Dive planning software to give NDL?

[stuartv]

That, this is why I recommend that all divers at least take a basic decompression course *no need for accelerated*, planning around NDL's is such a pain the butt. If your recommended 3 minute safety stop becomes a mandatory 5 minute decompression stop, who cares?

I understand. And I have already completed the book and classroom part of Deco Procedures. And read Deco for Divers. And I got BRW's book, Basic Decompression Theory and Application, which I have barely skimmed so far.

What you said is also why said what would be REALLY nice is if I could enter a Max Deco Time (whether it's 0 or some other number) to let the software tell me the maximum length of the last (or only, if there is only one) leg.

E.g.: I have an AL40 full of 100% for deco. I don't want my deco obligation to have me using more than half of that. I look at my SAC rate and determine that that means I can stay on my deco bottle for 25 minutes. So, I want to plug into the software that I want a Max of 25 minutes deco. Now tell me how long can I stay down on my last leg.

Or I could want 0 deco, so I set the Max Deco limit to that and it tells me how long I can stay with no deco (i.e. my NDL).

I realize the first example is not perfect because, if I have 25 minutes on 100% then I probably actually have some other deco to do at a lower depth. But, I think this kind of still makes my point.

Another way of putting it is: I have some hard limits when I'm doing my dive planning. The amount of back gas I'm going to carry. I'm not going to go out and buy bigger tanks if my dive plan says my current tanks aren't big enough. The amount of deco gas. Likewise, my deco bottle is what it is. Potentially, I might have a hard limit on total deco time I want to do. Or a hard limit on total dive time. It would be nice if there were some planning software that let me enter those hard limits and then it would tell me how long of a dive I can do. As it is, all the software I've tried or heard of requires me to put in a depth and time for every leg of my dive and then it tells me my ascent profile. Then I have to check that against all my different limits and go back and adjust the dive profile and recalculate over and over until the result fits within all my limits.

 

[tbone1004]

25 mins of O2 deco is a long bloody dive....

You said earlier double 120's and 40m depth. Rock bottom for that is 1200psi. In the 120's that gives you 160cf to play with, assuming total time at depth and SAC of 0.6, that gives you 18 minutes of O2 deco running EAN30 at GF 30/85. That is assuming that your average depth prior to ascent is 40m/130ft. Rather unrealistic. 45 minutes/41cf of backgas required for full decompression, so that limits your bottom time even more, because 41+68*rock bottom gas requirement*=110, so your bottom time is going to be limited by your tanks before it is going to be limited by decompression. I'm sure someone could program it calculate total decompression restrictions, but the Petrel is quite nice about giving you TTS and +5 TTS so you can see your deco racking up. Not quite as useful in a cave due to the swim out, but in the ocean it's nice because it factors in your total ascent time.

I like Pastodeco, it's free for the basic version, which is all you need. Sure you can't tell it to set maximum decompression times, but you plan the dive you want to do, and then with it telling you your deco requirements, you bring the right amount of gas.

 

[Dhboner]

In a way you are going about this backwards. You know your total gas supply, that is a finite number...you know your target e.g. wreck, cave penetration etc. so armed with that information, you plug the numbers into your planning software and end up with the tables for the dive (you can also cut contingency tables for gas loss situations). You also have to bear in mind any restrictions given your team members (gas supply, experience level, SAC rate). You can play around with the numbers in your plan to see how increases or decreases in bottom time affect deco.

 

[tbone1004]

What he said. Not to beat a dead horse, but this is basically how my OW planning goes.

I'm taking these tanks, how long can I spend at the bottom and still make it back without accelerated deco. With singles, the answer is usually not all that long. With doubles, the answer is a reasonable amount of time. 15-20 mins of accelerated deco is about all you're going to get on backgas, which is perfectly reasonable.

Edit:Oh, and to add insult to injury, I always multiply rock bottom by 1.1 to add in descent gas required, so that removes that total amount from the system, and rock bottom is obviously based on NDL, BUT, is good because if you're doing dives with planned backgas decompression of over 5 minutes, you should probably have O2 in the water anyway, and in that case, you can calculate backgas deco for one diver, and just get the OOA guy back to his deco bottle, OR you can bring a staged buddy bottle, and clip it off at the bottom of the anchor line if you're on a boat, and then you don't have to do rock bottom, just 5 minutes at the bottom to get the OOA diver to wake the hell back up, get him to the buddy bottle, and then he's on his own. Pros and cons of each system, but the point still stands that you are unlikely to rack up more than about 15 MAYBE 20 minutes of accelerated deco in the ocean for quite a while. 5-10 minutes is usually sufficient for most people.

 

[stuartv]

25 mins of O2 deco is a long bloody dive....

You said earlier double 120's and 40m depth. Rock bottom for that is 1200psi. In the 120's that gives you 160cf to play with, assuming total time at depth and SAC of 0.6, that gives you 18 minutes of O2 deco running EAN30 at GF 30/85. That is assuming that your average depth prior to ascent is 40m/130ft. Rather unrealistic. 45 minutes/41cf of backgas required for full decompression, so that limits your bottom time even more, because 41+68*rock bottom gas requirement*=110, so your bottom time is going to be limited by your tanks before it is going to be limited by decompression. I'm sure someone could program it calculate total decompression restrictions, but the Petrel is quite nice about giving you TTS and +5 TTS so you can see your deco racking up. Not quite as useful in a cave due to the swim out, but in the ocean it's nice because it factors in your total ascent time.

I like Pastodeco, it's free for the basic version, which is all you need. Sure you can't tell it to set maximum decompression times, but you plan the dive you want to do, and then with it telling you your deco requirements, you bring the right amount of gas.

Thank you. I get all that. Even using all that, it seems like entering my total available back gas (I.e. a different hard limit than the specific example I gave previously) and having it tell me the max time I can stay at my current depth would be nicer than having to enter a time and then tweak it back and forth until the resulting numbers fit all the various limits.

Second, I am working through your example and verifying that I can understand and derive the same numbers you came up with.

Where you mention 41 cu-ft + 68 (rock bottom requirement), am I correct in understanding that that is, essentially, saying I need to begin my ascent with that much back gas left to ensure that I have enough gas to, one, get my buddy and I both to the first deco stop (the rock bottom requirement) plus enough gas to do my own full deco on back gas? Is that really how you would normally plan things? Isn't the implication there that you're planning for having two major things go wrong - i.e. your buddy went OOA AND you lost your deco gas?

I'm taking the class now and have not yet done the "practicals" if you will. I am totally still working on getting it all sorted out in my head. But, off the top of my head, I would have expected to have my plan accommodate losing one of the 4 gases (my back gas, my deco gas, my buddy's back gas, or my buddy's deco gas), but not 2.

So, in the example, with a RB of 68 cu-ft and a requirement for 41 cu-ft for deco, I would think my turn point number would be 34 (half the RB) plus 41 = 75 cu-ft (translated to psi). As long as we only lose 1 gas supply at that point, I'll either have 34 cu-ft to ascend and then 41 to deco on (assuming I lost my deco gas), or I'll have 68 for my buddy and I to ascend on and we'll both deco on our own deco gases.

And, actually, what my instructor taught us is to plan the dive so that we need less than half our deco gas, so that a lost deco gas scenario just means sharing our buddy's deco gas - i.e. don't have to actually increase back gas allowances for a lost deco gas scenario. So, in the above example, the TP would be 68 cu-ft. If one of us went OOA and one of us lost all deco gas, the 68 cu-ft would get us to the first deco stop and we would buddy breathe the one deco bottle. Obviously, not ideal, but that's the scenario only if we totally lose one back gas and also totally lose one deco gas.

 

[tbone1004]

so yes, that example is two things going wrong, but they are going wrong independently. If you're truly team diving, one buddy has to be able to deco on backgas unless you plan on buddy breathing for 20 minutes on oxygen. Your buddy is OOA and it is priority that he completes decompression first on O2, then you can finish on the remaining O2 you have left. Keep in mind, even with the very conservative gas planning I gave, with RB+full decompression on backgas, you have almost 45 minutes of bottom time at 130ft, that is a long bloody time.

What is more realistic for me, since I don't actually ever dive with "buddies" in terms of gas planning in the ocean, is I calculate my rock bottom based on 1cfm SAC rate, and then I calculate my full deco requirement on backgas at GF70, and then if something hits the fan with my buddy, I'll tell the Petrel that I now want to dive at GF85, and since my buddies don't panic all that much, it doesn't leave much room, but enough to where it isn't much of a problem, and the odds of failure at the very last minute in the ocean with that aggressive of a profile is not something I would personally worry about. Point is I still have 45 minutes of bottom time at 130ft in HP120's, without having to carry a stage bottle. That is more than enough for me because I really don't like deco stops of more than 20 minutes in the ocean if I can help it. I'd rather do two shorter dives with 10 minutes of deco and a longer total bottom time, than one long dive. Personal preference, but I don't like being down more than about 2-2.5hours at a time if I can help it, and with a planned total run time of just under 90 minutes is a perfectly acceptable ocean dive for me, though I don't go to 130ft without being on trimix if I can help it, and that extends dive time a bit and I would carry a 50/50 mix since it cuts run time by 20 minutes using standard gasses.

 

[decompression]

Have a look at idecopro. I haven't used it in awhile but it might fit some of your needs.


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