As a rec diver, what to do if I breach my computer's NDL???

[lowviz]

...//...3) Gauges are notoriously inaccurate at the low end of the range, so if you want to keep a little reserve, 300 PSI might actually be 100 PSI, and 100 PSI might be fumes.

True.

All of my SPG's tell me I have 100 psi when they are disconnected. So when the needle actually lifts off the peg due to pressure, I might have 100 psi or something else. The manufacturers don't want you to see the error, so you really don't know. Me, I just assume the final 100 psi is "for the gauge".

 

[DevonDiver]

Do (yes really you must do this) try calculating your stops in a decompression software like v-planner...

This is a good idea for recreational divers - it doesn't hurt to have some contingencies. Use software for a while and you will start developing a much better 'feel' for how decompression works (whether with formal stops or no-stop profiles). It's a very useful educational tool.

I usually recommend using DecoChek for students to plan dives on my recreational-level courses. DecoChek isn't free-ware, but the trial version is unlimited duration and has full function down to 40m/130ft - so it's perfect for recreational divers. There is also a free, limited function, mobile phone (java) version.

There's also a good article included in the dive planner documentation: Diving and Decompression - A primer by Mark Ellyatt (at appendix 1)

All my training dives are team-planned with contingencies... no 'magic bracelet' dives. Contingency ascents normally cover +5min (just longer), +5m (just deeper, if applicable) and/or +5min/+5m (just deeper/just longer) beyond the planned max NDL and depth. The contingencies planned/carried should realistically reflect the dive parameters though. At the minimum there should be a single 'worst case' deco contingency.

Not only does this provide a more robust/refined (emergency) deco ascent profile, a 'worst case' contingency also works in tandem with gas management - to help divers plan the minimum gas they'd need to ensure safety. It also shows the ceiling... so that builds awareness also - both of when it exists and when it causes mandatory deco stops.

 

[TSandM]

There really is some good evidence that exertion, and especially exertion involving climbing or lifting (where one might Valsalva) can up the risk of becoming symptomatic from DCS. I actually agree with the recommendation to have someone else help you with any heavy lifting. Recreational computers are NOT programmed to generate good decompression profiles -- some of them more or less have an overlay that says, "Go do 8 minutes at 10 feet", not dissimilar from the tables. I would not be confident that my recreational computer was going to do a really good job of bubble control, and I would avoid heavy exertion after the dive.

The most important things about unplanned deco are 1) to have studied your computer's manual enough to know what it's saying to you; 2) to know that deco does not mean dead, it just means more time in the water, and 3) to make sure that any dive you do where the probability of deco is more than vanishingly unlikely, you have PLANNED ENOUGH GAS to spend a little extra time in the water. Doing deco never killed anybody. Running out of gas surely has -- one of the biggest areas of study for divers doing deliberate staged decompression dives is how to plan their gas, and how to plan for contingencies.

 

[Rhone Man]

Lots of sensible advice here. I'll try to add a couple more comments:

- When you start to develop a good SAC, it is highly likely sooner or later you will flip into accidental deco on a deeper dive, particularly if you have a conservative computer. In 85% of cases the deco obligation will clear of its own accord as your profile takes you shallower without you ever needing to worry about it.

- Just because your computer says you are over your NDL doesn't make it so. This is a very inexact science. You certainly need to be cautious and follow the computer's recommendation, and add a safety stop afterward. But if you are talking about a 2-3 minute deco obligation, even if you surfaced immediately (please don't) there is a pretty decent chance nothing would actually happen. So don't panic.

- If the worst comes to the worst and you have to surface and break the ceiling, for God's sake don't over react. If you are talking about a small deco obligation, then even assuming there is no oxygen or other assistance to hand, you are probably only talking about joint discomfort and probably some skin bends. Your odds of serious DCS symptoms for skipping a short hang are tiny.

- Also bear in mind that the onset of DCS symptoms is usually much slower than people expect. In around 60% of cases they take longer than 8 hours to present. Accordingly - again - don't panic. Your head will not suddenly explode when you break the surface. You have time to try and sort something out.

 

[DevonDiver]

Sounds like the dead horse of what is the safey stop for. If you are down and the puter yells deco you head up and your 3 min safety stop should take care of it for you.

"Should" is a very rude word, when it comes to deco and assumptions about algorithms.

I once managed to accumulate 18 minutes of deco on a rec computer, ascending from only 16m/52ft after slipping only 2 minutes beyond NDL before ascending. The deco continued to accumulate on ascent... not disappearing as it 'should'. This was due to my repetitive and multi-day diving profiles (algorithm determined a slower tissue compartment became controlling during that ascent).

As it happens, I was fully prepared for that; diving doubles and rule-of-thirds. I suspect many recreational divers, on a single cylinder and a gas plan no more complex than assuming to "surface with 500psi" would have blown that deco and faced a high risk of DCS. That risk of DCS not assumed by the 'number of minutes'... but by what those minutes represented, in conjunction with knowing why they were calculated as such.

Don't assume anything "should" happen - KNOW what will happen. If you don't KNOW... then accept your assumptions can really hurt you.

 

[dumpsterDiver]

A few weeks ago I took my 15 yr old on a dive with a 135 ft max depth. He stayed up around 120 or so I think. We were using computers (and nitrox) and we went a little into deco. We each had a pony bottle. We had not really planned to put him into deco and for some reason his computer showed more deco than me... maybe 6 minutes. He had an aluminum 80 tank.

It was no big deal really, but it did show me a problem with my somewhat causal attitude about teaching him this stuff (he was issued a PADI Jr. Diver card several years ago). Normally, I am pretty casual about safety stops and I generally lead and he follows and he often will stay 5-8 feet below me on ascent and on the hang. He can control his level easily (if he cares), but I've been pretty casual about it, I guess.

So we get up to the stop at 20 ft (a slow ascent has not reduced our deco time)and I have a reel and a float, so I have it easy, I just dump air from the BC, get heavy and hang down from the float with no need to pay attention to depth, since I am effectively tethered at the desired depth. Well the kid keeps sinking down and hanging like 5 ft below me. I signal to ascend a little and he does, but then 30 seconds later he is 5 ft below me. I quickly loose patience with him on this and my signals get more frantic and intense, and he still doesn't seem to "get it" that I want him hanging RIGHT with me at the desired depth. In retrospect, I should have handed him the float and that would have fixed his deco stop depth.

He now understands that when we do a "real deco" dive, he needs to be a little more precise in his drift diving deco stop depth precision. On the boat, I explained that the scubaboard Nazi's worry about holding stop depths to less than a foot and they seem to make it into some sort of competition.

So the precision of the stop depth is something that may be a little different for a recreational versus technical diver.

 

[DevonDiver]

He now understands that when we do a "real deco" dive, he needs to be a little more precise in his drift diving deco stop depth precision. On the boat, I explained that the scubaboard Nazi's worry about holding stop depths to less than a foot and they seem to make it into some sort of competition.

So the precision of the stop depth is something that may be a little different for a recreational versus technical diver.

What he, and you, obviously need to learn about is why stop depths are held accurately when technical diving, especially at 6m. You obviously haven't thought that through at all. You should... even if only to save yourself embarrassment.

I'd suggest getting technical training yourself, so that you'd understand better the issues you currently haven't grasped, but cannot refrain from lecturing on. You might have trouble finding a willing instructor though, given the slap-dash mindset you frequently display...

 

[rongoodman]

the scubaboard Nazi's

I think you owe the members of Scubaboard an apology for that comment. Seriously.

 

[Tigerman]

...
- When you start to develop a good SAC, it is highly likely sooner or later you will flip into accidental deco on a deeper dive, particularly if you have a conservative computer. In 85% of cases the deco obligation will clear of its own accord as your profile takes you shallower without you ever needing to worry about it.
...

Sorry, but WHAT??
Are you actually suggesting that because your SAC is getting better you're probably going to be less observant of your dive plan and that's OK??
If youre violating procedures, atleast fess up to it and admitt that you either did it because youre sloppy or it was intentional. Calling it an "accident" is severly inaccurate. (And yes, I do violate agency standards on a regular basis and I know it and I know I expose myself to more risk than my cert card want me to)

 

[doctormike]

Sorry, but WHAT??
Are you actually suggesting that because your SAC is getting better you're probably going to be less observant of your dive plan and that's OK??
If youre violating procedures, atleast fess up to it and admitt that you either did it because youre sloppy or it was intentional. Calling it an "accident" is severly inaccurate. (And yes, I do violate agency standards on a regular basis and I know it and I know I expose myself to more risk than my cert card want me to)

No, he is suggesting that it's very hard to go into deco with a high SAC rate because you are OOG before you get near NDLs. Given the exact same amount of situational awareness for a given diver, as the SAC rate improves, it takes longer to run out of gas, and makes them more likely to blow past the NDL. Interestingly (and controversially), this is one of the reasons some people prefer not to have newer divers without deco training dive doubles - easier to incur a deco obligation with more gas.