Overshooting NDL and mandatory deco stops

[KenGordon]

About now we would have expected Ross to turn up and tell you about his databases of dives from somewhere or other. Then Simon would point out that analysing the data after the fact make sure it a lot less useful with lot s of reporting bias and so forth. Also we have no idea whether these dives are representative...

So the data being asked for exists, it just is not actually useful.

On the issue of the injury rate in the man tests seeming very high compared with an anecdotal evidence, remember that the divers being tested are exercising more that a typical diver looking at fish on a reef.

There is no man testing of GF. It is just assumed that because it is more deco that full ZHL16 it will be safer. So we can’t really say how 30/70 vs 40/85 vs 50/95 (or whatever low is) compare as far as risk is concerned. There are some USN models which might help, but if you believe that then why do they just dive those models?

How to chose which GF number s to use is more art than science. There is a chart on page 201 of Mark Powell’s new book (Technical Diving an Introduction) which goes over it a bit, but from the point of view of doing deco dives.

Personally I only plan to surface with a GF of 95 if I am having a CO2 hit and would otherwise have nothing to breath.

 

[NAUI Wowie]

There is no single answer to your question as there are too many unknown variances. However, assuming a square profile, the computer (dependant on make/model) will show, either:
• a stop depth and the time required there, or
• the total time required to safely reach the surface, including stops.

I often plan to do up to 10 minutes of stops (it’s in my gas plan). My previous computer would jump from ‘zero’ to a 5 minute stop required. And when I found a body at 30m it quickly ramped up to 15 minutes in the 3 minutes we spent confirming we had found a diver. [This was the incident that convinced me to dive with a pony.]


Warning; No. Display a get out of the water plan: Yes.



No, I would switch to my pony; which isn’t included in my gas planning, even when planning deco stops.
---
To me the terms ‘recreational’ and ‘technical’ diving are meaningless, it’s all just diving.

I just ordered at 19cf pony and plan to sling it in front for ease of use

 

[NAUI Wowie]

Respectfully disagree. Owning a Perdix and not having a basic understanding of GFs, even the preset Recreation mode options, is kinda like owning a Bugatti and never taking it out of idle.

I bought a new dive computer for A.I. and for ease of use. Perdix is fifty times easier to navigate than a zoop. I also got it at my LDS at a major 25% off sale. Combined normal savings plus 20 percent. I definitely didnt pay bugatti pricing. The difference between a chevy and a bugatti is 40 grand vs 2 million dollars. difference between and average air integrated computer and what i paid for my perdix is maybe a hundred bucks if that.

I paid $720 for my perdix A.I. in 2018 during a large sale. If thats bugatti pricing for a dive computer id be shocked. I did however pay $260 for the transmitter separately. This is in California.

 

[NAUI Wowie]

Sure!



Maybe you meant "..all you need to know about a safe ascent rate.." which would be a bit more reasonable. But since new divers read these threads, it's worth correcting this statement.

New divers, please understand that even though an instructor is quoted as saying that "all you need to know about scuba is to go slower than your slowest bubbles", this is inaccurate. Even if this rule of thumb is approximately equal to a reasonable ascent rate, there are many things other than ascent rate that you need to be aware of. Here are a few, for example:

1) You need to be aware of your no-deco time, and not enter into a staged decompression obligation without the appropriate gear, gas reserve and training.

2) You need to be situationally aware and monitor your gas so as not to run out underwater.

3) You need to be aware of the maximum safe breathing depth for whatever you are breathing, especially if you are breathing enriched air.

4) A safety stop, while not mandatory, is still a good idea, and shouldn't be omitted. The reason for this is that many divers will inadvertently violate an ascent rate or NDL to a small degree during the dive, increasing decompression stress without their computer necessarily giving them a mandatory stop. The safety stop is an extra safety buffer in this instance. Also, people tend to ascend faster when they are almost at the surface, figuring that the dangerous part of the dive is done. By always doing a safety stop, you make this less likely.

5) You should avoid a sawtooth dive profile, with multiple ascents within a dive. This is how I ended up taking a chamber ride despite my computer saying that I was "safe".

Could you define how that ended you up in a hyperbaric chamber? what were the spikes in the dive in depth differences and how many? I dont want that to be me

 

[NAUI Wowie]

PDCs don't bend divers: divers bend themselves. PDCs are as dangerous as the person using them. Suuntos are de-facto more conservative than any other group of PDCs out there.. While they might provide a somewhat "safer" output, the biggest problem facing the diver is still the person wearing it. PDCs can't force you to ascend slowly, do a full five minute safety stop, avoid riding your PDC's NDL, avoid bounce dives and otherwise dive conservatively. Only you can accomplish that so your safety is all up to you. Your PDC is only a tool that can be used or misused. It doesn't matter if you understand the underlying algorithms or not. Your safety is entirely up to you. PDCs don't bend divers: divers bend themselves.

You eloquently made the point I was attempting to type. thanks, oh and public apology for anything I may have typed at you in the past you found offensive on other threads. And in p.m.s , and , well I think you get the point

 

[Storker]

I disagree

How to properly assess the relative conservatism of various algos/computers for Rec divers?

 

[NAUI Wowie]

Do you understand what the Gradient Factor does?

Do you understand why there are different recommended GF settings?

Do you understand that the GF settings change with different gases?

You where asked what GF settings you currently have, you didn't know, that means your computer could be at 100/100, which means you are diving pure Bhulmann?

As an example. if you where using 18/45 Trimix and pure oxygen. For a 50 meter 20 minute dive
at GF100/100 total time is 40 minutes
for GF20/70 total time is 67 minutes.
That's one hell of a difference in the decompression profile!
Now I have dived with those who run one computer 100/100 and the second set for the real dive profile e.g 18/45.
If it all goes to ratshit, you follow the 100/100, running the risk/likelyhood of a helicopter ride to the nearest chamber.


Addendum
I am pretty certain Shearwater used the ZHL16C which is more conservative than the B and A variants. Which according to both the Bhulmann table and the Suunto table I have, on a 30m dive is within a minute or so of the Suunto algorithm, for a 30meter air dive, so not quite as bad as I thought it was going to be.
I know the VR3 used the Bhulmann algorithm, but not which variant, probably C. I know that if you didn't increase the safety factor (which assumed a higher nitrogen load rather than gradient factors,) they where too chamber friendly. Especially when compared to the Suunto. But, then the VR3 was not really used for no stop dives.

Thats not true. Because I dont know a gradient factor my computer is not in default settings for rec divers? False. Most divers have no idea what GF even means let alone defining gradient factor. I have no intention of getting into any of that. And I will never ever be using tri mix nor would I pay the insane costs of trimix plus deco bottle with different 50 percent mixes etc. That is a richer sport than I can afford, pun intended ha. And more risk than I care to take. I understand trimix lessens danger at depth but I dont want to go there to find out lol.

 

[Storker]

So the data being asked for exists, it just is not actually useful

That depends on how those data have been collected.

If it's self-reporting by non-commercial divers, I agree. If it's complete logs including every dive performed by some actor (commercial, military, whoever has a duty to log everything), not so much. Of course, the data should be regarded with a healthy amount of scrutiny, but they'd be a lot better than one of Ross' self-report databases (which nobody but him had access to, but that's another discussion)

 

[NAUI Wowie]

I disagree, and I know cave instructors who refuse to teach students who have a Suunto.

I would also suggest that Suunto engineers disagree with you and are proud of their conservatism. Proud. I got to sit with two of them after they heard me make the very statement to a dive buddy at Dive Outpost in Luraville Florida. According to them, these PDCs were developed in the Baltic and North Seas to protect divers who dive in very cold waters. In fact, their HelO2 was developed with negative conservative factors in order to match Pelagic PDCs.

However, conservatism is only the beginning for safe diving with a PDC. I can't emphasize the diver's role in their own safety enough. I can dive more safely with a watch, a red ribbon and the rule of 120 than some divers I have seen with multiple PDCs on their persons. It's only a tool and it's up to each diver to figure out the best practices for them while using that tool.

No offense but you are starting to sound like my dad, he still refuses to use or buy cell phones because they arent necessary. Yes cell phones are not necessary to live and dive computers are not necessary to dive. When I got certified and for a few dives I used my diving watch and know how to use it. And I didnt die lol. BUT a dive computer is a great diving aid especially with air integration.

Who knows in 20 years divers may be diving inside diving air bubbles controlled via a hand computer.... And I may be you then saying nah I dont need that diving bubble to dive at all....

EDIT ah I see in your next post you are big into dive computers. It seemed on multiple posts on this thread you are almost kind of negative towards them.

 

[Storker]

Most divers have no idea what GF even means let alone defining gradient factor. I have no intention of getting into any of that.

If you want a proper answer to the question you posed in your OP (except "follow the instructions you got during OW class"), it'd be a good idea to learn some basic deco science.

You posted in Advanced, thus the answers you get will be... advanced. If you want the McDonald's answer, ask in Basic.

If you should want to learn and understand beyond PADI OW level, get Mike Powell's excellent book "Deco for divers".