What if...? Computer Issues

[boulderjohn]

Good point! But no, I was actually taking into account how much time has passed while at depth. If the timing wasn’t exactly 5 minutes since the last time I looked at my computer, I figured I should add a couple minutes (to be on the safe side) and I was considering the time that would pass to get my buddies attention and communicate. I agree an 8 minute safety stop is a bit much for this profile and also agree that 3 minute safety stop would have done just fine. Maybe I am a little too “conservative” in my thinking?

Going back and rereading the original scenario more carefully, I see that when you looked at your buddy's watch, on a strict profile, you have exceeded your NDL (rounding to from 82 to 90) by about 3 1/2 minutes. So , if you are playing exactly by the books, then an 8 minute stop at 15 feet followed by a 6 hour surface interval is correct.

That's a pretty big penalty. Because of that, some people would use their judgment and make a good estimate of the multilevel nature of the dive. With a hard sand bottom, and a wreck to explore, they might say they were only at 82 feet for a padded estimate of time and calculate a two level dive from that. Still others might say that because they were only a at 82 feet for a few minutes, they will round to 80 and calculate from that, in which case--as your computer suggests--you were within NDLs.

Note that I am differentiating between what is the official procedure and how some people would act--not that I am necessarily advocating it.

It might be helpful to understand the reason for the 6 hour surface interval. The PADI tables do not include any method for dealing with dives that exceed the NDLs. Once you exceed the NDLs, it has no way to calculate the next dive unless it is sure you are washed out and essentially starting over again.

I don't know what you know about theoretical compartments, so excuse me if this is a bit too basic for you. Decompression is based on the idea that different tissues absorb and release nitrogen at different rates. Theoretical tissues, or compartments, are used to estimate the amount of on-gassing and off-gassing in the different body tissues. The rate slows down as the nitrogen is absorbed or released, so a half-time concept, like radioactive decay, is used to calculate the rate. Thus a 10 minute compartment will be half way to equilibrium in 10 minutes, 75% there in 10 more minutes, 87.5% of the way there in the next 10, etc. It is said to be washed out (about 98%) in 6 times its designated "half time."

Based on the extensive research they did leading up to the RDP, PADI concluded that the 60 minute compartment should be the one on which normal recreational dives should be calculated. Therefore, if you have saturated the 60 minute compartment in your dive, a 6 hour surface interval (6 times the half time) will wash it out, and you should be safe to start over.

 

[Jewls]

No, but you really need to work on your situational awareness if you are getting that close to NDL's. A conservative diver wouldn't push the edges of NDL's.

The three minute safety stop is just that, a safety stop. It is not a mandatory stop.

100% agree with you about situational awareness and getting close to NDL's. If I remember correctly (either this is in the manual or my instructor told me this), once I reach a NDL of 5 minutes, I should start my ascent either to a shallower depth on a multi-level dive or make a safe ascent to the surface...while of course monitoring my air supply very closely. Would you say 5 minutes is good time to ascend or still cutting it too close?

 

[syntaxerrorsix]

100% agree with you about situational awareness and getting close to NDL's. If I remember correctly (either this is in the manual or my instructor told me this), once I reach a NDL of 5 minutes, I should start my ascent either to a shallower depth on a multi-level dive or make a safe ascent to the surface...while of course monitoring my air supply very closely. Would you say 5 minutes is good time to ascend or still cutting it too close?

That would really depend on the conditions, depth and your plan.

When you planned your dive how much time did you reserve for an emergency such as entanglement at depth?


Maybe that time plus 5 minutes would give you a good base line to start with and as your experience and awareness increases you could adjust fire from there.

Try not to get into the habit of flying the computer without planning your dive. Computers are a great reference tool but it shouldn't be a replacement for dive planning.

 

[Charlie99]

Would you say 5 minutes <of NDL left> is good time to ascend or still cutting it too close?

Depends upon the depth.

I would be much more concerned about being 5 minutes NDL remaining at 60' or 50' than being 5 minutes from NDL at 100' or 120'.

If I drop down to 100' and stay there for 15 minutes, my Oceanic computer will be showing about 5 minutes of NDL. But as I ascend and do a few minutes of stops, the fast compartments that are near their limits will rapidly offgas and I will surface with a large safety margin.

If I drop down to 50-60' range and stay the 75 minutes to 50 minutes that it takes to get down to 5 minutes of NDL, I am very heavily loaded in the medium halftime compartments (such as 30 minute/40 minute/60 minute). I will have to spend 10 minute or more on ascent to get what I consider to be reasonable margin of safety.

The way to really run up your N2 loading for a given amount of gas is to "ride the NDL" back up. In other words, go deep close to NDL, then go up 10 or 20' again until close to NDL. Keep repeating this and you will find yourself in the 50-60' range, close to NDL, with very heavy loading in the medium speed compartments that take a long time to offgas. So unless you have a lots of air and lots of time to hang out in the 20' and shallower region, you will surface with relatively heavy loading and not much of a safety factor. It will also make a big reduction in NDLs for the next dive.

If you have a dive simulator, you might plug in the profile below and see how you never get past NDL, but have very heavy loading:
8 minutes at 120'
5 at 100'
5 at 80'
7 at 70'
10 at 60'

The total bottom time at that point will be only 35 minutes, but you will be approaching NDL at 60'.

 

[rhwestfall]

I wish i had my Wheel with me to quiclky look at this..... darn job!!!!!!

 

[boulderjohn]

I wish i had my Wheel with me to quiclky look at this..... darn job!!!!!!

And I sure wish I could see you do it. It would make the second time in my life I have seen a wheel used outside of a classroom. Good to know they were not all wasted!

 

[Charlie99]

I wish i had my Wheel with me to quiclky look at this..... darn job!!!!!!

When you get back to your wheel, you can do another interesting exercise.

Do the same profile as a series of mini-dives with zero SI on the regular RDP. You will get the same results. All the wheel does differently is to add the the multi-level NDLs, add a bunch of rules about what are legal combinations of multilevels, and replaces the multiple rows of the SI credit table with just the one line.

The PADI NDL is 55 minutes at 60', but the MDL is only 46 minutes. The profile I described would exceed the MDL, and also would violate the rules of the PADI wheel because the next valid level shallower than 120' is 80'. And the next valid level above 80' is 60'. A dive computer that runs the DSAT model, such as most Oceanics, would be happy with the profile but would show low NDL. And it would seemingly take forever if you were to try and get the computer back into the green zone during your ascent and stops.

I just threw the profile above quickly together looking at a standard RDP.

If you want, you can do the wheel calculations on my flat table equivalent



Full size: ScubaBoard Gallery - "Flattened Wheel"

 

[Charlie99]

And I sure wish I could see you do it. It would make the second time in my life I have seen a wheel used outside of a classroom. Good to know they were not all wasted!

As a mental exercise I built a flat table equivalent to the PADI wheel after I had reverse engineered the dissolved ppN2 levels of the pressure groups.

After verifying that had exactly the same results as the wheel, I put both the wheel and my flat table equivalent back into the drawer and haven't taken them out since.

I find Table 2 on the table version of the PADI RDP with 10' steps of depth has sufficient resolution for doing the pre-planning of multilevel dives. Or more accurately, doing a sanity check on my plans, which are usually just clones of 3 or 4 fairly standard multilevel profiles you will find being used on dive boats everywhere.

 

[boulderjohn]

As a mental exercise I built a flat table equivalent to the PADI wheel after I had reverse engineered the dissolved ppN2 levels of the pressure groups.

If it is true that mental exercise wards off Alzheimer's, you have nothing to worry about.

 

[Jewls]

Well..... now I'm lost.

How do you guys know all this stuff??? It sure isn't in any of my dive manuals....(OW and AOW). Is this something you learn as dive master, instructor, or technical training?