How much gas in case of accidental deco.

[DevonDiver]

Not all excursions into deco are due to inattention or carelessness.

True... issues happen.

However, all divers are taught to dive conservatively. That should include allowing a healthy distance from their NDL, in line with their training and experience... and in relation to the complexity of the dive they are undertaking.

IMHO, there's way too much pre-occupation with depth amongst the recreational diving community, followed latterly by gas consumption - and far too little consideration of NDLs. Diving safely means ensuring a buffer from your limits... all limits.

How much of an 'issue' do you need to encounter to push your dive 15 or 20 minutes beyond a plan? I doubt many OW divers would encounter an issue of that magnitude.

Going deeper? Shorter NDLs? Fast Air Consumption? Narcosis? Experience, training and equipment should match the task. If it does, then appropriate resolutions should be in ample supply. It doesn't hurt to dive with a more experienced diver, as a fail-safe, when pushing your experience either.

 

[The Chairman]

he then had to sit on me and tighten my camband.

No wonder you're such good friends!

I discovered that weighting yourself neutral at 10 feet with 500 psi doesn't work very well if you have to shoot a bag and try to keep it upright,

One of the main reasons I hate open ended bags and love sausages with OPVs in them. My last time using such a bag was in very rough seas. The waves breaking over the bag constantly kept letting the air out. The boat never saw it either.

One of the TDI books suggests that all technical dives should be treated as a formal affair. I love that quote as it puts everything in perspective. If one of my limits is to stay within NDL, then I am going to stay within NDLs. If I have a gear issue, the first thing I do is to check gas and NDL left before I start. If you don't have enough of either, then do it during your safety stop or on the surface. I do this whenever I hunt, as well. Check your limits before you start any new task underwater. Obviously, entanglements and similar situations are the exceptions here.

 

[freeclimbmtb]

There are various ways of teaching people to do things. You can simply give them a set of rules and tell them not to break them. That'll work pretty well with the people who are temperamentally likely not to break rules.

You can give people a set of rules and explain why the rules are what they are, and what the consequences of breaking them are. That will induce obedience in some of the more thoughtful people.

Neither approach gives the student any tools to cope with a broken rule. And humans being what they are, rules are likely to get broken from time to time, even if that isn't planned.

But you can give the student a set of procedures and explain that these procedures apply to a subset of the possible situations the student might encounter, and that it is desirable for a student, with limited experience and training, to confine his explorations to those situations. If you also recognize that there are scenarios outside those protocols, and give the student some basic tools to cope with them, you will avoid the "OMG I BROKE A RULE" panic response, which we have seen written up on this board a number of times.

Deco isn't death. Tech divers do deco all the time. The reason recreational divers are told to stay out of deco is not because deco is horrible, but because recreational divers don't have the planning tools to know that they can do the deco they incur (gas, thermal protection, redundancy) and they may very well not have the diving skills to actually DO the deco (hangs in midwater with no visual reference, or a stepped profile that requires precise buoyancy control and timing). It is my understanding that CMAS includes some backgas deco at their higher levels of recreational diving; GUE teaches a "deco stop" approach to recreational dives, to make the transitions from no-deco to mandatory deco a pretty minor change.

I don't think the OP should plan to exceed his no-deco limits. But he is asking the right question -- IF you end up with a deco obligation, how do you know how much gas you need to execute it? I'd much rather see a recreational diver look at his computer and say, "Oh, drat, I went into deco, I'd better deploy my 40 and do my time," than have someone look at their computer, recognize they're in deco, have no idea what they need to do now, and panic -- it's happened, and people get hurt that way.

Kudos to the OP for asking the real question -- if you have the gas, you can do the time.

This is really what I was after. Instead of "dont do this for any reason" I prefer the "you shouldnt do this, heres why, and heres what to do in case plans fall apart" I dont like the mentality that I see in the majority of people ive dove with, just a complete and total fear of deco...i dont like being afraid of things so if i understand it and know what to do in that situation, im not any more likely to push it, just more likely to deal with it correctly. I agree 100% that the only thing i need right now is mileage, and that there is absolutely no substitute for it, but diving for the sake of diving doesnt seem as worth while to me as diving while keeping these things in mind. I would rather ask the questions early and learn as I go, than get to 1000 dives and say "ok, now i think i should learn about why ive been avoiding deco all this time"

And I did see the real answer to the title topic a few posts ago. Basically the answer being enough air left to hang at 15' for 15 minutes. I guess it really boils down to me not knowing how fast the duration piles up...like an extra minute at depth is 5 at the stop, but is 10 mins at depth 50 mins at the stop? Just a lack of knowlege on the topic on my part thats all. back to the books! (and more importantly the water!)

 

[ianr33]

I guess it really boils down to me not knowing how fast the duration piles up...like an extra minute at depth is 5 at the stop, but is 10 mins at depth 50 mins at the stop? Just a lack of knowlege on the topic on my part thats all. back to the books! (and more importantly the water!)

Download a copy of V-Planner V-Planner VPM & VPM-B & VPMB & VPM-B/E dive decompression software It's free for 30 days.
Alternatively try ideco App Store - iDeco Pro

Play around with a few profiles,get a feel for how quickly deco racks up. See how much gas you need for a 400 foot dive.

Hours of fun!

 

[freeclimbmtb]

IMHO, there's way too much pre-occupation with depth amongst the recreational diving community, followed latterly by gas consumption - and far too little consideration of NDLs. Diving safely means ensuring a buffer from your limits... all limits.

I agree. If there are two things I remember from my OW class, the first is check your spg often, the second is don't go past 60 feet. Sure they talk about NDL...but with 6-8 just gotten wet divers flailing around in the pool and the lake...we couldn't reach NDL if we tried...not on the air we had and the rate we burned it. And for that reason, it was easy (for me at least) to let NDL slip ranks on the priority list to well below depth and air. Getting into AOW is when it was really smacked back into prospective for me because at that point, like I said, my air lasted longer and was no longer the governing factor. My OW class identified the existence of computers, but was taught on tables, so we were taught basically to know how long we could stay at our target depth, and then how to determine repetitive dives. Now this might largely be a product of the way I personally learn...but it's really easy for me to let a skill go by the wayside if I dont use it. I know damn well how to clear my mask at all sorts of angles, because well, I can't see without it. But when I so very infrequently get anywhere near the NDL, its easy (albeit stupid) to let it slip to the back burner. As I said before, I only had one instance where it was a problem, and it compounded with other problems. When you go to an interview and they say "do you have any questions for me?" I might have 1 or 2...but when I get home I think of 100 more that I could have asked. For me, diving is much the same way. I dont know to ask the question until it presents itself to me. So here I am, alive and well, asking the questions!

 

[AfterDark]

I don't get accidently going into deco. I have 1973 logged dives some are deco as planned but most are NDL not once have I accidently gone into deco. I've only owned a PDC for 3 years. When I dive my 1st focus is gas, depth, time. Although I don't count I'd guess I check those three things dozens of times during a one hour dive. When I'm navigating the compass is on a console with a SPG and a depth gauge, doesn't take but a shift of the eyes to check 2 of the 3, my PDC and watch are on my left arm a quick look I have time and NDL min remaning. I take pictures but diving is my focus. I bring the gas I need to safely do the dive I planned. That does include a reserve for contingecies but I don't sling bottles unless I'm planning a deco dive and even then if I can avoid slinging a bottle I will. You need to re-examine your diving habits and create new habits. Dive safe, it's job one.

 

[DevonDiver]

This is really what I was after. Instead of "dont do this for any reason" I prefer the "you shouldnt do this, heres why, and heres what to do in case plans fall apart"

There are reasons why experienced divers err from giving a 'get out of jail' type response. The prime reason being that the accumulation of deco is dependent upon the individual's prior diving profile/s.

To say "it will only be a few minutes... XXXpsi is enough to cover it" is erroneous, mis-leading and can cultivate a very false sense of security. Having a false sense of security can lead people to complacency... and get them hurt... badly.

On most occasions, a slight deviation beyond your computers NDL limit will provide a minimal deco obligation... and that might even clear during a slow ascent to the stop depth. On other occasions, the deco obligation can accrue near exponentially...and continue accruing throughout the ascent, until you virtually reach the stop depth.

The variance in deco accumulation is dictated by the controlling tissue compartment. Most of the time, for recreational divers, that compartment will be 'quick' - thus deco clears quickly/on ascent. However, after multi-day and/or repetitive deep dives, slower tissue compartments become the primary controllers. They won't clear quickly...and can continue to calculate on-gassing even on ascent.

As an example, the other year I conducted a multi-day series of repetitive (2-3 dives per day) deep (100-140ft) dives. On the last dive of the series, I dove to only 50ft depth and noticed a 30 minute NDL. I made the decision to exceed my NDL, on the basis that I was a technically trained diver and had ample gas supplies for any deco that arose (I was on double AL80s). I was aware when I exceeded the NDL on my computer (a Suunto Vyper) and had predicted (wrongly) that a further 5 minutes bottom time wouldn't cause a major deco obligation. Only a minute later, I glanced at my computer and was shocked to see that the deco obligation was already at ~8 minutes. This wasn't as I expected, so I began my ascent there and then. As I ascended from 50ft to 15ft, the mandatory deco continued to rise...quickly. By the time I reached my stop at 15ft, I was owing around ~25 minutes of deco time.

That was my 'wake up call' from complacency. I believed I could second-guess my computer, based on 000's of dives, many of them deco dives... but I failed to consider the implications of the diving pattern I had been following prior to that specific dive. Luckily, I had the experience, equipment and... most vitally... ample gas supplies, so when I did get bitten in the ass, it didn't cost me anything other than pride and a delay in enjoying my post-dive banana.

If I'd been on a single tank... I would have blown that deco... no doubt about it. Even with good air consumption and short bottom time on the dive...

Also, that particular dive was on an anchored boat, with mild current and warm water... so I didn't get lost-at-sea, hypothermic etc. Again.. a whole spectrum of additional factors that need to be considered in the 'unplanned deco' scenarios...

That's why I hazard against giving 'simple answers' to what is, in effect, a very complicated topic. I'll discuss deco theory all day... but I won't give statutory advice about conducting deco to a stranger... or under the premise that one solution is applicable for every scenario.

My opinion is, and will remain, that adhering to No-Decompression Limits are a critical safeguard against the risk of serious injury or death. In that respect, unless conducting precise planning to conduct specific technical decompression dives (including fine gas management) - the most prudent and responsible advice to give is simply 'avoidance'. The emphasis being on the application of safe diving practices, a conservative approach to diving and situational awareness in the water.

I dont like the mentality that I see in the majority of people ive dove with, just a complete and total fear of deco...

What's not to fear about spending the rest of your life in a wheelchair... or never managing an erection again?

i dont like being afraid of things so if i understand it and know what to do in that situation, im not any more likely to push it, just more likely to deal with it correctly.

In this respect, the 'correct way to deal with it' is by avoidance. Either that or you train for, equip, plan and implement dives according to formal technical diving training.

There is no universal 'catch-all' solution to the question you asked.

...get to 1000 dives and say "ok, now i think i should learn about why ive been avoiding deco all this time"

You avoid deco because you are not trained to complete it. You avoid it because if you mess it up, you can get badly hurt. Training to safely cope with a decompression obligation is extensive and intense, only that preparation, in conjunction with a comprehensive equipment approach, should allow you to feel 'relaxed' about being in that situation.

You can, by the way, begin technical dive training at a relatively low experience prerequisite. Various agencies offer 'intro' and 'basic' courses to divers below the 100 dive experience level. That's a good 'heads up' to the issues involved... and also helps shape and enable future development when full technical training becomes accessible.

And I did see the real answer to the title topic a few posts ago. Basically the answer being enough air left to hang at 15' for 15 minutes.

If I'd done my 'bad' dive on that presumption, I could be wearing a colostomy bag right now. Just saying......

I guess it really boils down to me not knowing how fast the duration piles up...like an extra minute at depth is 5 at the stop, but is 10 mins at depth 50 mins at the stop?

It'd be worth doing some reading/research on how decompression is calculated. As a starting point, I recommend 'Deco for Divers' by Mark Powell. There's also a library's worth of materials available on the web.

Also Steve Lewis' 'The Six Skills and Other Discussions'.

The Wikipedia article on 'Decompression Diving' is actually very comprehensive too.. Decompression (diving) - Wikipedia, the free encyclopedia

 

[TSandM]

DevonDiver has some very good points, but it should also be pointed out that different models calculate decompression requirements differently, and what his computer told him on his 50 foot dive, another computer might present differently. This is why it's important to know more about decompression and how it is calculated, than simply to ride a computer.

I do think it's important to know that deco is not something to panic about. You shouldn't go into deco. I won't argue that at all; recreational divers are not educated to understand what it means to incur a decompression obligation, or how to arrange their profiles to minimize it once it's begun. They don't know how to plan gas for deco, which is the biggest problem, and they aren't taught to solve issues underwater, which is critical once deco has begun to accrue.

But all a decompression obligation means is that you have to spend more time in the water, and you have to spend it at prescribed depths. If you are so unfortunate or careless to go into deco when you didn't intend to do it, your obligation should be minimal, because one hopes you would notice what you had done before much time had passed at all. After all, one doesn't instantly go from 20 minutes of NDL to 8 minutes of deco . . . the clock counted down, and if you weren't watching it, then shame on you. If you were, and were delayed by something unavoidable, like my incident, then you have to cope with the results. And if you have enough gas -- which was the original question -- deco is primarily a nuisance. If you don't have enough gas, deco is a nightmare, which is why I applaud the OP for asking about the gas requirements.

I hope he does not intend to make incurring deco a habit (and it doesn't sound as though he does). But I will never forget my very best instructor ever, Joe Talavera, looking up at us when the boat captain asked him if he was okay for deco, and tell us the captain had asked the wrong question. Anything is possible, if you have the gas; if you're short on gas, your options have suddenly gotten fewer and less attractive.

 

[ianr33]

As an example, the other year I conducted a multi-day series of repetitive (2-3 dives per day) deep (100-140ft) dives. On the last dive of the series, I dove to only 50ft depth and noticed a 30 minute NDL. I made the decision to exceed my NDL, on the basis that I was a technically trained diver and had ample gas supplies for any deco that arose (I was on double AL80s). I was aware when I exceeded the NDL on my computer (a Suunto Vyper) and had predicted (wrongly) that a further 5 minutes bottom time wouldn't cause a major deco obligation. Only a minute later, I glanced at my computer and was shocked to see that the deco obligation was already at ~8 minutes. This wasn't as I expected, so I began my ascent there and then. As I ascended from 50ft to 15ft, the mandatory deco continued to rise...quickly. By the time I reached my stop at 15ft, I was owing around ~25 minutes of deco time.

DevonDiver has some very good points, but it should also be pointed out that different models calculate decompression requirements differently, and what his computer told him on his 50 foot dive, another computer might present differently. This is why it's important to know more about decompression and how it is calculated, than simply to ride a computer.

I agree with TSand M here. I don't think the deco in Devon Divers post was "real"

I have an Aeris computer and have found a foolproof way to throw it for a complete loop: Do a regular dive first ,maybe 60 feet. Then do a deco dive to perhaps 130. The computer will rapidly come back with insane amounts of deco. (V-planner wanted maybe 10 minutes, computer needed maybe an hour) My assumption was the computer didn't know what to do with those profiles ,became massively conservative just in case.

Now, if I hadn't planned these dives first on V- Planner I would have been very concerned about this.

Deco will not go instantly from nothing to 20 minutes on any "normal" profile. (But it's still a dumb idea to go into unplanned deco )

I would suggest anyone regularly doing dives that push NDL do an Advanced Nirox course. You will learn lots of deco theory and become a much safer diver . (But you may get hooked and end up spending thousands on new gear.......)

 

[DevonDiver]

I agree with TSand M here. I don't think the deco in Devon Divers post was "real"

I'm not sure quite what you mean by that, but I can assure you that the event described was very real. I'm not in the habit of posting BS for the sake of it, especially when it's an admission of my own fallibility.

Yes, computers do provide different algorithms - but not with the sort of deviation I highlighted. I thought I explained quite clearly why the profile occurred as it did - it wasn't an anomaly or quirk of programming.

Try diving to 120ft 3x a day for a week and checking out your residual compartment saturation....

Deco will not go instantly from nothing to 20 minutes on any "normal" profile.

Firstly, I didn't say "instantly". I said it accrued quickly... and continued to accrue on ascent. Secondly, what is a "normal" profile or a "normal" multi-dive routine... or a "normal" multi-day diving itinerary?

ianr33... I had about 4x your experience when that happened to me. I thought I knew a lot then. I managed to learn something. I'm still learning... so should you.