Decompression and dive computer

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Re: Decompression and dive computer

Post by spudgunman » Sun Aug 15, 2010 9:58 am

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Re: Decompression and dive computer

Post by Tubesnout23 » Thu Aug 12, 2010 8:08 am

CaptnJack wrote:
Tubesnout23 wrote:
CaptnJack wrote:
Tubesnout23 wrote: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.


In other words the Gekko gives you 'credits' and therefore is more 'flexible'?


Probably more biologically realistic. E.g. If you overstay at depth by 4 minutes and as a consequence have a 5 minute deco obligation showing on your computer at 100ft when you start swimming up slope... You will eliminate some or even all of the extra gas dissolved if you were to do a 10minute ascent from 100ft up to 20ft. Thus your gekko might even report a 2min non deco limit remaining as you amble around at 15ft. Deco isn't some magic thing that can "only" begin at the computer requested 5min stop at 10ft.


OK! Got it! Thanks!

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Re: Decompression and dive computer

Post by CaptnJack » Fri Aug 06, 2010 10:21 pm

Tubesnout23 wrote:
CaptnJack wrote:
Tubesnout23 wrote: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.


In other words the Gekko gives you 'credits' and therefore is more 'flexible'?


Probably more biologically realistic. E.g. If you overstay at depth by 4 minutes and as a consequence have a 5 minute deco obligation showing on your computer at 100ft when you start swimming up slope... You will eliminate some or even all of the extra gas dissolved if you were to do a 10minute ascent from 100ft up to 20ft. Thus your gekko might even report a 2min non deco limit remaining as you amble around at 15ft. Deco isn't some magic thing that can "only" begin at the computer requested 5min stop at 10ft.
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Re: Decompression and dive computer

Post by whatevah » Fri Aug 06, 2010 2:32 pm

spatman wrote:seems to me you're just making a case for becoming a better diver with proper training, not for smaller tanks.

:dontknow:


Hrm. I think I'm advocating divers being self-critical; giving themselves the time to gather experience and methodically refine their equipment and skills, thus naturally improving their gas consumption and safety. Conventional training will probably play some part early in the learning curve. The mass migration from 72, 80, 85 cuft cylinders to 100, 119, 130 cuft cylinders has me thinking that many people are focusing on insurance rather than competence. I've got nothing against larger cylinders per-se - but I've seen them used to cover up underlying efficiency problems which are correctable. Choose the right cylinder for the dive, and the dive will be more enjoyable - probably safer too.
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Re: Decompression and dive computer

Post by Tubesnout23 » Fri Aug 06, 2010 11:09 am

CaptnJack wrote:
Tubesnout23 wrote: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.


In other words the Gekko gives you 'credits' and therefore is more 'flexible'?

CaptnJack wrote:
Tubesnout23 wrote:'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.


Yes it has a bar colored green, yellow and red it works when it is Nitrox and decompression mode, as far as I understood.

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Re: Decompression and dive computer

Post by Joshua Smith » Thu Aug 05, 2010 9:38 pm

CaptnJack wrote:
Joshua Smith wrote: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.


Someone wise told me once (it may have been Simon Mitchell, I forgot where this was) that deco stress is basically bi-modal. There is a group of divers who are fine with pretty much anything "close" and a group that never seem to feel quite right no matter what they do or how they try to adjust their profiles.



OK. I'm enlightened enough to accept that nugget as wisdom. And I like it a lot, fwiw.
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Re: Decompression and dive computer

Post by Sounder » Thu Aug 05, 2010 7:56 pm

Geek wrote:
Maverick wrote:
Tubesnout23 wrote:This may be an 'off the wall' question...

My Gekko computer manual explains what happens to the computer when the decompression limit has been ignored and the dive becomes a decompression dive...Now I can read the manual several times and try to memorize its content but should I deliberately put myself into a decompression dive in order to fully understand what the text in the manual means or it is a foolish exercise?



dude thats the silliest thing i have ever heard. should someone get hit by a school bus just so the can understand reconstructive surgery they read about in a text book?


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Re: Decompression and dive computer

Post by Geek » Thu Aug 05, 2010 7:53 pm

Maverick wrote:
Tubesnout23 wrote:This may be an 'off the wall' question...

My Gekko computer manual explains what happens to the computer when the decompression limit has been ignored and the dive becomes a decompression dive...Now I can read the manual several times and try to memorize its content but should I deliberately put myself into a decompression dive in order to fully understand what the text in the manual means or it is a foolish exercise?



dude thats the silliest thing i have ever heard. should someone get hit by a school bus just so the can understand reconstructive surgery they read about in a text book?


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Re: Decompression and dive computer

Post by Maverick » Thu Aug 05, 2010 7:24 pm

Tubesnout23 wrote:This may be an 'off the wall' question...

My Gekko computer manual explains what happens to the computer when the decompression limit has been ignored and the dive becomes a decompression dive...Now I can read the manual several times and try to memorize its content but should I deliberately put myself into a decompression dive in order to fully understand what the text in the manual means or it is a foolish exercise?



dude thats the silliest thing i have ever heard. should someone get hit by a school bus just so the can understand reconstructive surgery they read about in a text book?
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Re: Decompression and dive computer

Post by CaptnJack » Thu Aug 05, 2010 5:58 pm

Joshua Smith wrote: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.


Someone wise told me once (it may have been Simon Mitchell, I forgot where this was) that deco stress is basically bi-modal. There is a group of divers who are fine with pretty much anything "close" and a group that never seem to feel quite right no matter what they do or how they try to adjust their profiles.
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Re: Decompression and dive computer

Post by ArcticDiver » Thu Aug 05, 2010 4:12 pm

Sitting here reading this thread on my Blackberry while my "travel agent" is planning my next dive trip. I am reminded of a conversation I had when I was a new diver.

I said that if no one knew factually vs theoretically what caused DCS and people used a fairly broad spectrum of algorithms to avoid DCS and, in any case, DCS is rare and there are "undeserved" hits maybe there is no such thing as an absolutely safe dive. Without an absolute safe dive definition there is no concrete definition of "conservative" or "aggressive"dives as applied to DCS.

There is also something to be learned on this subject from experience with aviators in hyperbaric chambers. At the total organism level there is quite a bit of variation between people and from day to day.

The upshot is that I have always believed that although its' conseqences can be severe DCS is a marginal risk. A marginal risk that can be mitigated by picking your favorite algrithm and following it. But the best defense against DCS and a host of other more likely problems is physical and mental fitness.
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Re: Decompression and dive computer

Post by Geek » Thu Aug 05, 2010 3:43 pm

Yeah, why do I have these stupid 119's again? Wait, how much lead do I need with them? Oh, yeah, None.

Your argument is not one that I can buy, and your selling to hard :angelblue:
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Re: Decompression and dive computer

Post by spatman » Thu Aug 05, 2010 3:31 pm

seems to me you're just making a case for becoming a better diver with proper training, not for smaller tanks.

:dontknow:
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Re: Decompression and dive computer

Post by whatevah » Thu Aug 05, 2010 3:23 pm

LCF wrote: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.


That is a valid choice of course. An LP95 is horribly negative when full - very similar to a HP130.

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.


If you're carrying five pounds of gas around that you're not going to use, you're compensating for that mass and it is costing you gas. At 4 or 5 atmospheres, that volume is going to add up. Further, whether that compensatory gas is in a wing or in a drysuit, it is increasing your drag and you are working harder. Carrying pounds of gas around that go well beyond a normal safety margin is worse than carrying those extra pounds as lead on your belt. For people whose air consumption is higher than they'd like, the best answer is to look at their equipment, their dive practices and their level of comfort - find ways to improve efficiency. Compensating with huge cylinders hurts their efficiency and may well steepen the curve if they're hoping to improve their air consumption with more experience. Wing size must also be adequate to float the equipment independently of the diver at the surface.

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 . . .


Improving strength and balance is not going to eliminate the risk - it helps - as does minimizing the complexity and mass of the equipment being carried.

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.


It sounds like you had the right amount of gas to handle the contingency. Should be glad you didn't have to deal with that problem with a Heiser 190. You should know that a simple problem like that can arise at any time, and allow for that. If <500psi at your safety stop was eating into your comfort zone, you needed to turn the dive earlier. The same problem could easily have occurred with a larger cylinder, and you might just as easily have ended with an uncomfortable reserve at your safety stop if you managed your dive the same way.

Nobody's arguing against having some insurance - but how much? One can have so much insurance that it increases your odds of making a claim. I frequently end dives and think to myself - that would've been more fun with less stuff hanging off me. This is why I have a selection of gear - single LP72s on up to HP100s and a set of doubled LP72s. I try to select the right gas capacity for the dive and the rest of my gear is configured to match. One of my favorite configurations involves my wetsuit and my freediving belt, along with an AL19, a first stage and one second stage. No buoyancy compensation necessary - don't even need an SPG. Occasionally I'll dive with someone new and they'll tell me they like to dive with a 119 or a 130 because their air consumption is pretty high - then I'll see them huffing and puffing their way to the water, starting the dive out at a loss. I'll look over at them during the dive while we're holding position in current and they'll be hanging there at a 45 degree angle with a big bubble in their wing, pumping gas through their reg at an astounding rate - eyes wide and kicking madly with up to 4lbs of lead strapped around each ankle. And I think to myself, "yes, I can see that your air consumption would be high" - good thing they carry lots of insurance.

To me, the oversized cylinders being used by many newer divers improve their odds of getting enough nitrogen loading to be a real problem when they either panic and bolt to the surface or they lose control of their buoyancy and cork because the big pocket of air they were carrying around in their suit/wing to compensate for the weight of the gas rapidly became uncorrectable. It's best to keep things simple, and learn to manage your dive so that you have enough gas left over when you reach the surface - it's done the same way with a 72cuft cylinder as it is with a 130cuft cylinder.
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Re: Decompression and dive computer

Post by Joshua Smith » Thu Aug 05, 2010 8:29 am

LCF wrote: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.
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Re: Decompression and dive computer

Post by LCF » Thu Aug 05, 2010 7:47 am

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.
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Re: Decompression and dive computer

Post by whatevah » Thu Aug 05, 2010 2:36 am

WASP7000 wrote: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.
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Re: Decompression and dive computer

Post by whatevah » Thu Aug 05, 2010 1:51 am

cardiver wrote: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.
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Re: Decompression and dive computer

Post by Sounder » Wed Aug 04, 2010 10:55 pm

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!!
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Re: Decompression and dive computer

Post by Joshua Smith » Wed Aug 04, 2010 10:19 pm

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.
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Re: Decompression and dive computer

Post by lamont » Wed Aug 04, 2010 9:21 pm

CaptnJack wrote:
lamont wrote: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. :pirate: 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...

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Re: Decompression and dive computer

Post by CaptnJack » Wed Aug 04, 2010 8:51 pm

Tubesnout23 wrote: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.

Tubesnout23 wrote:'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.
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Re: Decompression and dive computer

Post by Tubesnout23 » Wed Aug 04, 2010 7:42 pm

CaptnJack wrote:
Tubesnout23 wrote: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.

CaptnJack wrote: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?

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Re: Decompression and dive computer

Post by cardiver » Wed Aug 04, 2010 2:25 pm

Geek wrote:
CaptnJack wrote:
cardiver wrote: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 :tomnic:

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.
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Re: Decompression and dive computer

Post by Geek » Wed Aug 04, 2010 1:11 pm

CaptnJack wrote:
cardiver wrote: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 :tomnic:
If I'm killed by the questions like a cancer,
Then I'll be buried in the silence of the answer.


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Life isn't like a box of chocolate's, life is like a box of chocolate and horse bisket's and no matter which one you get you have to keep on chewing...

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