Decompression and dive computer

[CaptnJack]

Since buhlmann thinks such an ascent is far "too slow" and wants to race me up to 10ft, I've concluded buhlmann is hooey.

How do you explain the difference in overall deco, then, between a 20 min 150 foot dive and a 40 min 150 foot dive?

Just because gas dissolves into fluids (we are 98% water afterall) does not mean Buhlmann is conceptualizing how it comes out of solution in a meaningful way.

[sambolino44]

What a great thread! I really appreciate how everyone has been so, well, realistic about this and not just going off into the "No, no, no!" thing. I've talked to Tube about this (I swear she came up with the idea on her own!), and it's obvious that she has no intention of just going off and breaking all the other rules (like gas planning) just to do this experiment. She understands that if you're going over an edge like this, you'd better make damn sure every other possible variable is as nailed as you can get it.

I have a Suunto Gekko, and it went into deco once, and cleared before I got to the stop. But from that experience, I know what it looks like, and if it happens again, it will be a "oh, yeah, I've seen that before" instead of a "what the heck is that?!?" And that's really one of the main points about training, isn't it? You can read, discuss, and simulate all you want, but none of that sticks as well as actual experience. Sweat now so you won't bleed later, right? It's all about managing risk; accepting a higher level of risk in the short term for the purpose of lowering risk in the long term. Just make sure the payoff is worth it.

Others have made the point that it's actually kind of hard to get into deco and have enough gas on one tank. If I was going to plan a dive like this, it would be the second of a series, after a shorter-than-normal surface interval, saving the bigger tank for the second dive. But that's just coming off the top of my head without doing any actual planning, which I would certainly do, with plenty of contingencies, and "what ifs" before jumping in the water. But if we actually end up doing this, Tubesnout's going to do the plan, and I'm just going to "help". And watch from shore.

JUST KIDDING!!

Seriously, though, it might not be a bad idea to watch from ten feet above. With a pony. And oxygen in the car. And a cell phone with 9-1- already dialed. OK, kidding again.

[lamont]

Since buhlmann thinks such an ascent is far "too slow" and wants to race me up to 10ft, I've concluded buhlmann is hooey.

How do you explain the difference in overall deco, then, between a 20 min 150 foot dive and a 40 min 150 foot dive?

Just because gas dissolves into fluids (we are 98% water afterall) does not mean Buhlmann is conceptualizing how it comes out of solution in a meaningful way.

yeah, its just become trendy to completely ignore everything you can draw from a buhlmann model, including the basics of the dissolved gas model which are still underlying all of bubble (really dual-phase) models out there...

[lamont]

Others have made the point that it's actually kind of hard to get into deco and have enough gas on one tank. If I was going to plan a dive like this, it would be the second of a series, after a shorter-than-normal surface interval, saving the bigger tank for the second dive. But that's just coming off the top of my head without doing any actual planning, which I would certainly do, with plenty of contingencies, and "what ifs" before jumping in the water.

A better way to do this is to do a dive where you set your computer to 21% and you actually dive 32%. That way you don't actually have to go into actual deco in order to see your computer go into deco.

At some point after you do enough of these dives, and build up your confidence at following your own profiles instead of the computer's, and you bend the snot out of your computer repeatedly when you forget to set it to 32%, you may wind up putting your computer into gauge mode, but that's probably getting ahead of the story right now...

[whatevah]

Interestingly (or not), I've found that, if you follow gas rules and maintain a decent (correct) ascent profile, it's damn near impossible to get into deco on a single tank, especially tanks in the 72-80 cf size. If you strap on a 119/130, it's possible, but still needs some work.

That is an interesting thought, and I suppose that gas "rules" might vary. I know I can definitely reach and exceed my NDL on an LP72 filled with air - and ascending at a rate within typical recommendations, I would almost certainly violate obligations given by my computer and reach the surface with at least a third of my gas remaining. 119 or 130 cubic feet of breathing gas really does seem like a nutty volume to be carting around if you're not planning for decompression procedures and allowing for an appropriate level of redundancy.

[CaptnJack]

yeah, its just become trendy to completely ignore everything you can draw from a buhlmann model, including the basics of the dissolved gas model which are still underlying all of bubble (really dual-phase) models out there...

To me 16 compartment Buhlmann is just silly precise way to calculate where to cut with an axe. Uemis and others have figured this out and realized even half of those compartments give roughly the same answer. Then look at the bend rate on 100/100 Buhlmann and realize that oops maybe we should back off the m-values a bit, so gradient factors need to be layered in now. Oh wait it doesn't work at all for wicked long dives, maybe we can dive 30/120 GFs for those. In the end gas dissolves, gas comes out of solution and pretty much any computer model is just a rough guess of what you need to do on any given day.

[Joshua Smith]

In the end gas dissolves, gas comes out of solution and pretty much any computer model is just a rough guess of what you need to do on any given day.

It's ALL a rough guess, though- ratio, deco on the fly, buhlman, GAP, RGBM. What's your point?

[LCF]

I had a very, very interesting dinner the other night, with folks who make their living doing research in this area. A very intriguing statement from one of them was that, as research has continued into decompression, they are coming closer and closer to the idea that, as long as you do the time, it doesn't matter nearly as much as you might think where it is done. Obviously, there ARE some parameters, but obsessing over profiles probably isn't time well spent. As evidenced by the fact that people are successfully using a wide variety of programs with very different outputs. My Intro to Cave instructor does all his technical diving -- to WHATEVER depth -- with one deco gas, which is 100%. Pure, classic Buhlmann, and he does fine with it. Nothing could be more unlike the profiles I was taught in UTD Tech 1 . . . which also work.

[Tubesnout23]

Has anybody ever thought about adding a glossary to NWDC board? I don't understand the majority of your abbreviations-jargon! :angry:

My action plan is to re-read the Gekko manual and then look at the simulator and then take it from there. I have noticed how some people keep pounding away about Gekko being an almost 'pain-in-the-neck' conservative computer...Well I like it!

(and by the way if Grateful Diver is reading this when the manual told me to set my O2 % to 31 instead of 32 it also: 'As a safety precaution the O2 calculations in the computer are made with an O2 of 1% + setO2%' )

[WASP7000]

I guess it would be nutty if you were the type that breathes like a bird and comes up with 1/3 of their gas left on a 72. Some people are a little bigger, breath a little more, or do other activities under water which require more gas and are not planning for a deco dive.

I would gladly dive a 119 or 130 if I had one. All it does is give me longer bottom time and extra gas in a contingency. And it's not like it weighs any more when you have your BC inflated...

[CaptnJack]

Has anybody ever thought about adding a glossary to NWDC board? I don't understand the majority of your abbreviations-jargon! :angry:

Why not ask for an explanation of what you don't understand, probably good items to discuss.

My action plan is to re-read the Gekko manual and then look at the simulator and then take it from there. I have noticed how some people keep pounding away about Gekko being an almost 'pain-in-the-neck' conservative computer...Well I like it!

The gekko is pretty conservative on repetitive dives and deco along with giving you modest amounts of credit for a slow ascent up to "the stop" - in fact if you slowly ascend up a slope its likely to clear the obligation ~20ft before you get to the stop depth. In any case, if you are clearing that I would feel pretty good about that. Its not an algorithm I would stress about being "in the yellow".

[cardiver]

Has anybody ever thought about adding a glossary to NWDC board? I don't understand the majority of your abbreviations-jargon! :angry:

Why not ask for an explanation of what you don't understand, probably good items to discuss.

My action plan is to re-read the Gekko manual and then look at the simulator and then take it from there. I have noticed how some people keep pounding away about Gekko being an almost 'pain-in-the-neck' conservative computer...Well I like it!

The gekko is pretty conservative on repetitive dives and deco along with giving you modest amounts of credit for a slow ascent up to "the stop" - in fact if you slowly ascend up a slope its likely to clear the obligation ~20ft before you get to the stop depth. In any case, if you are clearing that I would feel pretty good about that. Its not an algorithm I would stress about being "in the yellow".

The Uemis, even when set on the least conservative mode, is more conservative than the Suunto. When Bassman and I do semi-deep dives my computer is screaming at me at least 5 minutes before his Cobra....

[CaptnJack]

The Uemis, even when set on the least conservative mode, is more conservative than the Suunto. When Bassman and I do semi-deep dives my computer is screaming at me at least 5 minutes before his Cobra....

Holy cow
are these repetitive dives? air? nitrox? Really curious what's in that yellow box now...

[cardiver]

The Uemis, even when set on the least conservative mode, is more conservative than the Suunto. When Bassman and I do semi-deep dives my computer is screaming at me at least 5 minutes before his Cobra....

Holy cow
are these repetitive dives? air? nitrox? Really curious what's in that yellow box now...

Nope. First dive of the day on air.

[Geek]

The Uemis, even when set on the least conservative mode, is more conservative than the Suunto. When Bassman and I do semi-deep dives my computer is screaming at me at least 5 minutes before his Cobra....

Holy cow
are these repetitive dives? air? nitrox? Really curious what's in that yellow box now...

The Uemis is air integrated and does real time calculations based on workload so it can vary quite a bit. One reason it appears so conservative.... of course that's just my exp and what I've gathered from useing mine for the last 7 months, I could just be imagining things

[cardiver]

The Uemis, even when set on the least conservative mode, is more conservative than the Suunto. When Bassman and I do semi-deep dives my computer is screaming at me at least 5 minutes before his Cobra....

Holy cow
are these repetitive dives? air? nitrox? Really curious what's in that yellow box now...

The Uemis is air integrated and does real time calculations based on workload so it can vary quite a bit. One reason it appears so conservative.... of course that's just my exp and what I've gathered from useing mine for the last 7 months, I could just be imagining things

You're probably right. I don't mind it being conservative and on shore dives at all. Any deco obligation that I get goes away on the way back in to shore.

[Tubesnout23]

Has anybody ever thought about adding a glossary to NWDC board? I don't understand the majority of your abbreviations-jargon! :angry:

Why not ask for an explanation of what you don't understand, probably good items to discuss.

Actually I don't care that much about the jargon, I was just joking but I will ask for explanations whenever I would feel totally lost.

The gekko is pretty conservative on repetitive dives and deco along with giving you modest amounts of credit for a slow ascent up to "the stop" - in fact if you slowly ascend up a slope its likely to clear the obligation ~20ft before you get to the stop depth. In any case, if you are clearing that I would feel pretty good about that. Its not an algorithm I would stress about being "in the yellow".

Here you go...'modest amount of credit'...uuhhmm I have heard that somewhere already but its meaning is still confusing.

'algorithm being "in the yellow"' I think I have some kind of an idea of what you are talking about but does it matter to know that in practical terms?

[CaptnJack]

Here you go...'modest amount of credit'...uuhhmm I have heard that somewhere already but its meaning is still confusing.

Deco is like a debt you have to repay (in time). So if a computer is giving you "credit" at intermediate depths during your ascent its paying like back the debt early. Vs. some computers which require you to be at or with a few feet of the stop depth before you start working off the deco penalty. Like a mortgage which doesn't allow payment until 10days before the due date.

'algorithm being "in the yellow"' I think I have some kind of an idea of what you are talking about but does it matter to know that in practical terms?

Some computers have a graphical "accumulated gas" bar which starts off at zero, gradually works its way through "the green", then there's a "yellow" caution zone, and finally the accumulated gas enters the "red zone" which means the gas load is substantive enough to require deco stops as shown on a numeric display. If I recall correctly the Suunto's have such a bar although I'm not sure if the edge of the screen is colored green/yellow/red or not.

[lamont]

yeah, its just become trendy to completely ignore everything you can draw from a buhlmann model, including the basics of the dissolved gas model which are still underlying all of bubble (really dual-phase) models out there...

To me 16 compartment Buhlmann is just silly precise way to calculate where to cut with an axe. Uemis and others have figured this out and realized even half of those compartments give roughly the same answer.

I was always taught in physics courses to carry through all calculations on a calculator with precision and to only round off at the end.

Then look at the bend rate on 100/100 Buhlmann and realize that oops maybe we should back off the m-values a bit, so gradient factors need to be layered in now.

Which is a better model...

Oh wait it doesn't work at all for wicked long dives, maybe we can dive 30/120 GFs for those.

..but still clearly not perfect.

I don't like the fact that the surfacing gradient factor is going to be completely different on a T1 dive vs. a T2 dive vs. a very long shallow T2 dive. Its obvious to me that its just a mathematical hack to make the model better.

In the end gas dissolves, gas comes out of solution and pretty much any computer model is just a rough guess of what you need to do on any given day.

Yeah, but that doesn't mean you can throw away everything. Newtonian gravity was shown to be wrong by Einstein and even General Relativity gets silly when you get down to the quantum level or in areas of space which it predicts to curve infinitely. I'm still not going to go jump off the top of the house thinking that gravity is all wrong...

[Joshua Smith]

I must admit that I find the sheer amount of time people spend pondering decompression to be somewhat baffling. I use a combination of my Shearwater computer and good ol' vplanner for decompression dives. I've done many, many dives in the 150-250' range, and enough in the 250-300' range to be pretty comfortable with them. I've never had so much as a niggle. I guess I just don't see much of a need to modify what I'm doing, or to try and game or manipulate something that works pretty darn well, as far as I can see. Like, if I devoted a whole ton of time and resources to it, I *might* be able to modify the motor in my van enough to make it 3% more powerful, or get another 1.7 mpg out of it, but I'm just not that interested. Seems to me the whole deal has been figured out as well as is reasonably necessary.

[Sounder]

Paraphrasing a wonderful quote: "Unexploded goats are not necessarily an indication of a sound decompression algorithm."

Great post btw Josh.

FWIW - this thread which could have turned very ugly has, instead, yielded an awesome amount of information. Thanks to all who are contributing!!

[whatevah]

I don't mind it being conservative and on shore dives at all. Any deco obligation that I get goes away on the way back in to shore.

One would hope that's true on surfacing from a boat dive also. No reason they should be different.

[whatevah]

I guess it would be nutty if you were the type that breathes like a bird and comes up with 1/3 of their gas left on a 72. Some people are a little bigger, breath a little more, or do other activities under water which require more gas and are not planning for a deco dive.

I would gladly dive a 119 or 130 if I had one. All it does is give me longer bottom time and extra gas in a contingency. And it's not like it weighs any more when you have your BC inflated...

Where does it all end? Diving a T cylinder to cover all those contingencies once and for all? The trend I'm seeing is for basic recreational divers to haul bigger and bigger cylinders - because it's "safer". Well, I disagree. My air consumption has improved through experience and optimization. When I dive larger cylinders it hurts my efficiency because ultimately it means that I have risked a fall (and broken bones) to lug a huge, heavy cylinder down to the shore entry where I put several cubic feet of air into my whopping great wing (aka sail when the current picks up) so that I can stay afloat. Then I'll have to fiddle with larger adjustments throughout the dive. Boat diving with big cylinders offers only slightly less drama. Cylinder size is definitely subject to the law of diminishing returns. I enjoy choosing the right size cylinder from my collection for any given dive. I'm not sure what these special underwater activities are that require more gas - perhaps you could elaborate. Spearing certainly doesn't fit, and salvage - filling large lift bags or operating pneumatic tools, should be done with a separate cylinder and regulator.

[LCF]

I don't use a different wing with a 130 than I do with my 95, and I wouldn't go to a smaller one if I used smaller tanks. The lift on my wing is primarily set by my exposure protection. After all, even with a 130, you're only compensating for about 9 pounds of gas.

I do hear the bit about the increased risk of getting to and from the water. I often dive doubles on simple dives here, just to stay in practice with my drills for the cave diving I do, but I don't take them to sites like Day Island Wall, where the access raises the risk of falling. I have already broken one bone diving and have no desire to break any more. But I think the answer is strength and balance training, not less gas . . . I view the extra gas I haul around as insurance, in case I get delayed. I have had one dive (with Bob) where we had to deal with a tank that came loose at about 95 feet. It took some time to convey the problem and some time to fix it. I was diving a 95, and ended that dive (which never went into deco) doing a safety stop with less than 500 psi. Nobody ever ended a dive wishing they'd brought less gas.

[Joshua Smith]

I had a very, very interesting dinner the other night, with folks who make their living doing research in this area. A very intriguing statement from one of them was that, as research has continued into decompression, they are coming closer and closer to the idea that, as long as you do the time, it doesn't matter nearly as much as you might think where it is done. Obviously, there ARE some parameters, but obsessing over profiles probably isn't time well spent. As evidenced by the fact that people are successfully using a wide variety of programs with very different outputs. My Intro to Cave instructor does all his technical diving -- to WHATEVER depth -- with one deco gas, which is 100%. Pure, classic Buhlmann, and he does fine with it. Nothing could be more unlike the profiles I was taught in UTD Tech 1 . . . which also work.

I missed this post the first time through. That IS very interesting. It also makes sense to me, somehow.