Multi level deco on the fly

[Red Sea Explorer]

OK so you are coming out from a 15 minute dive at 60 or 80 meters and you are going through the deep stops. Half way through you see something interesting or have a situation where you need to stop ascending for a significant amount of time lets say ten minutes.

How would you change your Deco?
When you are done with the stop and you start ascending, do you continue as if you are still in the deep stops or do you resent the Deep stop counter to 80% of the last depth you spent the time at?
What about the profile on the top, would you recalculate a new average depth for the entire dive and deco accordingly?

That is what I do. It seems to work, but I am wondering how other think of it.

[Hassan Adly]

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[Hassan Adly]

Hi again,

Here's a reference from the recreational world: As a teaching aid, I will use my trusty PADI wheel

Seriously though, when using tables such as the wheel to plan a multilevel dive, it basically works by treating the next segment of the dive just like a repetitive dive. So when you go up to the next level, you know your pressure group at the end of the deeper segment, and treat the shallower segment like a repetitive dive with no interval, using the pressure group to calculate the new no decompression limit. This way you can dive to say 30m and stay there till you reach the NDL, then ascend to say 18m, and still have x amount of no deco time.

Now the question arises, how to apply this to decompression diving? After all, we are going quite far beyond the so-called NDL, and when we stop for a while halfway to explore something or attend to some problem, we could just treat it as a repetitive dive. But what do we do with the decompression that we haven't yet done? Perhaps we could calculate it using deco on the fly for 2 dives, simply adding up the result to get the total deco, plus whatever conservatism we apply for repetitive dives.

I still have no conclusions, just sharing my thoughts....

Hassan

[peter_steinhoff]

Hi guys,

That's quite interesting.

Let's say you did 60m/200ft for 15 minutes, ascended and started some deep stops at 45m/150ft. At 40m/130ft you stopped for 10 minutes.

Since this is a deep to shallow profile I would have calculated the average (52m/170ft or so) and done the deco for a 52m/170ft for 30 minute dive. Since that also symbolises the gasloading of the dive I would calculate a new deep stops from 80% of ATA from this average depth, which in this case is the next stop up from 40m/130ft.

Having played with this a bit in the past I know that in theory using average depth will be conservative for longer bottomtimes and aggresive for very short times. In practice I haven't been able to detect any differences but I haven't tried the extremes either. Since there is really little we know about decompresssion I use a little extra conservatism doing average calulations for longer dives involving decompression.

So I guess that's quite similar to what you do. We actually discussed this on this forum not long ago but from a recreational point of view doing multilevel dives without a computer.

Come to think of it I think might have to travel down to Egypt to discuss this some more...I'm in need of some warmer climate

Best,
Peter

[Red Sea Explorer]

Hassan.
I appreciate your analysis but I am looking to see what people are actually doing. Many people on this list are diving regularly and must have some techniques. I personally average it out in my head and Deco accordingly and it has worked fine with me.

So if anyone is has a formula for this type of multi level Deco on the fly, please share. Mainly address the following points.

- When ascending from the shallower stop do you continue the deep stops of the deep end or move to 80% of the shallower depth?

- How do you adjust the deco if you did not have the multi level dive planned?

[boomx5]

Excellent topic Faisal!

[Red Sea Explorer]

Hi Peter,

I am faced alot with this scenario as many times we are coming up the reef and we meet other groups and decide to hang with them or we find something interesting on the reef. I usually do as we mentioned above.

Only I usually move the deep stops up from the new level as if that is where the dive was. My thinking was that having spent the 10 mintues at this depth I am already relativly cleared to push the gradient a bit rather than stay and deep and on gass. But I think the way you are looking at it makes more sense, and is more conservative from a Deep Stop perspective.

Quote: (Originally Posted by peter_steinhoff)

Hi guys,

Since this is a deep to shallow profile I would have calculated the average (52m/170ft or so) and done the deco for a 52m/170ft for 30 minute dive. Since that also symbolises the gasloading of the dive I would calculate a new deep stops from 80% of ATA from this average depth, which in this case is the next stop up from 40m/130ft.

I like this idea of starting the seep stops from the 80% of the new average depth I will give it a try.

Do you do that for standard profiles as well. Basically if you spend 10 minutes at 70 and then drop to 90 and spend 10 mintues your average is 80. but do you calculate the first deep stop at 80% of 90 or 80? I know the diffirence is minor but it is the thinking I am after.??

Quote: (Originally Posted by peter_steinhoff)

Come to think of it I think might have to travel down to Egypt to discuss this some more...I'm in need of some warmer climate

If you make it down here we will love it. You are always welcome. We are doing some fatastic diving. A few weeks ago we did a few dives on the SS Maidan. It is a steam ship that went down in 1923. Only a handfull of divers have been on it and it sits on a platuae between 85m and 120m with its bow hanging over the platuae. you can scooter under it. It offers great penetration opprtunities and it wsa loaded with Cargo coming from Africa. Definatly worth checking out.

[Chris B]

Quote: (Originally Posted by peter_steinhoff)

Hi guys,

That's quite interesting.

Let's say you did 60m/200ft for 15 minutes, ascended and started some deep stops at 45m/150ft. At 40m/130ft you stopped for 10 minutes.

Since this is a deep to shallow profile I would have calculated the average (52m/170ft or so) and done the deco for a 52m/170ft for 30 minute dive. Since that also symbolises the gasloading of the dive I would calculate a new deep stops from 80% of ATA from this average depth, which in this case is the next stop up from 40m/130ft.

Having played with this a bit in the past I know that in theory using average depth will be conservative for longer bottomtimes and aggresive for very short times. In practice I haven't been able to detect any differences but I haven't tried the extremes either. Since there is really little we know about decompresssion I use a little extra conservatism doing average calulations for longer dives involving decompression.

So I guess that's quite similar to what you do. We actually discussed this on this forum not long ago but from a recreational point of view doing multilevel dives without a computer.

For what its worth, this is what I would do; keep averaging the depth for the time to the point in the dive and re-calc deep stops from where ever I am now.

The danger of changing things significantly during the dive is ensuring your gas reserves match the new deco required......

Chris B

[Hassan Adly]

Agreed, though it is worthwhile using for dive planning, there is a dive I would like to do that involves exploring a wreck at 95m for about 15mins, then ascending (including deep stops) to 60m and scootering at 60 along a reef wall for about 15 mins, viewing a couple more wrecks on the way

jeez I probably would need more tanks for that...

[peter_steinhoff]

Quote: (Originally Posted by Red Sea Explorer)

Hi Peter,

Do you do that for standard profiles as well. Basically if you spend 10 minutes at 70 and then drop to 90 and spend 10 mintues your average is 80. but do you calculate the first deep stop at 80% of 90 or 80? I know the diffirence is minor but it is the thinking I am after.??

I would use the max depth since that is where were were and we haven't done any offgasing as we did in the previous example. So deep stops starts at 70m (90-18-2=70). If we would take a more extreme example like spending 15min at 30m followed by 15 min at 90m I think it makes even more sense to start deep stops at 70m instead of 45m.

Also, from playing with multilevel dives from a dissolved gas point of view 10min@70m followed by 10min@90m is NOT at all the same as 10min@90m followed by 10min@70m even if the average depth is the same. Going shallower we are to some degree offgasing the deep part of the dive but not the other way around.

The most conservative way would be to consider it a 20min@90m dive. The best compromise I found however was that 75% of the difference is closer and fits well in with tests I've run. So a 10min@70m followed by 10min@90m would be 20min@85m. When the depths are close it's not a big difference but if you would do for example 40m followed by 80m the equivalent profile would be 70m instead of 60m which is the average.

There are probably better ways of doing this but I think this is a methods that seems to work fine and I haven't experienced any problems with it so far.

Best,
Peter

PS. I'm off for a few days so don't if I won't reply until I get back...