's not quite so straightforward as there's also SI and other fudge factors it uses to compute its "reduced gradients").

 

[Basking Ridge Diver]

On the profile I posted I get a ceiling after 7-ish minutes at 90-ish fsw, that would be the "long enough" by your definition.

This conversation is a slow train wreck and as much as I keep trying to ignore it - I keep reading it... Someone slap me...

Someone needs to go back to OW class - sell their Shearwater and buy a dive table. Learning dive tables will go a long way in understanding how your computer works.

Lol...

 

[scubadada]

If your computer stays in NDL until you surface, it was a no stop dive, safety stop is optional. If your computer goes over into deco, no matter how short the time, no matter if it clears on the ascent, this was a deco dive. I believe it would be recorded that way, on computer, or downloaded.

 

[dmaziuk]

The difference between the tables and a computer is like the difference between a no-stop dive and a planned deco dive: in one case you've already decided that X will happen, and you are working the numbers back from there. In the other: you're in the water trying to figure out when X would happen based on current time and pressure and a bunch of things that have not happened yet. Like whether you stay at the same depth, or stick to a particular ascent rate.