Question about Dive Planning and Computers

[Kim]

As far as I always understood it - the first dive is controlled by a fast compartment - subsequent dives are controlled by increasingly slower ones. The amount of SI has a big effect on how quickly the controlling compartment moves to a slower one. This is what I see on the Suunto graphs in any event.

 

[Diver0001]

Just a point of clarification.

The PADI tables (NDL's) themselves are calculated based on a multiple compartment Haldane/Spencer model. In the case of the PADI RDP if I'm not mistaken the tables were calculated using 12 compartments.

The *surface intervals*, however, decay based on only one compartment. On the PADI tables that's the 60 minute compartment. This is what Charlie was trying to say (I think).

This choice makes the table easy to use and cover most diving situations but can lead to higher than anticipated tissue loading when making several long shallow dives in a row.

R..

 

[Kim]

Just for interest I checked my logbook and looked at the tissue loading over 4 dives in one day. The Suunto model uses 9 compartments which I will number from 1 being the fastest to 9 being the slowest. The controlling compartment in each dive is the most saturated throughout the dive starting from about 10 minutes into the dive.
1st dive - comp 3. 2nd dive - comp 5. 3rd dive - comp 6/7 (about the same loading). 4th dive - comp 7

 

[scubatoys]

If using an Aeris or Oceanic Computer, they use the same algorithm as the Padi Tables. As far as planning, the computer will tell you at any given time, how long you can go do a 60 ft dive, or 70 foot dive if you went in right now. Problem is if you just finished a dive, and know you are going to dive again in an hour, how much time will you have for a 60' dive then?

Really, you can get close to the answer by simply checking your allowable time to do a 60 foot dive on the computer when you exit from the first dive. Then by comparing that to your tables, you can figure out what pressure group you are in. For example if you see upon exiting from dive 1, you computer says you can do a 60 foot dive for 25 minutes, that will put you around a K or L diver. When in doubt, assume you have more nitrogen so call yourself an L. Then you can plan your surface interval and next dive.

Of course, when doing the dive... trust the computer - not your calculations - since this is a bit sideways... but if you test it out, you will see it is actually pretty darn accurate - especially if using the Aeris and Oceanic computers.

 

[Don Burke]

Very interesting... I'm not pretending I understand the physiology you and Charlie99 are discussing... but what you're saying is, dive tables are based on only one "compartment" (which means a theoretical group of tissues that absorb and release nitrogen at the same rate). And with more and more repetitive dives, you're saying that the "controlling compartment" may -- or probably does -- change... which tables don't account for. And that therefore tables may become less and less accurate.

Precisely

While computers take into account the varying absorbtion and release rates of various compartments... and should track that sort of thing better over repetitive dives.

They certainly have the potential for it and as near as I can tell, that is exactly what most computers attempt to do.

Which means that, on one hand, computers do give a smaller margin of safety for one or two dives... but then become safer, as compensation, for subsequent repetitive dives. Pretty close?

I gets pretty obvious if you run a program that shows nitrogen loading as a bar graph. As the number of dives increases, the bar that sticks out of the pack changes to longer and longer compartments. The Oceanic simulator makes it very obvious, although I can show the same thing with DecoPlanner.

Too bad I don't make that many repetitive dives!

The PADI tables get you into "fudge factors" if you take it to the limit at sixty feet on the second dive. If you spend twenty minutes at one hundred, one hour at the surface, thirty eight minutes at sixty, you then have to wait an hour to spend five minutes at thirty five feet due to the fine print. Otherwise you could jump in after three minutes.

 

[Don Burke]

Just a point of clarification.

The PADI tables (NDL's) themselves are calculated based on a multiple compartment Haldane/Spencer model. In the case of the PADI RDP if I'm not mistaken the tables were calculated using 12 compartments.

The *surface intervals*, however, decay based on only one compartment. On the PADI tables that's the 60 minute compartment. This is what Charlie was trying to say (I think).

This choice makes the table easy to use and cover most diving situations but can lead to higher than anticipated tissue loading when making several long shallow dives in a row.

R..

Good catch. Thank you.

 

[Charlie99]

The time the tissues are under pressure has everything to do with the controlling compartment. If the time under pressure does not increase with the number of dives, we need a waiver from the laws of physics.

Two different statements Don. Your first statement was

For repetitive dives, the tables use a particular compartment to determine a pressure group. If we take it on faith that the compartment chosen is the best one for the second dive, we also must agree that it is probably not optimum for the fourth dive.

. That statement I take issue with because the pressure groups and times are all based upon 1 and only one compartment.

Your later statement about controlling compartments

The time the tissues are under pressure has everything to do with the controlling compartment. If the time under pressure does not increase with the number of dives, we need a waiver from the laws of physics.

is a different statement, and is correct.

Even with the first dive, the controlling compartment depends upon depth. Thats why, for example, on a dive to 80', that one is not allowed to exceed Pressure Group R. The 60 minute compartment will only have reached 71% of the maximum allowed value, while the 10 minute compartment has reach 100%, and the 20 minute compartment has reached nearly 100%.

As a general rule, the controlling compartment halftime is about 55% of the NDL for any given depth (You can derive this from Hempleman square root law or just verify it by cranking through numbers on any deco program).

 

[Don Burke]

That statement I take issue with because the pressure groups and times are all based upon 1 and only one compartment.

Absolutely incorrect.

The no-stop times are based on one of however many compartments are used in the algorithm.

The tables pick one compartment and use that as the controlling compartment for repetitive dives.

Modern computers use all their compartments for repetitive dives and monitor the theoretical gas load in each. The concept is to keep all compartments below the danger line.

In no case is a single compartment used as the basis for all pressure groups and times.

I have come to the conclusion that you are intentionally being obtuse.

Have a nice life.

 

[Charlie99]

The PADI tables (NDL's) themselves are calculated based on a multiple compartment Haldane/Spencer model. In the case of the PADI RDP if I'm not mistaken the tables were calculated using 12 compartments.

The *surface intervals*, however, decay based on only one compartment. On the PADI tables that's the 60 minute compartment. This is what Charlie was trying to say (I think).

This choice makes the table easy to use and cover most diving situations but can lead to higher than anticipated tissue loading when making several long shallow dives in a row.

SHORT VERSION: Pressure groups are based solely upon the 60 minute compartment. NDLs consider all compartments.

LONG VERSION: While the PADI/DSAT model has 12 compartments, the actual numbers on the PADI tables (all 3 table sections) are based SOLELY on the 60 minute compartment. The slower-than-60 minute compartments only affect very long, repetitive, shallow dives, and the effect of the those compartments show up only as the W, X,Y, Z special rules that extend SI to 1 or 3 hours.

The faster compartments that limit deeper than 40'/12 meter dives are incorporated into the table by not allowing one to reach Pressure Group Z in the deeper dives.

To use a more concrete example, look at the PADI table for 80'. Note that the 30 minute NDL is only pressure group R. After 30 minutes at 80', the 60 minute compartment has not reached its limit, but the 10 minute and 20 minute compartmentss have. This is why, as you go deeper, that the table entries for the higher pressure groups are blanked out.



Were the PADI model to be revised such that the 10 minute compartment's M value were decreased 20% to 71fsw, then it would hit its limit after 19 minutes @ 80', not the current 30 minute limit. The effect on the RDP would be to leave leave the 80' pressure group numbers the same for PG A to PG I, with the NDL number of 19 minutes being inserted into PG J. Pressure groups greater than J in the 80' column would all be blank, indicating that one is not allowed into that zone.

 

[Charlie99]

As far as I always understood it - the first dive is controlled by a fast compartment - subsequent dives are controlled by increasingly slower ones. The amount of SI has a big effect on how quickly the controlling compartment moves to a slower one. This is what I see on the Suunto graphs in any event.

You will also see that the deeper your first dive, the faster the controlling compartment.

Another interesting, and important effect on controlling compartments comes about when one "rides the NDL" -- in other words, goes deep until hitting NDL, then slowly comes up while just barely staying within NDL. The controlling compartment is intially a fast one, then as you continue to "ride the computer" or "ride the NDL" up, the controlling compartment gets slower and slower.

The practical effect of this is that, if one goes to NDL while deep,then does a normal ascent and stop, you will signficantly offgass the most heavily loaded compartment and will exit the water with a relatively large margin between the max limit and your actual loading.

If you ride the NDL, you will have a slower tissue near its limits,and it will take long hang times to get a significant margin. If you have a Pelagic/hockey puck, it will take you a very long time to get the tissue loading bargraph to back off into the green.