first deco dive

[Mr Carcharodon]

I thought it was 2 minutes past NDL, not two minutes of deco. I've had that give me as much as 20-30 minutes of deco time when nitrogen saturated in Bonaire.

Tracy,

If you have 30 minutes of deco please think about what you are going to do if you have an equipment failure. Will your equipment get both you and your husband to the surface after 30 minutes? I have been pretty relaxed on this thread about the risks of two minutes of deco. And I think that is reasonable given how fuzzy the line is, the high probably of not getting hurt even if you missed that 2 minutes of deco, and +2 minutes and -2 minutes not being a whole lot different. But if you had an equipment failure and omitted 30 minutes you would be looking at some of the more dire outcomes. You really need to know you have the gas to get to the surface even in the face of a failure. If your regulator or computer failed would you be OK? The chances of equipment failures are very low, but since potential results include paralysis and death having a plan to deal with potential failures is a necessity.

You know it could be the 30 minutes is just a wacky number your computer is spitting out. I do not have any insight into how your dive was planned. But you should, and if you want to do dives that require that much deco you should really know the difference between what a reasonable plan is and is not. I hope you do, if not please do get the training.

Andy

 

[tracydr]

We did. We both had well over 1,000 psi. Did it twice. But, it might very well br a computer whack out although we both had similar numbers on different brands.
I'll go check my log tomorrow and make sure I'm remembering right. It was awhile ago and not something we've done more than a couple of times.

 

[Garth]

Cubic feet of gas will help you determine whether you have enough...

 

[Blackwood]

Cubic feet of gas will help you determine whether you have enough...

Only if you know your consumption rate in volume units. If you know it in pressure units for your cylinders, pressure is perfectly workable.

 

[hg frogman]

I thought it was 2 minutes past NDL, not two minutes of deco. I've had that give me as much as 20-30 minutes of deco time when nitrogen saturated in Bonaire.

As a Divemaster candidate, you probably know that PADI/DSAT "recreational" dive tables have been made around 1989 specifically for no-deco dives. With these tables, and if you stick to no-deco diving (i.e. you stay within the NDL), you can do more than 2 dives a day, which is interesting business-wise (and that was probably one of the main reasons behind these tables). Also you can have short surface times between the dives, e.g. 1 hour. All that is man-tested and valid, provided none of these dives is a deco dive; in case you exceed the NDL (thus doing a deco dive), PADI tables stipulate that you have to wait 6 or 24 hours (according to how much you exceeded the NDL) before diving again.

All deco dive tables for air (with deco also on air) that I know (and I know quite a few of them) have been man-tested and validated for 2 bounce dives a day, not more. On-gassing is more important during a deco dive, and off-gassing is slower than on-gassing, and not linear (though no one exactly knows how much slower, and how non linear). The more you are loaded, the slower it unloads. So it takes significantly more time (sometimes MUCH more) to off-gas after a bounce deco dive. And it takes a hell of a time (up to weeks !) to off-gas after a saturation dive.

Single, isolated, bounce deco air dives have been intensively (and quite successfully in a given range, up to around 25' at around 60 meters for air) researched by dive tables' designers. For such dives, the amount of off-gassing happening underwater (during the stops) is quite well taken into account by decent computers, which match the results of the tables for setting the number of deco stops, and their duration, at the end of the dive. But researches have not been as intensive and accurate about the off-gassing after the dive (i.e., the "surface stop") which is a complicated affair, also for dive tables' designers who have to face too many combinations.

This calls for some conservatism : long surface intervals are paramount for deco diving then. This has been taken into account by the designers of deco dive tables, hence the limit of 2 dives a day while using these tables.

Dive computers just extrapolate the data of the tables. They are not more reliable or trusty than the tables, they just "cut time in slices" and "don't round any number" (that's a very simplified image). If you do more than 2 dives a day with at least 1 of them being a deco dive, you are just out of the range of man-tested dive tables and starting a new (and hazardous) experience on yourself, whatever your computer is telling you.

Bottom line is, if after a one-gas-only dive (air or Nitrox) you get 30 minutes of deco stops with a no-nonsense dive computer (e.g. Uwatec) and you do more than 2 dives a day, or you dive again shortly (e.g. 1 or 2 hours) after such a deco dive, you may well be hit by DCS rather sooner than later.

For severe deco dives done with only one gas, e.g. air and also air for deco (of course it depends upon one calls "severe"), divers normally do only 1 dive a day (e.g. 15' at 65 meters is the utmost limit with air for French Navy tables, which in that case don't allow a second dive in the day ; anyway it's an "emergency" profile I don't recommend and neither does the Navy !).

 

[tracydr]

Thanks Frogman. The two times we did this we ended diving for the day and didn't start until the next morning.
FYI, I was not a DM candidate at the time. We had done some deep dives that week, one to 150 feet for a few minutes with a total bottom time of 51 minutes.
I checked the NDL dive and it was towards the end of the week. We were at 104 feet on the 3rd dive of the day. Went 2minutes into NDL and started to slowly ascend which caused us to continue to accumulate Deco time. Ended at 10 feet with 30 minutes of deco time. We both surfaced with about 1100 psi. I dive a short tank and when we started ascending I had 1800 psi.
Not the smartest thing in the world to do but probably not reckless either as we both had enough air for both of us to complete our deco.
We our planning to do GUE Fundies this year and I have also just ordered the book " Deco For Diverers".
I just got my dry suit and will be completing DM and drysuit class this month. Will be working with getting used to a Bp/wings and doubles after that.
Can't do everything at once but sure wish I could!

 

[Rhone Man]

For severe deco dives, divers do only 1 dive a day.

Good post, but I am not sure that the last line is true. If you read stories about the hard core divers diving on the Doria during the 80s, divers used to regularly do two dives a day, going as deep as 240 ft (on air, and decompressing on air). Not exactly an advert for safe diving practice, but it certainly used to be done.

As your own sig line says: Decompression diving was common, long before the "Tech" agencies were born.!

 

[hg frogman]

Good post, but I am not sure that the last line is true. If you read stories about the hard core divers diving on the Doria during the 80s, divers used to regularly do two dives a day, going as deep as 240 ft (on air, and decompressing on air). Not exactly an advert for safe diving practice, but it certainly used to be done.

As your own sig line says: Decompression diving was common, long before the "Tech" agencies were born.!

Thanks for your comment.

Well, that's a matter of what risk one takes.

For no-deco air dives, the occurrence of DCS is around 1/100000.

According to stats from the COMEX commercial diving company, given around 1992 to the French Ministry of Labor, occurrence of DCS (mostly type I, i.e. bends) while using COMEX dive tables for air bounce dives with deco also on air (which is a different story than deco with Nitrox and O2) was found to be around :

1/10000 for "moderate" deco dives (e.g. 20' at 39 meters/130 feet)
1/1000 for so-called "standard" deco dives (e.g. 20' at 51 meters/170 feet)
1/100 for "severe" deco dives (e.g. 25' at 60 meters/200 feet).

I guess these Doria divers (not more than 2 dives a day, right ?) were certainly in the "severe" case. Their second dive may have been done on the top of the wreck (at around 57 meters/190 feet if I remember well). Or not ("china fever" has led to a lot of harm on that wreck).

Recent stats of the French Navy for its MN90 air dive tables (more aggressive than the COMEX tables) with deco also on air give a risk of DCS = 1/30000 globally, but 1/3000 for the range 45-60 meters/150-200 feet. The population of these statistics consists in uber-fit young Navy divers, while stats of the COMEX describe commercial divers of various age, laboring underwater. COMEX tables don't allow deeper than 51 meters/170 feet for the second (and last) dive of the day.

For some people, 1/100 or even 5/100 seems to be acceptable, for others (including me) it is not. Fairly common surgical operations have this amount of vital risk, though, but often there isn't much choice in their case, alas. I think we may agree on that an important point is to know the risk (and its amount) beforehand. Ignorance is a killer.

My signature comes from some really gross crap I read in the Encyclopedia of some well-known agency, about deco diving being "taboo" and coming out only in the nineties, with tech diving. As if diving had been limited to no-deco dives to 39 meters/130 feet maximum, all around the universe, ever since the invention of scuba. Such a lie really irritated me. Deco tables came with Haldane as early as 1908 ; Cousteau and his buddies were already doing leisure deco dives in the fourties ; deco dives have been the mainstream amongst European divers since then ; and even in North America there was no shortage of experienced deco divers on a single tank of air, I guess, though they seem to be much wiser now .

PS1: I edited my previous post to make it more accurate.

PS2: Tortuga68 (see post thereafter), please don't forget that bottom time is as important (and maybe more) than depth for assessing the severity of a dive. Actually, the severity scale used by COMEX was defined as max depth (in meters) x square root of bottom time (in hours). A weeny 5 minutes at 50 meters/165 feet is very different from a severe 30 minutes at that depth. Also I am writing here about deco on air, not on Nitrox/O2, which is a different story.

 

[LanceLisenby]

What model Oceanic? I'd like to see if I can find an online manual to take a look at.

Its a veo180nx. I looked in the manual it gives dashes only below 330 then switches to a working depth gauge when you go back above 330.

 

[Cave Diver]

Its a veo180nx. I looked in the manual it gives dashes only below 330 then switches to a working depth gauge when you go back above 330.

Thanks. That's the out of range indicator, makes sense.