not bent yet?

[SailNaked]

My Sac is such that it has become easy for me to exceed the NDL of either a computer or certainly tables.

I have been thinking about how to phrase this question and not sure I have it yet but will let the SBI make the decision.

I learned tables when dive computes were either new or non existent

we were taught that we could use our max depth and total time and that to dive any other way was risking being bent. multilevel diving was not even mentioned.

well I recently had a dive to 130ft and about 50-60 minutes, by the tables I should be very dead. by my computer it was not even a deco dive. we did 130 for about 5 minutes (trying to catch a lion fish in a plastic bag). and then we went up to about 80 or so for 10 minutes then up to about 50 for maybe another 15 then up to about 35 for about 20-25 min till we had to do our SS.

when I downloaded the profile to look at. it it seems I was within about 1-3 minutes of NDL for most of the dive until we went above about 40 ft. then we had many hours of time.

I understand that the tables were designed originally for the Navy and that they do square profiles and that multilevel diving has more freedom.

What I am really trying to understand is how did we make the transition from using tables the way we did and how we now dive and use computers to wiggle along the NDL line. and why do we suddenly get released from any obligation at about 40 ft? (as a rec diver only).

I think if I actually exceeded a limit, it would have still cleared at 40 or maybe 30 ft. recently we did dives that were deepish (80-90) until we hit 1500# (25-30min 32%) and then we would come up to 25-30ft and use the rest of the tank and we never got close to any NDLs even though on the tables they were 90 ft dives for 60 minutes.

This post is to understand the mechanics of very slow ascents from recreational profiles. I am not sure what I do not understand yet, but it seems to me something is going on here that I definitely do not understand and is not what I was taught. Obviously this is only useful in reef diving where there is something to see and do both deep and shallow. not really a wreck diving scenario.

 

[Bossman]

The tables assume you spent your entire time at max depth (and using air?). So the tables assume you went to 130 feet for 60 minutes. Theoretically you could calculate your time spent at each depth with no surface interval to understand accumulated nitrogen loading at the various levels. That would be a difficult task.

Your computer models your nitrogen loading based on the actual time spent at each level. Some computers sample as often as every few seconds.

 

[RJP]

This post is to understand the mechanics of very slow ascents from recreational profiles. I am not sure what I do not understand yet, but it seems to me something is going on here that I definitely do not understand and is not what I was taught.

Beyond what Bossman points out about actual profile vs square profile, it seems like it's probably that we better understand offgassing, and where/when that takes place and the various compartments, etc.

Do you have V-Planner? Download the shareware version and play with some multilevel dive profiles. The program will tell you when offgassing starts. What you'll see is that on the shallow end of the dive you are OFFGASSING the N₂ from the deep part of the dive faster than you are LOADING N₂ on the shallow part of the dive, which is why the computer starts giving you back more time shallow, even if you were getting closer and closer to NDL while you were deeper.

The shallow level of the dive is essentially a "deco stop" from the deep part of the dive:

As an example, here we see that a dive to 100ft for 30min on 36% puts you over NDL and gives you a mandatory deco stop at 20ft. It's only 2:20, but it's mandatory deco...

Dec to 100ft (2) Nitrox 36 50ft/min descent.
Level 100ft 28:00 (30) Nitrox 36 1.42 ppO2, 75ft ead
Asc to 20ft (32) Nitrox 36 -30ft/min ascent.
Stop at 20ft 2:20 (35) Nitrox 36 0.57 ppO2, 10ft ead
Surface (35) Nitrox 36 -30ft/min ascent.

Do the same dive, but add another 20min at 40 ft and you end up with NO deco time.

Dec to 100ft (2) Nitrox 36 50ft/min descent.
Level 100ft 28:00 (30) Nitrox 36 1.42 ppO2, 75ft ead
Asc to 40ft (32) Nitrox 36 -30ft/min ascent.
Level 40ft 20:00 (52) Nitrox 36 0.78 ppO2, 26ft ead
Asc to 20ft (52) Nitrox 36 -30ft/min ascent.
Surface (53) Nitrox 36 -30ft/min ascent.

How does a LONGER dive - with the same amount of time spent at 100ft - end up avoiding deco? Well, according to V-Planner, offgassing for the 100ft portion of the dive starts at 51.2ft. So anytime spent at 40ft is actual netting you less and less N₂ load as each minute goes by. Up until a point of course, but that point is well beyond your gas supply

Obviously this is only useful in reef diving where there is something to see and do both deep and shallow. Not really a wreck diving scenario.

PS - You need to do a better job of selecting your wrecks...

:cool2:

Fujikawa Maru
Depth
Superstructure 30 ft. 9 m.
Deck 60 ft. 18 m.
Bottom 112 ft. 34 m

Heian Maru
Depth
Hull 40 ft. 12 m.
Bottom 120 ft. 36 m.

Kansho Maru
Depth
Superstructure 60 ft. 18 m.
Deck 80 ft. 24 m.
Bottom 130 ft. 35m.

Shinkoku Maru
Depth
Superstructure 40 ft. 12 m.
Deck 65 ft. 20 m.
Bottom 125 ft. 38 m.

 

[jpbutler]

How does a LONGER dive - with the same amount of time spent at 100ft - end up avoiding deco? Well, according to V-Planner, offgassing for the 100ft portion of the dive starts at 51.2ft. So anytime spent at 40ft is actual netting you less and less N2 load as each minute goes by. Up until a point of course, but that point is well beyond your gas supply

Not trying to start anything. I think that your description is great, but not all tissue compartments will be off gassing at 51.2 ft, only the leading compartment(s). Slower tissue compartments are still on gassing nitrogen. Their rate of on gassing will be slow enough on a dive as described by the op such that when surfacing they will not be an issue.

JPB

 

[RJP]

Not trying to start anything. I think that your description is great, but not all tissue compartments will be off gassing at 51.2 ft, only the leading compartment(s). Slower tissue compartments are still on gassing nitrogen. Their rate of on gassing will be slow enough on a dive as described by the op such that when surfacing they will not be an issue.

JPB

Right, but given that the compartment model is theoretical anyway it's close enough. Which is why people get bent when their computers say they are fine, or end up fine even when their computer says they're bent.

 

[TSandM]

The tables were designed for dive planning. There isn't a lot you can know about the dive you are going to do, before you do it -- but you can certainly know your maximum depth, and that will give you a maximum time before you exceed the nitrogen absorption that allows you to proceed directly to the surface.

Dive computers, on the other hand, are real-time calculators, which are keeping track of precisely where you have been and how long you have been there. Instead of one, all-inclusive determination for the dive, they are doing constantly iterative calculations of nitrogen saturation in a number of theoretical compartments, and comparing each to the maximum nitrogen tension allowed before you can no longer surface.


Each of the theoretical compartments has a maximum acceptable nitrogen tension for surfacing, known as an M-value. If, at a given depth, a saturated compartment doesn't exceed the M-value, then in theory, your no-decompression time for that compartment at that depth is unlimited. This is true for most of the medium and slow compartments. For the fast compartments, if you exceed the M-value at a depth, but the allowable ascent rate will permit clearance of the excess, your time is still theoretically unlimited.

If this stuff interests you, I would highly recommend Googling Erik Baker's paper, "Understanding M-values", or purchasing Mark Powell's book, Deco for Divers.

 

[SailNaked]

Thanks yall, excellent responses to a not so good question. I have the full version of vplanner and never thought to play with it like that.

RJP let me know when you want to go to Truuk, be very happy to dive those as soon as I get my full 3X training.

 

[SailNaked]

Hmm you went to Truuk without me :depressed:

so what is the calculation that is used to determine off gassing depth? what is the theory behind it?

 

[Mr Carcharodon]

Offgassing starts when the pressure in a given tissue is greater than ambient pressure (you already knew that!). So you need to know what the ambinet pressure is and that part is easy too. Then you need to know what tissure pressures are. For a constant depth you use the Haldane equation which is: P = Po + (Pi - Po)(1 - 2^(-t/half-time)), where Po is the starting tissue pressure, and Pi is the inspired partial pressure of inert gas. And you have to do this for all the compartments you are considering, typically 8 to 16. For ascents or decents at constant rate you also need Schreiner's equation: P = Pio + R(t - 1/k) - [Pio - Po - (R/k)]e^-kt which is almost the same as Haldane's. R is the decent rate, and k is ln(2)/half time. So the math is really not too hard but it gets to be a lot to keep track of for a multilevel dive. You just add together the pressures for each segment of the dive.

Then you compare the tissue pressure to the "M" value which is maximum allowed pressure gradient. In this case the gradient is just the difference between ambinet pressure and the tissue pressure. M-M0+deltaM*depth so it is really simple too. For a NDL dive you make sure that your M's are at or above the surface. It really comes down to a bookeeping exercise. A computer program like v-planner helps. Although v-planner is running a bubble model which book keeps volume and limits ascents based on volume rather than pressure.

 

[SailNaked]

So it was RJP's idea and I have been having fun.

I took a dive to 130 for 40min (28%). the total calculated dive time was 110 minutes.

then i added a stop at 80 for 10 minutes (40%) and a stop at 40 for 10 min (50%) and then one at 30 for 5, and 20 for 5 and got the total dive time down to 82 minutes. but still a 7:40 deco obligation on O2. at one point I had the deco down to :40 and i am still playing with it to minimize both the deco obligation and the total dive time.

It will take me some re-reads to understand what was posted above. I am a little slow that way. I have to see what is behind the formulas I can not just absorb them as is.