Deep Dive Strategy

[mccabejc]

I was futzing around with my Cobra simulator on the PC, and simulating dives with max depth of around 90 ft. I've never gone deeper than 60ft., so I'm trying to learn how to plan for a deep dive.

Anyway, it seems to me that if you're going down that deep, your plan at the outset has to be something like "we'll hang around at 90 ft. no longer than 5 minutes or so and then start back up" if you don't want to make it a decompression dive.

Also, when your max depth is 90 ft., it looks like you need to ascend to about 40ft. before you start adding/recovering NDL time.

In contrast, when your max. depth is only 60ft., it looks like you begin adding/recovering NDL time as soon as you start ascending above 60ft.

This simulator gizmo is cool....

 

[cdtgray]

I've never gone deeper than 60ft., so I'm trying to learn how to plan for a deep dive.

Please, take a class. A class is the only good way to learn how to properly plan for deep dives.

 

[Dive-aholic]

Simulators are cool, but there's nothing like planning out your dives using the wheel. You know exactly what you're doing and why. We use the simulator as a way to backup our calcs. We agree--take the class, and make sure it's from a good instructor who is willing to go over those things with you. We took a deep class, but didn't learn about SAC rates until we purchased the deep diving book ourselves for our own information after completing the dives and reading about them there.

 

[lostinspace]

In contrast, when your max. depth is only 60ft., it looks like you begin adding/recovering NDL time as soon as you start ascending above 60ft.

right idea, mis-placed conclusion - you are probably adding NDL time because of the length of time you were down at 60ft. as others have said, learn to plan using tables first, then play with the gizmo once you have grasped the concepts.

 

[mccabejc]

Does anyone understand the concepts and equations well enough to give me a short synopsis? No disrespect to PADI or other classes, but my experience with my OW class was that they are long on procedure (find your depth on this column in the tables, then move across 'til you find the time...), and short on really explaining the underlying concepts. Maybe everyone who dives deep does make a point to first sit down with their tables or wheels and plan, but I kinda doubt it. My guess is we tend to do things by simple rules of thumb whenever possible. But maybe I'm way off base on this...

I agree about the class, and will not be diving deeper than 60ft. until I do take a deep dive class, but this is just out of curiosity.

 

[Charlie99]

Does anyone understand the concepts and equations well enough to give me a short synopsis? No disrespect to PADI or other classes, but my experience with my OW class was that they are long on procedure (find your depth on this column in the tables, then move across 'til you find the time...), and short on really explaining the underlying concepts.

While there have been lots of advances in decompression theory since JS Haldane's work early last century, if you have a good understanding of the basic multicompartment dissolved gas model you will have a pretty good understanding of decompression.

A short, but very informative paper is Erik Baker's M-values Article.. He also has a good paper on deep stops.

To better understand what is going on in those dives you are simulating, you need to look at the various compartments, both slow and fast. The 90' NDL is limited by a relatively fast compartment. The 60' dive by a slower one. When looking at the Cobra simulator on your PC, pay attention to the % of max value in each of the compartments. That is generally more informative than just the raw NDL number.

 

[Green_Manelishi]

your plan at the outset has to be something like

Your plan has to be "am I going to (can I?) carry enough gas to actually do the dive I am planning and what if my time/depth exceeds my plan?"

It's from that starting point that you proceed. "Staged" deco (or not) is merely the price you pay for the dive you planned.

 

[mccabejc]

Excellent !!! Nothing warms the heart of engineers like me more than tech papers with equations and stuff. Damn, it's got graphs, too. Excuse me, I think I'm gonna cry...

Thanks.

 

[-hh]

Does anyone understand the concepts and equations well enough to give me a short synopsis?

The classical model is that the human body is modeled as series of 'compartments' that fill (and empty) at different rates. Further, each compartment has maximum saturation value that it can tolerate back at the surface, and the rule of thumb is that the faster the compartment, the higher this value is.

Okay, let's take a super-KISSed example:

I'm going to invent a 2 compartment model. We'll say that Compartment F (for Fast) has a 5 minute halftime and Compartment S (for Slow) has as 60 minute halftime. I'll show how halftimes work in a minute.

Next, we're going to assume all of our ascents and decents are instant. Its not realistic, but it makes this all a lot simpler to understand.

Finally, we're going to assume that the Maximum value that F and S can have without you getting bent back at the surface are F=70 and S=20. These maximum values for each compartment are called "M-values", and are often fudged by a dive computer manufacturer to add conservatism (safety margin). Exceeding one of these M-values during a dive isn't fatal - - it just means we've gone over the no-deco limit and into decompression.

Okay, at the beginning of the dive, time=0, depth=0, F=0 and S=0.

We jump in and immediately teleport to 90fsw (d=90).

After 5 minutes, Compartment F has seen one full "halftime" (because time=halftim). This means that it has filled halfway between where it was and the value at which it would reach a new equilibrium. I'm assuming these M-value numbers are equivalent to depth in feet, so here, it goes to halfway between 0 and 90, so its value is (5min/5min)*(90-0)/2 = 45.

Meantime, Compartment S has seen 5/60ths of its halftime, so it will have gone 1/12th of halfway: that would be (5min/60min)*(90-0)/2 = 4

Summary: t=5, d=90, F=45, S=4.

We stay another 5 minutes. So we take our current Compartment loadings (45 and 4) and add on their additional halftime loadings.

EDIT: I really did this above too, but since our starting values were zero, I was lazy and didn't bother to show it in the math.

F = 45 + (5/5)*(90 - 45)/2 = 67.5
S = 4 + (5/60)*((90 - 4)/2) = 8

Summary: t=10, d=90, F=67.5, S=8.

The math looks a bit yucky, but its simple: you're taking the current balance and adding to it the appropriate halftime fraction...FWIW, it might be easier to understand it if I rewrite the formula this way:

Compartment = PriorBal + (TimeSpent/CompartmentTimeDefinition) * (depth - PriorBal)/2 = NewBalance

...the division by2 in this formula is because it works in halftimes. If you had nuclear physics in school, you'll remember this concept as "half-life" for atomic decay.

Okay, at this point, our F is getting close to its M-value of 70, so we're probably down to just a few more minutes before we're into Deco...so we decide its time to ascend.

Ascending to depth 50ft: t=10, d=50, F=67.5, S=8.

We're now going to spend 5 minutes here.
F = 67.5 + (5/5)*(50 - 67.5)/2 = 58.75 (lower!)
S = 8 + (5/60)*((50 - 8)/2) = 10 (higher)

Summary: t=15, d=50, F=59, S=10.

What happened here is that because F > d, this meant that the Fast compartment started to offgas, and because S < d, the Slow compartment continued to load.

And since our dive computer will calculate "time remaining" based on all of this, it will tell you that you have more bottom time...at least for now. This is because what's called the "Controlling Compartment" (for your no-stop time) changed. Here, we had little choice: it went from F to S.

Lets skip ahead and assume we've run these numbers for around ten more 5 minute chunks and that we're now at:

t=65, d=50, F=~50, S=~18 ...and now the slow compartment says we're out of no-deco time, so its time to ascend to the surface.

We first have to see if its safe to come to the surface. Since we kept things no-deco, it should be, but we'll check anyway:

F= ~50 (below its M-value of 70: safe to surface), S= ~18 (below its M-value of 20: safe to surface).

Okay, we're now at the surface. When we arrive, we're at: t=65, d=0, F=50, S=18.

During our surface interval, if our model was perfectly symmetrical, then the same rules would apply for our off-gassing. Let's assume it is and give our SI its first 5 minutes:

t=70, d=0 and:

F = 50 + (5/5)(0 - 50)/2 = 25 (that dropped a lot)
S = 10 + (5/60)(0 - 10)/2 = 9.6 (that dropped only a little).

And so on. If you followed the basic concept here, then you're ready to dip underwater to see the rest of the iceburg of decocompression modeling.


-hh

 

[Charlie99]

As you transition from the excellent KISS tutorial above to the real numbers in the Erik Baker article, a few tips to help you along:

1. fsw is absolute pressure.

2. Don't forget to apply the fraction of N2 in your breathing gas when figuring out your inspired ppN2. For example, at 60', you are at 60+33=93 feet absolute.
93* 79% N2 = 73fsw for the ppN2 of air at 60' depth.

3. When you start your first dive, your compartments are already at sea level saturation. (0+33)'* 79%= 26fsw. Actually, since your lungs have about 1.6-2fsw of water vapor pressure in them, more accurate calculations subtract that amount from the depth before continuing the calculations. In practice, this doesn't make much of a difference other than for very shallow dives.

While in hindsight, the above points are obvious, on my first attempt to read and understand Erik Baker's articles, I ignored points #1 and 2 and absolutely nothing made sense.