A few Table Use Questions

[Rick Murchison]

Walter once bubbled...
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?

SSI does not teach or advocate multilevel dive computations using the Navy Tables.
In my original YMCA course we did learn to do decompression dives, but all table use assumed a square profile... All the multilevel stuff we used to do was developed outside any agency, and had nothing to back it up other than it worked the way we did it. We used both the "common group" and the "zero surface interval" methods (our terminology), whichever allowed the most bottom time.
Rick

 

[Walter]

YMCA has never included multilevel table use, but now it specifically prohibits it.

 

[Doof]

QKRTHNU once bubbled...
#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?

Here's my stab at this, in laymans terms (since I'm a layman - and trying not to get into half-times).

When designing the tables, they use theoretical tissue compartments. Each compartment is capable of being saturated by nitrogen, but each does so at a different rate. Each also has a minimum pressure at which it can become saturated. So for instance, a pressure group that can only become saturated at a pressure equivalent to 100 fsw will not become saturated when you're only in 60 fsw. But when you are at 100fsw or deeper, it can become saturated. And at that pressure, it absorbs nitrogen much faster than the compartments that can become saturated at shallower depths (I'm going to just start referring to depth now instead of pressure).

The greater the pressure required for saturation, the faster it can become saturated (but only once it's at the required depth).

The less pressure required for saturation, the slower that particular compartment becomes saturated.

So on a 30 foot dive, it's the "30-foot compartment" that controls the dive, because it's the fastest compartment that is able to become saturated at that depth. At 100 feet, it's a different compartment. While the 30 foot compartment is capable of becoming saturated at 100 feet, it still takes the same amount of time for it to become saturated at 100 feet as it does at 30. So for the 100 foot dive, a different compartment is "in control".

At least, that's how I understand it. Like I said, it's a real non-technical explanation, but I believe it to be accurate. If not, I'm sure somebody will straighten me out. I sure hope I'm right though, because this was a really difficult thing for me to grasp and I sure hope I don't have to do it again. ut:

 

[MikeFerrara]

Doof once bubbled...
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.

They're not hard to use. If you can't tell if the arrow is touching or percing then the difference is a minute or a presure group. The error isn't of any real consequence when diving. However, they're no fun to take a test on because of this.

 

[Walter]

"I sure hope I don't have to do it again."

........you need to do it again.

"Each compartment is capable of being saturated by nitrogen, but each does so at a different rate."

Correct.

"Each also has a minimum pressure at which it can become saturated."

Incorrect. All compartments become saturated at pressure after 6 increments of their respective half-times. For example, I am completed saturated right now at 1 ATMA.

Also drop "tissue" from your compartments unless you are speaking in historical terms.

 

[Doof]

I was afraid of this.


Ok. I'm wondering if I'm using the term "saturated" incorrectly in this instance.

By saturated, I mean that, for a given compartment, it has aborbed as much nitrogen as it can and still allow a safe ascent to the surface without bubble formation. Or should I say, dangerous bubble formation.

So at 100 fsw, by the time you've stayed for 20 minutes (or whatever the NDL is - don't have my tables handy), the controlling compartment is as saturated as it can get and still allow you a direct ascent without the formation of bubbles.

Is that closer?

If so... is it true that the compartment that would control a 130' dive (in the example above) is not under enough pressure to ever become "saturated" in the way that I've defined it above? Even if I stayed for an hour at 100'?

My terminology is really bad - I realize that's at least part of my problem. I just hate using terms like M-Value, because it doesn't really have any meaning in and of itself.

 

[MikeFerrara]

Doof once bubbled...
I was afraid of this.


Ok. I'm wondering if I'm using the term "saturated" incorrectly in this instance.

By saturated, I mean that, for a given compartment, it has aborbed as much nitrogen as it can and still allow a safe ascent to the surface without bubble formation. Or should I say, dangerous bubble formation.

So at 100 fsw, by the time you've stayed for 20 minutes (or whatever the NDL is - don't have my tables handy), the controlling compartment is as saturated as it can get and still allow you a direct ascent without the formation of bubbles.

Is that closer?

That compartment is said to be at it's "M" value. The most N2 load that in theory still allows a direct ascent to the surface.

If so... is it true that the compartment that would control a 130' dive (in the example above) is not under enough pressure to ever become "saturated" in the way that I've defined it above? Even if I stayed for an hour at 100'?

My terminology is really bad - I realize that's at least part of my problem. I just hate using terms like M-Value, because it doesn't really have any meaning in and of itself.

Sort of...but ...the controling compartment is determined by both depth and time. In an extreme case you are correct. For instance at 30 ft I think you could stay there fore ever and not be able to drive the fastest compartments to their M value because the ambient presure just isn't great enough. But at any depth youhave the potential to drive any compartment to it's M value if...the M value is less than the the ambient presure. Which compartment is leading is going to depend on how long you are at that depth. Don't let the term M value through you ist's just a presure gradient diven in units of presure.

 

[QKRTHNU]

Thanks guys. I forgot about the compartment factor.
I'm still somewhat confused though.

Sounds like the quicker compartments are affected by pressure more than the slower compartments. In that I mean it sounds like they have a greater increase in gas transfer with increased pressure whereas the slower compartments don't change as much.

That's the only way I can make sense of the tables. Otherwise if all compartments increased absorption rate in a linear fashion you would be in the same pressure group regardless of the depth when NDL was reached.

Does this sound right?

 

[chrispete]

hrm... I may be off base here as I am still in the very early stages of learning deco theory, and please feel free to set me straight here, but I am under the impression that any compartment could become saturated at any pressure (even 1 foot underwater,) given enough time to do so. Take into account a 30 foot dive to saturation, in this case it wouldn't be the fact that you are saturated at such a depth that would control bubble formation on surfacing, but the lack of a large enough pressure gradient between 30' and the surface to cause substantial bubble formation...

Again, maybe I'm misunderstanding a basic concept here...

 

[Diver0001]

QKRTHNU once bubbled...
Thanks guys. I forgot about the compartment factor.
I'm still somewhat confused though.

Sounds like the quicker compartments are affected by pressure more than the slower compartments. In that I mean it sounds like they have a greater increase in gas transfer with increased pressure whereas the slower compartments don't change as much.

That's the only way I can make sense of the tables. Otherwise if all compartments increased absorption rate in a linear fashion you would be in the same pressure group regardless of the depth when NDL was reached.

Does this sound right?

Affected more? If you mean that the more pressure you put them under the more gas they will absorb in same length of time, this is true.

However, the length of time it takes to saturate at a constant depth doesn't change. Saturation of the compartments doesn't work in a linear fashion. They work with 1/2 times. I think someone already explained this but a 1/2 time is the length of time it takes for a compartment to go from it's current state 1/2 way to saturated.

Concrete example:

Say you have a tissue compartment with a 2 min 1/2 time and you decend to a constant depth.

After 2 min it's 50% full
After 4 min it's 75% full (50% + 25%)
After 6 min it's 87.5% full (50% + 25% + 12.5%)
After 8 min it's 93.75% full (50% + 25% + 12.5% + 6.25%)
After 10 min it's 96.875% full (50% + 25% + 12.5% + 6.25% + 3.125%)
And finally after 12 min it's 98.4375% full (50% + 25% + 12.5% + 6.25% + 3.125% + 1.5625%)

Maybe stick those numbers in a graph in a spreadsheet and you'll see what the statruation/offgassing graph looks like assuming your depth remains constant.

In terms of the model you can keep going like this forever but the difference becomes insignificant. Therefore after 6 iterations the compartment is considered saturated.

IIRC the PADI RDP uses 14 compartment model with 1/2 times varying from 5 min to 480 min.

That's the bit you seem to understand.

So getting to the thing about pressure groups on the RDP. The RDP was designed with the idea in mind that you would directly descend to the deepest depth you want to reach and at the end of your dive you ascend directly to the surface. Based on that you can pretty easily map what happens in terms of gas-loading in each of your 14 compartments at each depth & time on the table and behind the scenes you can see how high the gas loading actually got in all the compartments based on the depth and total bottom time.

After surfacing (we're almost there now) the group designation is just a mapping of the residual gas loading in the most highly loaded of your 14 compartments (called the controlling compartment) to a letter of the alphabet. The deeper/longer you dive, the more nitrogen you absorb in a given length of time, the higher the residual gas loading after the dive and therefore the higher the group designation.

R..