A few Table Use Questions

QKRTHNU · May 5, 2003

#1. Why is it that Group desinations don't match up @ NDLs?
ie. On the USN tables:
70ft NDL is 50min and leaves you at group I
90ft NDL is 30min but only puts you in group H

Why doesn't the 90ft NDL put you in the same pressure group as the 70ft NDL?

#2. This question is in regards to using the tables for multi-level dives. I read a post that said it was accurate to be "Reading the Tables Sideways" to calculate a multi-level dive. http://www.scubaboard.com/showthread.php?s=&threadid=9625&highlight=multilevel+dives+AND+Tables

If I wanted to use tables to plan a multi-level dive, would this be the correct way to do it?

Ficticious dive profile:
90ft for ~18min (GroupF)
Move up to 40ft
Look at table find equivilent 40ft time for group F (50min)
I now have 150min left on NDL according to USN NDL of 200min @ 40ft.

Is this an accurate way to use a table for a multi-level dive? This makes sense to me, but it flys in the face of the group desination/NDL question above.

Thanks!

Walter · May 5, 2003

"Why doesn't the 90ft NDL put you in the same pressure group as the 70ft NDL?"

You don't have the same amount of nitrogen disolved in your tissues after a 90 ft dive to the NDL as after a 70 ft dive to the NDL.

"If I wanted to use tables to plan a multi-level dive, would this be the correct way to do it?"

There is no "correct" way to use tables to plan multilevel dives. While it is possible to use them for multilevel dives, it is not recommended by any authority of which I'm aware. I stopped teaching multilevel table use a long time ago. Buy a computer if you want to dive multilevel. Tables weren't designed for multilevel use.

QKRTHNU · May 5, 2003

"Why doesn't the 90ft NDL put you in the same pressure group as the 70ft NDL?"

You don't have the same amount of nitrogen disolved in your tissues after a 90 ft dive to the NDL as after a 70 ft dive to the NDL.

Please explain this more. I thought that NDLs were all related to Residual Nitrogen in that once a certain level of Nitrogen was disolved into your body in-water deco was needed.
And that NDLs were shorter at greater depth simply because the higher pressures saturated you more quickly.

So why would deco be required for a 35min dive @ 90ft if there is less Nitrogen (lower pressure group) in your body than a 50min 70ft dive?

Rick Murchison · May 5, 2003

Walter once bubbled...
There is no "correct" way to use tables to plan multilevel dives. While it is possible to use them for multilevel dives, it is not recommended by any authority of which I'm aware. I stopped teaching multilevel table use a long time ago. Buy a computer if you want to dive multilevel. Tables weren't designed for multilevel use.

Before the advent of computers, we had several ways to use the Navy tables for multilevel diving - but - let me echo what Walter is saying here. The tables weren't designed for it, and now that cheap computers are readily available that'll do multilevel calculations for you, get one and use it. (You needn't buy new - there are plenty of divers with old air computers laying around who've moved on to Nitrox and are willing to part with the air models at very reasonable prices)
That said, I wouldn't abandon playing with the tables working out multilevel profiles - they'll give you good ballpark numbers you can use to make sure your computer isn't totally off the wall... "To err is human... to really screw up takes a computer."
Rick

Walter · May 5, 2003

Tables use several compartments which in-gas and off-gas at various rates.

The first issue is slower compartments won't have as much time to approach saturation on a 90 ft NDL dive as on a 70 ft NDL dive.

The second issue is faster compartments are approaching saturation at 90 fsw as opposed to 70 fsw, so the pressure is different for the two dives. If you need a more in depth explanation, you'll need to wait till I have more time or someone else will need to delve into the theory.

Walter · May 5, 2003

Rick, you are correct. One of the reasons I stopped teaching multilevel table use is YMCA now prohibits it. Does SSI have a policy on the issue?

Anyone really interested in the topic can dig up an old Dennis Graver article on the topic. Might take some real digging, I don't believe I've seen a copy since about '85.

MikeFerrara · May 5, 2003

QKRTHNU once bubbled...

Please explain this more. I thought that NDLs were all related to Residual Nitrogen in that once a certain level of Nitrogen was disolved into your body in-water deco was needed.
And that NDLs were shorter at greater depth simply because the higher pressures saturated you more quickly.

So why would deco be required for a 35min dive @ 90ft if there is less Nitrogen (lower pressure group) in your body than a 50min 70ft dive?

It isn't just the amount of nitrogen that matters but which tissue compartment is the controling compartment and to what extent it's loaded.

In the example above the 90 ft 35 min dive requires decompression because the leading/controling tissue compartment will have a presure gradient greater than it's "M" value (max allowable gradient) if you ascend directly to the surface. Yet...the SI required to clear can be less because the leading compartment is a faster compartment and has a shorter halftime.

If you are now completely confused let me know and I'll try again.

SubMariner · May 5, 2003

That said, I wouldn't abandon playing with the tables working out multilevel profiles - they'll give you good ballpark numbers you can use to make sure your computer isn't totally off the wall... "To err is human... to really screw up takes a computer."

Chalk it up to a difference in opinion, but I wouldn't advocate using the "Flat" PADI Recreational Dive Planner at all for multi-level diving.

The PADI Wheel is the best way to calculate multi-level profiles and is much easier to use.

Why re-invent the wheel? [pun intended]

~SubMariner~

Doof · May 5, 2003

I've heard the Wheel described in many different ways, but "easy to use" has never been one of them.

Is the arrow touching or piercing? Do the dots line up? Ugh... too much guesswork for my tastes.

The tables work fine. Treat a multilevel dive as if it were a multiple dive with a 0 minute surface interval and use the table from there.

I'd be interested to see how that compares with the Wheel, though. Maybe when I get home tonight I'll run a few comparisions. If I can figure out how to use the Wheel again.

Walter · May 5, 2003

"The PADI Wheel is the best way to calculate multi-level profiles and is much easier to use."

I'd argue in favor of a computer.

"Why re-invent the wheel?"

Actually, multilevel table diving was in practice for many years before the wheel. The wheel is the reinvention.

A few Table Use Questions

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