No Deco on Trimix?

[Gilldiver]

For example with a particular bubble gradient model, 30 minutes at 165' on air with 50% O2 for a deco gas would require:

2 min @ 80' (on air)
2 min @ 70' (50% O2 for all remaining deco)
2 min @ 60'
2 min @ 50'
4 min @ 40'
5 min @ 30'
9 min @ 20'
17 min @ 10'

For a total of 45 minutes of deco

Just an aside here:

Using the old Navy Tables we all used in the 70's, I get a dive to 170' for 30 minutes on air as:

4 minutes @ 30
13 minutes @ 20
26 minutes @ 10

60 feet/minute accent and no O2 switch

Total Accent time of 45.8 minutes
Total Run of 75.8

You are into a Bend Me, Fix Me off gassing mode but the acctual occurance of hits using the Old tables and doing square dive calculations was very low. Not that I would do that now a-days.

 

[Bismark]

That would be a long 26 minutes at 10'......Interesting how the shape of the curve changes but the deco time stays pretty close to the same. With all the most recent information about deep stops who knows what the future holds?...........

 

[DA Aquamaster]

That would be a long 26 minutes at 10'......Interesting how the shape of the curve changes but the deco time stays pretty close to the same. With all the most recent information about deep stops who knows what the future holds?...........

The same bubble gradient model (D-Plan) with 170' for 30 minutes and air for deco requires 83 minutes of deco rather than the 43 specified by the US Navy tables.

So the curve is different but the deco is also far longer on air. The current bubble gradient and variable permeability deco models are much more conservative, but you can do repetitive dives with a very low probability of getting bent.

In contrast the US Navy tables had a hit rate around 1% on single deco dives and around 4% on repetitive deco dives. Which is why the US Navy did not do repetitive deco dives and normally did not do deco dives at all unless an on board recompression chamber was available.

So again, the big decrease in deco times (compared to air) is not due to deeper stops but to more efficient deco gasses. The end result of using higher % O2 gasses for deco is that you get short deco times comparable to old, very liberal and not very safe air decompresion tables but with a much better margin of safety.

 

[Bismark]

The same bubble gradient model (D-Plan) with 170' for 30 minutes and air for deco requires 83 minutes of deco rather than the 43 specified by the US Navy tables.

So the curve is different but the deco is also far longer on air. The current bubble gradient and variable permeability deco models are much more conservative, but you can do repetitive dives with a very low probability of getting bent.

In contrast the US Navy tables had a hit rate around 1% on single deco dives and around 4% on repetitive deco dives. Which is why the US Navy did not do repetitive deco dives and normally did not do deco dives at all unless an on board recompression chamber was available.

So again, the big decrease in deco times (compared to air) is not due to deeper stops but to more efficient deco gasses. The end result of using higher % O2 gasses for deco is that you get short deco times comparable to old, very liberal and not very safe air decompresion tables but with a much better margin of safety.

I may have been unclear in that my reference to deep stops was a comment on some of the most recent research (I have three seperate articles) which, in short, questions the validity of deep stops as a means of reducing bubble growth. My point mostly was that we haven't got all the answers yet and who knows what the future holds for decompression theory. Also, my reference to the shape of the curve in the aforementioned example was that the deco profiles I use are not as heavily weighted to the shallower, longer stops yet the end result for my deco time and the time in the example was very similar. Just different shapes.

 

[Gilldiver]

I always keep the old Navy tables with me as a get out of Hell back up.

With the Navy tables you would use the next deepest dive, so for a 165 foot dive, you use a 170' table.

I don't think the hit rate was 1% on the first deco dive, more like 0.1% or less if I remember, I would need to call Bill Hamilton to check. But with the Hundreds of thousands of dives conducted over 30+ years on those tables we just do not see the high number of hits that a 1% rate would produce.

The biggest problem with the Navy tables was in the 60 to 90 foot range. If you look at max NDL and go over that time by 2 or 3 minutes, your deco obligation as a percent of total bottom time was the largest. I would need to recreate the curves we used to teach deco back then to show you. But the end effect was that you were much more likely to get bent by exceeding the NDL in that range then in any other.

The other thing was that these tables were very empirical in nature. They dove goats and then men on differing schedules until they found what seemed to work and then back off some. What this did was take you to micro bubble formation (Bend Me) then got you to Deco those bubbles out (Fix Me) which is why I call it "Bend Me, Fix Me."

Back to what I was thinking about. A standard dive for us on the USS Bass was 20 minutes on the bottom. This figured to be:

20 min @ 160 gives

3 min @ 20
11 min @ 10

We bumped that to

5 min @ 20
15 min @ 10


I did this many, many times but the second dive was always much shallower. We said - Two dives on the Bass = One dive in the Chamber.

But if all else fails and I have at least one deco gas and my O2 was at 20 or 21% use the air tables and get home. Doing the 5 and 15 deco on 50 or 100% gets you a long way toward the safety of the modern tables.

In fact that is how we first started doing mix in 1991 or 92, use any mix as long as the He and N2 added up to 79% - and the He was less then 35%, use one higher O2 deco gas (O2, 32%, 36%, 50%, etc) and dive air tables. This still works, use any computer deco table and dive say 21/30 with a deco gas, then run the same depth and time with air and the same deco gas and look at the run times. They should be within a minute or so of each other.

 

[Bismark]

Very interesting Pete. Thanks for taking the time to post this.

 

[DA Aquamaster]

I always keep the old Navy tables with me as a get out of Hell back up.

With the Navy tables you would use the next deepest dive, so for a 165 foot dive, you use a 170' table.

I don't think the hit rate was 1% on the first deco dive, more like 0.1% or less if I remember, I would need to call Bill Hamilton to check. But with the Hundreds of thousands of dives conducted over 30+ years on those tables we just do not see the high number of hits that a 1% rate would produce.

I should clarify. The hit rates were obtained when square profiles were utilized - which was exaclty why so many fudge factors were built into how the tables were actually used (rounding deeper or longer to the next depth or time and going to the next greater depth and/or time for hard working dives, cold water dives, etc.) When the tables were dove conservatively with the fudge factors added in, you ended up with a much lower hit rate - but the tables themselves as written did produce the preceding figures when pushed to the limits of the table.

 

[Kevrumbo]

I am not a Trimix diver nor in a class, but I have been doing some reading on Trimix and doing a little math, and so a question.

Assume a dive to 165 feet using 21% O2, 45% N2, and 34% He. At 165 feet the equivalent N2 air depth is 80 feet and the NDL is 30 minutes. For the He, the equivalent Heliox (80% He, 20% O2) is only 50 feet and the NDL is 2+ hours.

And so it would seem that a diver on this Trimix could diver to 165 feet and as long as the dive was less than 30 minutes the diver not incur a mandatory deco stop.

What have I missed?

No it doesn't work that way . . .the dynamics going on in a trimix decompression dive are much more complicated than the intuitive construct above (nice try, but it's not correct).

Read this thread on Helium Offgassing Rate for a qualitative description of what's going on. . .

 

[ivankahn]

Thanks Kevrumbo! Your reply was most useful and the referenced thread helped a great deal!

 

[Blackwood]

My point mostly was that we haven't got all the answers yet and who knows what the future holds for decompression theory.

Rocketship ride to the surface to suck the gas out of solution followed by a re-descent to shrink them enough that they can be exhaled.

It's the way of the future.