continuous repetitive trimix diving

[aleksi]

Hi all!

My previous thread did not receive much interest so I will try and rephrase the question so that it will be generally more interesting.

My team of two Finnish normoxic trimix divers is travelling to Chuuk lagoon. We are diving there for seven days in a row. The general rules of repetitive trimix diving naturally apply, like doing the days deepest dive last. We are thinking of doing one (maybe two) 30+m dive with bottom time around 45-60 mins and one 50-60 m dive with 20-30mins BT every day.

Plans calculated with V-planner seem to permit doing these kinds of dives regarding both CNS exposure and off-gassing. On this forum there have been discussions about doing dives to 60+ meters with very short surface intervals successfully. On the other hand there have been some "undeserved" oxtox incidents (at least one in Finland) that suggest the CNS clock might not completely reset in 24 hours as suggested and thus the risk would raise continuously.

Any theorethical points of view to long-time accumulation or personal experiences of prolonged repetitive diving in the tech 1 or 2 range would be very helpful.

-Aleksi

[Brian Allen]

Prolonged multi day 70m tech-2 diving has never caused me a concern where it was 1 dive per day. If I was doing a second dive I wouldn't push the limits and probably uneccesarily pad the upper part of the deco on the second dive by 50% but that's just me, i'm over cautious.

But I always thought that the dive operations around Trukk lagoon had their own set ways of running deco, based around surface supplied O2. Maybe I'm out of touch but they were certainly doing this 3 years ago.

What has the dive operation said about running your own deco?

B

[aleksi]

You are right Brian, they do still offer deco stations and surface supplied oxygen. Despite of that we will plan and run our own deco and carry all the necessary gas with us and that is ok with the operator. We are booked aboard ms Thorfinn.

When doing two dives a day, one shallower and one deep, padding the shallow deco seems reasonable. The problem is padding oxygen deco by 50% increases CNS considerably and the risk of oxtox might be very real by the end of the week. Do you think this is manageable or even not an issue at all?

-Aleksi

[jerry.mobbs]

Quote: (Originally Posted by aleksi)

You are right Brian, they do still offer deco stations and surface supplied oxygen. Despite of that we will plan and run our own deco and carry all the necessary gas with us and that is ok with the operator. We are booked aboard ms Thorfinn.

When doing two dives a day, one shallower and one deep, padding the shallow deco seems reasonable. The problem is padding oxygen deco by 50% increases CNS considerably and the risk of oxtox might be very real by the end of the week. Do you think this is manageable or even not an issue at all?

-Aleksi

have fun on the thorfinn

the capt is a real larger than life character

hes been there 20 odd years and it a bit of a hoot to be around

unless lance moves the boat every time (very unlikely as its a true museum piece), you'll be diving off small boats, and useing the thorfinn as a base

i'm not sure how they will run deco for you off the back of these smaller boats, as my memory is a bit rusty

i'd carry all my deco gas with me on every dive out there, but use the surface supply if its there (which should be every time)

we did 3 dives a day, 2 deep and 1 shallow night dive

check out www.deepweek.com for a write up

and

http://homepage.mac.com/jerry.mobbs/...Theater24.html

for some video

make sure you visit the thorfinn engine room when your there, it will put the engine rooms of the wrecks into perpective

you'll enjoy it

jerry

[Mats Edenfeldt]

I see no problems with doing the same deco and the same dive 2 times a day. I have successfully done 2*65 meter during a 3 day period, same deco and its no big deal. It depends on how you are feeling and how cold the water is and so on.

For warm water diving I guess you could cope with the deco better then in a cold water environment like in the Baltic but that is just a guess as I have not done the same in warm water. Ask the warm water gurus here on the board what they do. But for Baltic condition 2*65-70 per day and with at least 4 hours of surface time and the same deco is no problem. I would add a few extra minutes on the oxygen on the second dive for the Baltic dives if I where to do this for more then 1-2 days. It all depend on how I am feeling and de water temp and the sea conditions.

Have a nice trip.

[aleksi]

Thank you for the replies Jerry and Edenfeldt!

I am sure that diving in warm water makes the dcs risk much lower.

Jerry, your dive shedule in Truk seems quite similar to what we are planning. Did any of you feel any pulmonary oxygen toxicity effects or DCS symptoms during your trip in Truk?

-Aleksi

[Ahmed Adly, Marlin Inn DC]

Hey Aleksi,

It is a great surprise that you didn't get more attention with a similar thread before. This is, I think at least, about one of the most important issues within technical diving.

Now first of all, let us be sure that we are on the same track and we agree that first of all: there is no spoon.

OR, you could also say, there is no real absolute theory on O2 tox and so it would be pretty hard to comment on it in an absolute fashion.
What we know for sure is, that to keep your exposures low, you be non-active as possible on your O2 stops, stay warm and try using "air breaks". Now although air breaks are time consuming and pretty serious (especially if using hypoxic mixes) they seem to work for both O2 tox and decompression. I know of many a dive where I figured passing 100% (will not say how far) as clear as day and never felt a "jitter".
There are a few methods of air breaks which include 12/6 or 15/5 and even 20/5 and so on. I think you should worry equaly about Pulmonary O2tox.
Although there is not much to back it up, I feel it when I work out and especially when I run after a day where I decompressed on O2. Like I am weak or something, no strength, out of breath easy. Wierd but not the same as sub-clinical DCS.
So my final word on O2 in your week of fun would be to watch more for pulmonary. Might be better to lay off the other 30 meter dives or keep them short.

Your next consideration (wise choice) is decompression.
I think repetative profiles really only affect divers who are seeking the "no decompression" limit for single gas air dives.

I think because the speed at which gas absorbs into tissues is governed by the laws of half-times, we do not really come that close to saturating more on the second dive than on the first, especially when decompressing on O2. Unless we are talking say 90 meters for 20 minutes a pop. But then the day of diving would be limited more by sunlight and money (thus the lack of data the deeper you go).
I could be wrong, but with my un-educated mind, that is all I can figure out, till someone tells me otherwise (or I get bent like a pretzel)

I would summerise by saying that go ahead and do a couple of 60 meter dives with a decent plan and a few hours in between but lay off the other dives you might want to do.
Maybe best to do the shallower dives in the start of the week say 3 or 4 a day and then go for the deeper targets at 2 a day. Should work out fine. (I hope)

Oh, and try to make your bottom mixes as poor as possible. say 18/45 or maybe 17/45. Just to keep your working PO2 as low as possible.

I hope the long post was not boring, I think if you have any questions, I will be able to make short enough answers, so don't feel threatened to ask.

[jerry.mobbs]

Quote: (Originally Posted by aleksi)

Thank you for the replies Jerry and Edenfeldt!

I am sure that diving in warm water makes the dcs risk much lower.

Jerry, your dive shedule in Truk seems quite similar to what we are planning. Did any of you feel any pulmonary oxygen toxicity effects or DCS symptoms during your trip in Truk?

-Aleksi

no

we also used nitrox on the night dive

[aleksi]

Hi Ahmed,

thanks for your reply! It certainly was not boring, I love deco-talk
I am on the same page with you that there is no spoon, that's why your and others personal experience is great. I'd still want to know if you have done a similar series of dives (I bet you have as you live at the Red Sea).

Quote: (Originally Posted by Ahmed Adly, Marlin Inn DC)

There are a few methods of air breaks which include 12/6 or 15/5 and even 20/5 and so on.

Airbreaks will naturally be used, we do 12 on, 6 off as recommended also by gue.

Quote: (Originally Posted by Ahmed Adly, Marlin Inn DC)

I think you should worry equaly about Pulmonary O2tox.

That's a good point, by the end of the week this is quite probable. I discussed with a Finnish mix-instructor yesterday and he suggested doing some of the dives with just ean50 to let the lungs rest a while. This might work on the shallower dives, in warm water long deco is not a big problem.

Quote: (Originally Posted by Ahmed Adly, Marlin Inn DC)

Oh, and try to make your bottom mixes as poor as possible. say 18/45 or maybe 17/45. Just to keep your working PO2 as low as possible.

Yes, we were planning on using 18/45.

Quote: (Originally Posted by Ahmed Adly, Marlin Inn DC)

Might be better to lay off the other 30 meter dives or keep them short.

Maybe best to do the shallower dives in the start of the week say 3 or 4 a day and then go for the deeper targets at 2 a day. Should work out fine.

This is very interesting. Can you explain a little more of the rationale. My thinking is that if we do a 60m dive, say at 18:00 for a 60 runtime and a 30m dive at 10:00 the next morning, the most of the bubles would allready be gone during the 15 h surface interval. If so, it would be ok to do a 30m and a 60m. But if there were bubles left in the morning, there would probably be shunting past the lung filter to the venous side. Doing two dives of the same depth would prevent this. Is this what you were thinking?

Thank you for all the replies, please keep them coming

Aleksi

[Ahmed Adly, Marlin Inn DC]

Quote: (Originally Posted by aleksi)

This is very interesting. Can you explain a little more of the rationale. My thinking is that if we do a 60m dive, say at 18:00 for a 60 runtime and a 30m dive at 10:00 the next morning, the most of the bubles would allready be gone during the 15 h surface interval. If so, it would be ok to do a 30m and a 60m. But if there were bubles left in the morning, there would probably be shunting past the lung filter to the venous side. Doing two dives of the same depth would prevent this. Is this what you were thinking?

I think you miss read my incomprehensable babble.

The suggestion is that you limit yourselves to a couple of dives a day. Originally you proposed doing a couple of deep dive followed by a couple of long shallow dive per day. I suggest that you stick to either the deep dives or the long dives.

I have spent up to 2 weeks of doing 1 deep dive on air or trimix every day. But not for a week of multi deep dives. I have also done several days of 1 deep(75m+) 1 shallow(30m-).

In no way do I assume to know anything about "bubbular" habits and their "ins&outs" regarding tissues. And as regards shunting, let me remind you that we have ladies reading this forum