High O2 deco stops

[MonkSeal]

You can also print the tables using GAP with different depths and run times (of course with your deco mix included), very easy to use. You can prepare/print appropriate tables at home. However, deco training is the most valuable thing.

 

[voop]

My son and I are going to Bikini Atoll in July to do some wreck diving. All of the dives are deep (over 40 m) and require long deco stops. The stops are being done at a surface-supplied deco station, with regs dispensing 75-80% O2. The times and depths of the stops will be calculated using air tables (or, in most cases, a computer set for diving on air), to increase the safety margin.

I would be concirned about breathing "75-80% O2" on a deco-stop. Wouldn't it be possible to know the exact mix you'll be breathing, then calculate stops accordingly?

Pure O2 for deco stops is a good idea, in general, but I'd be reluctant to use an air computer for determining my stops. O2 exposure, residual nitrogen etc. as tracked by the computer will be far off from reality -- and then a good part of the advantage of using a computer is lost. Better, then, to get a set of tables and use them correctly, I think.

Most of all, have fun and be safe

 

[Curt Bowen]

I would be concirned about breathing "75-80% O2" on a deco-stop. Wouldn't it be possible to know the exact mix you'll be breathing, then calculate stops accordingly?

Pure O2 for deco stops is a good idea, in general, but I'd be reluctant to use an air computer for determining my stops. O2 exposure, residual nitrogen etc. as tracked by the computer will be far off from reality -- and then a good part of the advantage of using a computer is lost. Better, then, to get a set of tables and use them correctly, I think.

Most of all, have fun and be safe

I fully disagree. I feel using the 75-80% Oxygen for the shallow 20-10 foot stops while using an air computer is a great idea. Why, because of the extreme remote location they are diving in. If you take a hit out there it is a LONG way for help.

On many of my expeditions into remote locations where medical assistance is far away or just lacks good quality, safety through prevention is of most importants. I normally run 100% O2 at 20 and 10 and either pad my tables or just run the air computer.

I do not worry about CNS because of my personal experince with high PP Oxygen for extreme dives I have conducted in the past. Small 30 to 50 exposures should not cause any problems.

Better safe than sorry

 

[MikeFerrara]

I too often configure my deco on the conservative side.

I don't go all over the world like Curt does but I'm often in the hills of Kentucky or in the national forest of Missouri. Cell phones don't always work in those places and help could be a long way off.

Assuming you watched your O2 exposure the only concern I would have about using 80% or 100% O2 on an air computer is if for some reason you needed to get out of the water. If a buddy was hurt and you were decompressing on 80% using an air computyer the difference in deco could be HUGE. You wouldn't have a clue wat your real risk was by cutting deco a little short.

Tere's a cave that we dive where we hi almost 300 ft but there's a long swim from the deep section to the entrance and we're on deco gas the whole time. We do about a half hour at 20 ft outside the cave because we have to haul our gear up a huge hill (Missouri mountain) to get back to the truck. However I know I have the deco padded and wouldn't hesitate to cut that 20 ft deco down to almost nothing in an emergency.

If I just used an air computer all I would know is that the thing would still be telling me that I owed a couple of hours of deco.

In a nut shell...pad your deco if you want but know when you think you could get out if you had to.

 

[voop]

I fully disagree. I feel using the 75-80% Oxygen for the shallow 20-10 foot stops while using an air computer is a great idea. Why, because of the extreme remote location they are diving in. If you take a hit out there it is a LONG way for help.

On many of my expeditions into remote locations where medical assistance is far away or just lacks good quality, safety through prevention is of most importants. I normally run 100% O2 at 20 and 10 and either pad my tables or just run the air computer.

I do not worry about CNS because of my personal experince with high PP Oxygen for extreme dives I have conducted in the past. Small 30 to 50 exposures should not cause any problems.

Better safe than sorry

I agree with the "Better safe than sorry" statement. And I agree that high-O2 mixes for deco are a great idea. What I disagree with is (i) using an essentially unknown deco mix (75-80%??? Which is it, then...?) and (ii) using a high-O2 deco mix with an air computer without tracking O2 exposure and without knowing how long the mandatory stops really are.

Calculating deco on air may give some quite long, shallow stops. Breathing a high-O2 mix is good (for evacuating N2), however a long-term exposure to O2 might not be desireable unless done correctly (air-breaks, tracking exposure and all that).

Bottom line is, that IMO, just breathing O2 and running a dive-computer generated air-deco schedule isn't really prudent nor necessarilly beneficial. Since there are additional conditions (besides "doing what the screen tells me to do) to take into consideration anyways (airbreaks, etc.) calculating and following a deco schedule "correctly" for the gasses in use is just as easy.

I do agree that knowing the deco schedule, then padding it to be as conservative as is desired (as a function of distance to EMS, recompression chambers etc) makes a whole lot of sense. However this should be done knowing what is done and why -- as Mike says, knowing when one can relatively safely get out of the water in a hurry. However just breathing high-O2 on an air-stop doesn't leave you with much of a clue as to when the stop goes from being "mandatory" to being "padding".

Yeah, if I have a shallow stop to do, I'll do it on O2. I'll also compute the stop-duration accordingly -- then I may pad it to be extra conservative. However I'll still know when I have completed the mandatory part of the stop, in case I need to get out in a hurry....

Anyways, that'd just be my preferences. Others may prefer otherwise....

 

[vjongene]

I agree with the "Better safe than sorry" statement. And I agree that high-O2 mixes for deco are a great idea. What I disagree with is (i) using an essentially unknown deco mix (75-80%??? Which is it, then...?) and (ii) using a high-O2 deco mix with an air computer without tracking O2 exposure and without knowing how long the mandatory stops really are.

Just a quick note: the "75-80%" O2 figure given in my original post was taken from the Bikini Atoll Web site. I assume that once we are there we will get precise information about what mix is dispensed at each of the deco stops (e.g. 75% at the 9 m stop, and 80% at the 6 and 3 m stops).

After reading the feedback given here, this is what I plan to do:

- Using the GAP software, calculate tables for the day's profiles, with recommended deco times for each of the stops and mixes. This will give me the minimum time I will have to spend at each stop. I will write the deco schedule on a slate.

- Dive using an Aladdin Pro (air), and follow the deco schedule calculated by the computer. If for any reason I have to bail out, I can reduce the times to those on the slate.

Does this make sense?

 

[DA Aquamaster]

I think a couple people are missing the point on using accellerated deco on an air deco schedule to pad the deco and are getting a little too hung up on oxtox concerns. Nitrox 80 at a PPO of 1.4 (depth of 24 ft) can be tolerated by most people for 150 minutes assuming proper air breaks. The deco times involved on this trip are going to be relatively moderate in the 30-40 minute range even on an air schedule. The accellerated deco on an air schedule idea may not be the ideal arrangement but for issues other than O2 exposure. To answer the original question, O2 toxicity should not be a problem.

However, the air computer in question may or may not go out of range on anything greater than mild deco, so that needs to be checked prior to the trip to see if it can handle the expected profiles. I am sure the boat staff would not be thrilled with a computer in violation mode.

The deco model used by the computer would be another concern. If it is a more traditional bend an treat model, you would face some realtively long 10' and 20' stops that in the end may not be terribly conservative even if the deco were done on Nitrox 80.

You would be better served by software such as Palm VPM or D-plan that would rely on deep stops and shorter overall deco and could be adjusted for a higher degree of conservatisim if desired. (this gets to be very relavent and important for gas planning a few paragraphs farther down)

My preference would be to use D-plan or a similar application to generate a table based on the max planned depth and maybe a couple more contingency tables. The dive times will be limited by the backgas supply anyway so you should always have a very good idea what you can do for maximum depths/bottom times while maintaining an appropriate 1/3 reserve. Air consumption is a very personal thing, and you never want to assume someone else's plan will leave you with enough gas.

The "is it 75% or 80%" question can be resolved on site if you use a PDA or plamtop to cut the tables onsite. If not, plan the deco based on the assumption you will be using 75% and monitor the O2 exposure based on the assumption that it is 80%.

The primary concern in my opinon would be to complete the gas planning (and a contingency table if appropriate) on the assumption that you will not be able to access the surface supplied O2 and will have to complete the dive either on backgas or with a deco bottle with your own deco mix. Surface supplied O2 is nice, but it is much more comforting to be able to conduct the dive without wondering what you will do if for some reason it is not available or you have to surface elsewhere.

Having to follow an air computer that does not take into account a current bubble model will require much more time in water and will require much more gas in reserve if you plan it properly and maintain adequate backgas to complete your own deco independently. This will potentially translate into either much shorter bottom times or leave you with an indadequate reserve and in a position where you absolutely have to make it back to the ascent line to complete your required deco.

A 30-40 cu ft stage bottle with Nitrox 80 would similarly allow you to be able to increase your bottom time and/or reserve by enabling you to complete independent deco on an accellerated schedule. I'd check to see if this is a possible option on the trip.

Given that you can get a new monochrome display PDA like a Palm 500 off e-bay for $50-$60 dollars and can get a copy of D-plan for another $60, it's hard not to at least consider this option for doing your own tables on site rather than relying on an air computer.

 

[vjongene]

Thanks a lot, Aquamaster! This is the most detailed and helpful answer I have had so far. I have no idea whether stage bottles will be available on site, but this would definitely be a big plus safety-wise. I was a bit worried myself on what would happen if we missed the ascent line and had to do long decos on air using only our back gas.

See my other post re tables. I definitely plan to take a Pocket PC (which I already have) with GAS to generate appropriate tables for each dive.

 

[will_tekkie]

I agree with DA...just a comment...the scheduled dives in bikini seems to be multilevel. (as they said in their website)..so...this should be taken in your dives profiles if you decided to use a PDA based program in your dive deco procedures in order to improve as much as you can your safety and bottom time....this is why, i said the bikini operator should have this tables already properly generated and HAVE to be explained in the meeting previous to dive....I think ALL people involve in these dives have to know exactly all the important details related to the wreck, dive caracteristics, depths, times, deco stops, bailout gases, emergency procedures...agressive sharks!!!... ...

I don´t like too much using an air only computer and a nitrox gas in deco together because your deco is too long ..you can get more than enough safety using other methods like deco tables properly generated for the dive that you will actually do...i agree is safer and more then possible to use both (if you have logged your %CNS track..which is not nuclear engineering)..but is not the best way to dive..well it´s just my own opinion....

buying Dplan or another dive program is not a bad idea anyway if you use it or not in your trip..but remember these programs have to be used properly before dive..deco stops are not the only thing in game...

enjoy your trip..i would like to dive there as well...i just will wait a little (after have sons and save more money)...i don´t trust too much in the lawrence livermore labs reports related to the radiological conditions there... :06:

 

[Curt Bowen]

What I disagree with is (i) using an essentially unknown deco mix (75-80%??? Which is it, then...?) and (ii) using a high-O2 deco mix with an air computer without tracking O2 exposure and without knowing how long the mandatory stops really are.

Calculating deco on air may give some quite long, shallow stops. Breathing a high-O2 mix is good (for evacuating N2), however a long-term exposure to O2 might not be desireable unless done correctly (air-breaks, tracking exposure and all that).

Using any other oxygen enriched mix besides air while using an air computer will build in extra safety. I could really care less if its 50% or 99% as long as its above 21.

Yes tracking O2 exposures are not accurate, but to tell the truth, I never track them anyway unless its a serious exposure dive. The average 150-250 feet for 20-25 minutes using a 1.4 is not an extreme exposure dive.

And yes, you need to add in your deep stops on the air computer.

Sheck Exley taught me to cut my deco on the 20 and 10 foot stops by 33% 1/3 while using an air computer and breathing 100% O2. You can stay longer if you felt the exposure needed it.