Question about Dive Planning and Computers

Peculiar Clown · May 29, 2005

Hi,

I have recently bought a Dive Computer, and I could do with some advice on how to use it when planning multiple dives.

I usually plan my dives using the dive tables, but given that dive profiles are never as square as the tables allow, I want to use my computer to adjust the plan for the second and subsequent dives.

E.g. If I plan a 16m dive using the dive tables, what adjustment can I make if I only dive an average of 14m ? Can I use 14m as the depth and adjust the plan for the next dive ?

Thanks in advance for any advice you can give me.

Kim · May 29, 2005

I am not completely sure what you are asking here but if it's what I think it is:

Your computer will log exactly what you did as opposed to the theoretical plan. My computer has a planning mode so if I want to look at the various NDL times I just key in the different depths and it will give me the times. It uses all the information from the first dive + the SI time to calculate this. Obviously the information changes as the SI time increases so if I know that I want a certain time at a certain depth then I simply wait until the computer says that's what I've got.
As you said - tables give square profile information. Computers calculate multi-level actual dives on the fly so will often give you more time than the tables would.

Diver0001 · May 29, 2005

Peculiar Clown:
Hi,

I have recently bought a Dive Computer, and I could do with some advice on how to use it when planning multiple dives.

I usually plan my dives using the dive tables, but given that dive profiles are never as square as the tables allow, I want to use my computer to adjust the plan for the second and subsequent dives.

E.g. If I plan a 16m dive using the dive tables, what adjustment can I make if I only dive an average of 14m ? Can I use 14m as the depth and adjust the plan for the next dive ?

Thanks in advance for any advice you can give me.

You shouldn't go back and forth between tables and the computer for planning. Use either one or the other. Your computer very likely has a planning mode too.

R..

Don Burke · May 29, 2005

Peculiar Clown:
Hi,

I have recently bought a Dive Computer, and I could do with some advice on how to use it when planning multiple dives.

I usually plan my dives using the dive tables, but given that dive profiles are never as square as the tables allow, I want to use my computer to adjust the plan for the second and subsequent dives.

E.g. If I plan a 16m dive using the dive tables, what adjustment can I make if I only dive an average of 14m ? Can I use 14m as the depth and adjust the plan for the next dive ?

Thanks in advance for any advice you can give me.

Switching back and forth between tables and computer has some risk to it, but you can track what the computer is doing with the tables and be ready for a computer crash.

I would not try to use average depth to modify the plan. Using the bottom times the computer is calling for the next dive does have some merit.

For instance, if your plan called for you to be in pressure group "D" after the first dive and after the actual dive the computer is allowing times that would be in group "C", the plan can be modified to reflect that.

You are going to stay within the limits of the computer. The modification would just be a way to better predict what the computer is going to output later on.

After two dives, all of the algorithms are in uncharted waters, so the risk goes up a bit.

scubapro50 · May 29, 2005

It would help if we knew what brand and model of computer you purchased. Some computers are better than others and work differently. What's nice about scubaboard is that there are divers that probably use the type of computer you purchased and can help on any techinical questions you may have about operation or the difference between it and your tables.

Marek K · May 29, 2005

Don Burke:
After two dives, all of the algorithms are in uncharted waters, so the risk goes up a bit.

Don--

Why do you say that?

With tables, you can theoretically iterate as many times as you can take it. In fact, it's a standard problem, after a second dive, to calculate RNT + ABT = TBT so that you can plan a third dive... and so on. As safe-sided as tables are (rounding up for depth and bottom time, assuming square profile), I would imagine the risk would stay very low.

Or is there something the table makers aren't telling us? :11:

I agree that as the number of repetitive dives increases, the more a computer's planning will deviate from table planning

I can also see that with several repetitive dives, a computer's less-safe-sided calculations are likely to become more "fuzzy."

--Marek

Don Burke · May 29, 2005

Marek K:
Don--

Why do you say that?

Partially because of the phase of the moon, but mostly because of the way the tables were created.

Marek K:
With tables, you can theoretically iterate as many times as you can take it. In fact, it's a standard problem, after a second dive, to calculate RNT + ABT = TBT so that you can plan a third dive... and so on. As safe-sided as tables are (rounding up for depth and bottom time, assuming square profile), I would imagine the risk would stay very low.

The algorithms that were used to create the tables and that are used in computers are approximations. 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.

Marek K:
Or is there something the table makers aren't telling us? :11:

They told us. Read all of the fine print on a PADI table. A fudge factor is applied to the third and subsequent dives.

Marek K:
I agree that as the number of repetitive dives increases, the more a computer's planning will deviate from table planning.

That is not what I said, but I do agree with it.

Marek K:
I can also see that with several repetitive dives, a computer's less-safe-sided calculations are likely to become more "fuzzy."

I could make a pretty good case that you are probably better off with the computer on the fourth dive and might be better off on the third. My Oceanic looks at all the compartment gas levels that it has computed before coming up with a time for the next dive. I would imagine most or all modern computers do the same thing.

Charlie99 · May 29, 2005

Don Burke:
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.

The pressure groups of the PADI table are ALL based upon the 60 minute halftime compartment. The pressure groups of the USN table (and USN derived tables such as NAUI, SSI, YMCA, etc) are all based upon the 120 minute compartment. It has nothing to do with whether it is the 1st, 2nd, or even 5th dive of the day.

Read all of the fine print on a PADI table. A fudge factor is applied to the third and subsequent dives.

You need to read the fine print more finely. Those rules only apply under certain rather unique conditions ..... very long shallow dives.

Don Burke · May 30, 2005

Charlie99:
The pressure groups of the PADI table are ALL based upon the 60 minute halftime compartment. The pressure groups of the USN table (and USN derived tables such as NAUI, SSI, YMCA, etc) are all based upon the 120 minute compartment. It has nothing to do with whether it is the 1st, 2nd, or even 5th dive of the day.

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.

If you intend to convince me that the controlling compartment does not change with the time under pressure, you are in for a long siege.

Charlie99:
You need to read the fine print more finely. Those rules only apply under certain rather unique conditions ..... very long shallow dives.

...and that proves that the tables do not have problems near the edges of the simulation?

Marek K · May 30, 2005

Don Burke:
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. [...] I could make a pretty good case that you are probably better off with the computer on the fourth dive and might be better off on the third. My Oceanic looks at all the compartment gas levels that it has computed before coming up with a time for the next dive. I would imagine most or all modern computers do the same thing.

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.

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.

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?

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

--Marek

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