Non-technical dive-planning software?

[divengolf]

Daniel,

One thing that I took long ago from Lamont's RB paper and always use:

Depth X 10 plus 300 is the minimum pressure at any depth. Assuming an 80 or 100 CF tank. So at 80 feet, you need 1100 psi for a save ascend with a buddy on your octo.

Remember that at 25 dives, one thinks he has this sport figured out. At 100 dives one realizes how little he knows about diving.

Question everything until you understand, but pay attention to those who have gone before you.

 

[rstofer]

Well, that 2.5 CFM/ATM SAC rate is pretty high. Calm, experienced, divers are well under 1.0. I thought the magic number was around 0.5 for an experienced diver. So, you could make the surface easily with more modest breathing. I realize we're talking high stress but still, the SAC of even two divers could be under 2.5. Of course, the combined rate could be 10 for all I know. It all depends on what numbers you want to grab. And the divers...

Then too, the 30 FPM isn't set in stone! The PADI ascent rate is 60 fpm. That saves a bunch of air! Now, it's true that the NAUI rate is now 30 fpm but I'll bet there are more PADI divers than NAUI divers. In fact, you would probably (and I use that term cautiously) be ok if you ascended at 100 fpm. The Navy used to do that and most divers still do, by mistake. That's why the safety stop was created. Divers didn't control their ascent.

And, there is no real need to make a safety stop on a dive that doesn't exceed NDL. I guess if I were getting low on gas, I might negotiate a little on the 3 minutes. Like all of them, if necessary. I certainly wouldn't hang at 15' while I was drowning! "Gotta wait 3 minutes, gotta wait 3 minutes...". No way!

The PADI requirement for a safety stop is questionable in the context of requiring a stop on an NDL dive. It kind of flies in the face of "No Decompression". Sure, it's a good idea. But people dove for decades without doing safety stops and it can't be shown that they were injured more often than the current crop of divers. The 1989 NAUI tables don't mention a safety stop but they did slow the ascent to 30 fpm from 60 fpm in the 1987 tables.

So, yes, 750 psi will very likely get you and a buddy to the surface. It won't be a slow leisurely pace, you can't hang around at 100' very long and it won't include a safety stop but you won't run out of air. If you keep your wits about you... If you panic, all bets are off!

Take all my numbers, and any other set of numbers you see, with a huge dose of skepticism. We're just grabbing them out of the blue and you are the diver.

Just for giggles, next time you are down at 100' try air sharing on an ascent and see how many psi it requires. Then work out how may CF so you can calculate the info for other tank sizes. There's nothing like an experiment to settle the mind. It works, or it doesn't.

Richard

 

[rjack321]

I think that what we're seeing in this thread is an error of risk assessment, i.e. the risk of DCS is assigned too high a value vs. gas management.

Exactly. At this point in your career Daniel I would focus on learning as much as you can about how much gas it takes to do things. Start by calculating and knowing your SAC rate. Use that to estimate how much gas you <should> have at 5 mins, at 10 mins, at 15, etc. Track where you are relative to your plan during your dives. Know when to turn around. Know when you <must> ascend.

Hours of deco is mostly just boring, cause having enough gas to do it is known.

In your case, 5 or 10mins of deco because your surface interval was a bit short or you miscalculated residual nitrogen (if you were using tables) is not a particularly angry monster to focus on. Getting blindsided because you or your buddy suddenly ran out of gas at depth IS a boggie man.

 

[rstofer]

Another way of looking at the NAUI vs PADI ascent rate/safety stop is this: NAUI ascends at 30 fpm and doesn't mention a safety stop. PADI ascends at 60 fpm and recommends a 3 minute safety stop for all dives and requires it if the diver is within 3 letter groups of the NDL. Either way, they're trying to slow the diver down.

All things considered, I would take the PADI approach and ascend at 60 fpm. That gets me a lot closer to the surface if I do run out of air. It also gets me shallow quicker so I use less ATMs of air, on average.

When I got to 15' I would see if I had the air for a safety stop. It wouldn't take much for a 3 minute stop. If I didn't have it, oh well... I'd surface.

The point is this: sure, you should be on your way up when you get to 1/3 of psi. But, if you mess up and find yourself a little short of 1/3, don't panic. You can still make it safely. You just need to be calm and breathe normally.

But you need to know when you get to 1/3 and if you don't watch your gauges, no amount of planning will help.

Richard

 

[divengolf]

I thought that most agencies were moving toward 30 fpm for recreational ascents with a 15-20 ft SS. With the emergence of the "tiny bubble models", I believe that research has shown that 30 fpm is an optimum rate. Faster or slower is not as good. The book "Deco for Divers" has a good discussion in Chapter 6.

Myself, for any recreational dive over 60 ft, I'll do a deep stop at half of max. depth for a minute. Then proceed to the SS, staying at 30 fpm +/-.

Of course I'm talking non-emergency, non-mandatory deco ascent.

 

[Blackwood]

Well, that 2.5 CFM/ATM SAC rate is pretty high. Calm, experienced, divers are well under 1.0.

...

Richard

Note that the calculation in my post wasn't directed at "calm, experienced divers," but rather someone with fewer than 24 dives and likely diving with similarly inexperienced divers.

I don't dive often with new divers, but I have read a lot of posts about new divers sucking down tanks in 15 minutes flat and having to share with DMs, etc.

The PADI requirement for a safety stop is questionable in the context of requiring a stop on an NDL dive. It kind of flies in the face of "No Decompression".

No more so than does imposing a maximum ascent rate.

Calling it "no decompression" is the fallacy.

 

[eponym]

Rstofer opined:
Well, that 2.5 CFM/ATM SAC rate is pretty high. Calm, experienced, divers are well under 1.0. I thought the magic number was around 0.5 for an experienced diver. So, you could make the surface easily with more modest breathing. I realize we're talking high stress but still, the SAC of even two divers could be under 2.5. Of course, the combined rate could be 10 for all I know. It all depends on what numbers you want to grab. And the divers...

Quite. Thanks for pointing that out, R.

I started out computing rock bottoms assuming two stressed divers both using 1 cf/min/atm, double our usual rate. Then one day I was on a wreck when a solo diver ran out of air and linked up with another pair of divers. By the time the three were ascending up the buoy line, the donor diver had switched to sharing air with his buddy because of how quickly the OOA solo diver was hoovering down the donor's gas.

I can only pray I might be capable of such creative and calm problem-solving in similar circumstances.

Way off the OP's question but good stuff, everyone.

-Bryan

 

[Blackwood]

I thought that most agencies were moving toward 30 fpm for recreational ascents with a 15-20 ft SS. With the emergence of the "tiny bubble models", I believe that research has shown that 30 fpm is an optimum rate.

30FPM may work well on tables generated for 30FPM, or 60FPM, but would certainly be questionable on a 15FPM table.

A no-stop model has an assumed maximum ascent rate. The goal is to dive within the limits of the model.

 

[daniel1948]

A few comments, and then I'm going back and reading all the links in the above posts:

1. I realize now that the 750 psi figure was given to us because we were always making a two-level dive on the deep dives. We'd spend maybe 10 minutes at 80 feet on the deepest dives, so we were ascending from that depth with much more air. We were also spending time at 50 or 40 feet, which has the effect of a stop, even though we were swimming and enjoying.

2. With 24 dives under my belt I certainly do not think I know it all. I consider myself a rank beginner. I know the theory taught in the PADI OW book. I am comfortable under water, but well aware that I have no experience to speak of. But I'd say that my gas consumption is pretty good for a rank beginner. Being a jogger probably helps. And being a snorkeler for a number of years, and a swimmer all my life, helps my comfort level.

3. I expect to make a dive to 100 feet with an instructor, because I plan on taking the AOW course. Apart from that, I have no plans for diving that deep until I have a lot more experience, if then. The pretty fishes are mostly shallower. I like shallow.

4. It's lunch time now. After lunch I'm going to read those links above. I am going to study gas management. You've all convinced me that that's the most pressing study issue for now.

 

[eponym]

Daniel, sounds like a good plan.

The idea of designing non-deco dive planning software sounded interesting at first. But the reality is, can you hold to your planned depths? Do you really know what your consumption will be? What about gas needs for surface swims?

For me the result of your thread will be to go back onto tables for some dives, with my computer in gauge mode, as a check on my own monitoring abilities after almost a decade of letting the computer tell me my remaining non-deco time (except for the occasional planned deco dive).

-Bryan