Decompression Planning

[beeger]

Let’s talk decompression.

Hypothetical dive scenario: We take a boat to the Madeira site. We tie up on the stern, drop down the line, head out past the pilot house, and explore a new piece of wreckage found at 130'. We want to stay there 25 minutes in order to make a sketch and examine it further, or something like that (I'm just making this whole thing up for sake of example). That puts us at a run time of, say, 30 minutes when our trip back home starts. How would you plan this dive with regards to mixes/decompression, and why?

[DRE]

The scenario described below will only work if you're trimix certified, so I encourage people who aren't to post their ways of possibly doing this dive.
Up front I must admit that there is a wide variety of possible scenarios available, dependent upon team size (whther there are support divers, surface supplied O2, etc.). Using support divers for a dive such as this might be a bit exaggerated, but it would be great practice for bigger dives.

With a team of 2 or 3 divers I'd be using TRI 21/35 for backgas. The reasoning behind it is twofold: I consider an END of 130ffw too deep to accurately take measurements or survey. With 21/35 I've got an END of 73ffw (considering the O2 to have the same narcotic effect as N2), which is fine for surveying. Also, the helium makes offgassing far more easier and better for the body compared to using N2, so I know I'll have an added deco advantage. I'd use EAN50 and O2 for deco starting at 70ffw and 20ffw respectively, which would yield me a total bottom time of 53 minutes.
The reason for using 21% O2 in the mix as compared to a higher O2 percentage has to do with two things: first, 21/35 is a standard gas that is used by many deep exploration teams. I believe in uniformity of execution, and it is far easier to match gasses for everyone on the team and mix these gasses over and over, than to use a custom blend for every single dive. Secondly, by keeping my bottom PO2 very low (1.0) I won't run into problems with regards to the CNS clock and could do this same dive 3 times in a day with a 3 hour surface interval without having to change the overall run time of the dive (ie I could run the same profile for each dive). Doing this dive 3 times in a day is maybe not the best idea (mainly because of the repeated submersion in a cold water environment), but at least 2 times shouldn't be a problem whatsoever.

Like I said, this might be a little bit more advanced way to be doing this dive, but there are other safe ways to do this one as well without using helium - so fire away!

[beeger]

DRE - Suppose for some reason you only wanted to take one deco gas. Which would you take: 50% or 100%?

To everyone else: I'm just trying to get a little discussion going on decompression planning for dives that would generally be considered as residing a little bit outside the typical recreational profile. DRE and I are kind of on the same page with this stuff, so I'm looking to get other input. I think his post sets this thread up nicely for a little compare and contrast. Actually, I would be more interested in hearing what people are actually doing and why, rather than use this particular thread to debate it (we could save that for another time). I'm curious what profiles people are using around here - kind of a survey I guess you could say.

Also, on a related question, how are people planning their "technical" dives? Are you using software? If so, which program? Tables? Whose? Are you just wingin' it? Again, this is more of a survey than anything else. I'm curious.

[DRE]

Depends on logistics - O2 is a lot easier to "mix" than EAN50, in terms of actual deco the two seam fairly similar with regards to diving profiles. With the EAN50 you do have the advantage of the 1.6 earlier on in the schedule, after which you can use the gradient to offgas. Also, if for some reason you have to cut deco short (weather, etc.), you probably have a much better chance getting out of the water without a type II hit if you had used the EAN50 - O2 should always be on board anyway.

[davisj]

I have an advanced open water certification, with basic NITROX to 40%. I am in no manner an intructor or a guru. I post this info based upon what I have learned, and would like to have an open dialog.

My first point in planning this dive would be to figure the "best mix" for the dive, assuming that NITROX is the only gas used, and that only one dive is planned.

To mimimize nitrogen exposure, which will reduced decompression obligation and also the effects of nitrogen narcosis, I will want to maximize the oxygen content of my breathing gas, as long as I stay withing the oxygen exposure limits.

For this dive, I have chosen 28% O2 for my best mix. At a depth of 130 ft., my PO2 should be about 1.4. This should give me a single dive limit of about 150 minutes, which should be plenty to complete the dive.

Running this dive plan on my PC shows a 2 min stop at 30ft, 3 min stop at 20. and a final stop at 15 ft fot 25 minutes, for a total dive time of 64 minutes. With a working SAC of 0.70, and a DECO SAC of 0.50, this equates to a gas consumption of 127cuft.... If using the "Rule of Thirds", you couldn't find cylinders large enough to complete this dive. What have I figured wrong ????

Accepting all coments.....

JDavis

[tbrown]

JDavis, I think you need to recheck your figures. I don't have my tables in front of me, but I think your 150 minutes of bottom time might be erroneous. Or else I could have misread your plan, as my eyes are a little bleary. Also I'm not sure that it has been proven that Nitrox will help prevent narcosis, but again, I'm not quite firing on all 8.

[DRE]

"I have an advanced open water certification, with basic NITROX to 40%. I am in no manner an intructor or a guru. I post this info based upon what I have learned, and would like to have an open dialog.

My first point in planning this dive would be to figure the "best mix" for the dive, assuming that NITROX is the only gas used, and that only one dive is planned.

To mimimize nitrogen exposure, which will reduced decompression obligation and also the effects of nitrogen narcosis, I will want to maximize the oxygen content of my breathing gas, as long as I stay withing the oxygen exposure limits."

Thinking that a higher O2 % in your mix will actually reduce the narcosis is old school thinking. Although the reasoning behind it seems correct, the reality is that it doesn't (just try it: mix up some trox and also have a tank of air and see how you feel). Instead of talking about Equivalent Narcotic Depth for a nitrox mix (which I'd reserve for helium based gas mixtures), I'd rather talk about Equivalent Nitrogen Depth as the only benefit is in terms of deco.

"For this dive, I have chosen 28% O2 for my best mix. At a depth of 130 ft., my PO2 should be about 1.4. This should give me a single dive limit of about 150 minutes, which should be plenty to complete the dive.

Running this dive plan on my PC shows a 2 min stop at 30ft, 3 min stop at 20. and a final stop at 15 ft fot 25 minutes, for a total dive time of 64 minutes."

What software are you using. I don't like this profile very much, as it pulls you to close to the surface too soon. I'd prefer to have my first actual stop at around sixty feet, or about halfway from max depth, with 10 second stops starting at 80% of max depth. Your total run time for the dive would be very similar, but the distribution of your stops would be different. The deeper you start your stops the cleaner your deco will be.


"With a working SAC of 0.70, and a DECO SAC of 0.50, this equates to a gas consumption of 127cuft.... If using the "Rule of Thirds", you couldn't find cylinders large enough to complete this dive. What have I figured wrong ????"

You haven't, that's the amount of gas you need, which means you have to carry at least a set of doubles (which is necessary for any real deco dive). Bear in mind that the rule of thirds demands that you keep 1/3 of your gas supply for emergencies, which in this case means the 127cf of gas consists of 2/3 of your total volume.
That's why for dives like this an Advanced Nitrox/Deco Procedures course is very beneficial.

[davisj]

DRE, thanks for your reply. The software is used is Voyager 2000, from DiveRite. The stops can custom set if you like. The default stops are every 10ft feet. The only change I made to the default was to make the last stop at 15ft, instead of 10ft.

In your reply you also indicated that you would make 10second stops starting at 80% of the max dive depth. Would these stop be every 10ft ? And if so, wouldn't slowing your ascent to your first stop accomplish the same thing ?

I ran the profile again, using 1minute stops starting at 60ft, then agian at 50,40,30. The first "stop" was at 20ft, and required only one minute. The last stop at 15ft was for 22 minutes. Total dive time was 63 minutes. The dive time didn't really change, nor did the final gas usage.

Jeff

[davisj]

tbrown. The bottom time for this dive wasn't 150 minutes. The 150 minutes I noted was the maximun O2 exposre time on EAN28 at 130ft. Sorry for the confusion.

Jeff

[DRE]

"DRE, thanks for your reply. The software is used is Voyager 2000, from DiveRite. The stops can custom set if you like. The default stops are every 10ft feet. The only change I made to the default was to make the last stop at 15ft, instead of 10ft."

I would even recommend making your last stop 20ft instead of 15ft, and then to go from 20 ft to the surface in about 3 minutes time. From 20ft to the surface is one of the more critical parts of the deco (although you are "cleared" according to the software) as any residual inert gas will escape in bubble form. If you go too fast, these bubbles will trap and cause a very mild form of DCS, ie post dive fatigue, etc. Going up really slowly makes sure you get rid of these last bubbles.

"In your reply you also indicated that you would make 10second stops starting at 80% of the max dive depth. Would these stop be every 10ft ? And if so, wouldn't slowing your ascent to your first stop accomplish the same thing ?"

The 10 seconds every 10 ft makes sure your ascent isn't too fast. It basically accomplishes just that. Make sure your ascent isn't any faster than 30ft/minute.

"I ran the profile again, using 1minute stops starting at 60ft, then agian at 50,40,30. The first "stop" was at 20ft, and required only one minute. The last stop at 15ft was for 22 minutes. Total dive time was 63 minutes. The dive time didn't really change, nor did the final gas usage."

Actual deco time will not decrease the way I suggested to run the deco, but it will give you a much cleaner, more optimal deco. See if you can play with the gradient factor settings of the software - I still think the profile gives you your stops too shallow, and should instead lengthen some of the deeper stops. Playing with the conservatism of the gradient factors might allow you just to do so. The problem is that most software that's out there runs straight Buhlmann models (as do all computers), which will pull you as close to the surface as possible. However, recent studies have shown that no matter what the dive, bubbles will always be present in nuclei form. Therefore it is very important to start doing deep stops in order to keep these bubbles small. Variable Permeability Models do calculate deco from a bubble instead of a tissue group perspective, but very little software is available to take that into account. It is however possible to play with the conservatism settings of Buhlmann models to approach the VPM models.
In essence good deco means a balancing act between taking advantage of the oxygen window (creating the highest possible gradient, ie Buhlmann model) and to offgas in bubble form (VPM based) without letting the bubbles get too big so they cause DCS.