Decompression and dive computer

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Decompression and dive computer

Post by Tubesnout23 » Mon Aug 02, 2010 12:14 pm

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?

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Re: Decompression and dive computer

Post by Sounder » Mon Aug 02, 2010 12:32 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?


Without training in decompression diving, it's pretty inadvisable to deliberately put yourself into decompression. May I suggest one of Bob's articles about "Uh oh, I'm in deco" to read through instead: http://nwgratefuldiver.com/articles/deco.html

Some dive computers come with simulators so you can run a dive from the safety of your couch to see how the computer will behave and others have online simulators you can play with... but I think you'll find very few people recommending you deliberately dive outside your training level, and incur a decompression obligation, just to see how your computer will function.

Check out the article - I think you'll find it helpful.
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Re: Decompression and dive computer

Post by Tubesnout23 » Mon Aug 02, 2010 1:13 pm

Sounder 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?


Without training in decompression diving, it's pretty inadvisable to deliberately put yourself into decompression. May I suggest one of Bob's articles about "Uh oh, I'm in deco" to read through instead: http://nwgratefuldiver.com/articles/deco.html

Some dive computers come with simulators so you can run a dive from the safety of your couch to see how the computer will behave and others have online simulators you can play with... but I think you'll find very few people recommending you deliberately dive outside your training level, and incur a decompression obligation, just to see how your computer will function.

Check out the article - I think you'll find it helpful.


I had the feeling that that was the case. Thanks for the article!

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Re: Decompression and dive computer

Post by Tubesnout23 » Mon Aug 02, 2010 9:25 pm

I have read Bob's article. It is straight to the point, simple and clear. Thanks Bob! I am still pondering about this, though and I haven't made up my mind yet.

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Re: Decompression and dive computer

Post by lamont » Mon Aug 02, 2010 9:50 pm

Sounder wrote:Some dive computers come with simulators so you can run a dive from the safety of your couch to see how the computer will behave and others have online simulators you can play with... but I think you'll find very few people recommending you deliberately dive outside your training level, and incur a decompression obligation, just to see how your computer will function.


+1 on using the simulator and putting it into deco from the safety of your couch.

don't worry too much about trying to use the computer to "execute" a decompression dive. just focus on being able to recognize what it looks like when you are about to go into deco and what it looks like when you go into deco. then if you see that at ~100 feet you need to ascend to <= 30 feet and it'll rapidly clear.

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Re: Decompression and dive computer

Post by Kirby » Mon Aug 02, 2010 9:52 pm

I like to think of decompression as swimming up to a hole in the side of a very large ship wreck. You can poke your head in the hole, or even go into the first room pretty safely, but you can't go straight up to safety any longer if you have a problem. How far into the hole are you willing to go? People get bent sometimes when they are well within the no deco limit. I was diving right up to the no deco limit of my computer for a long time before I tied off the line and went deep into the wreck. I don't think I was any safer walking the no deco line on an 80 than I am now with 30+ minutes of deco with duals and slings. Different computers set at different levels of conservatism will clear at much different times, and some will give much deeper stops. If you feel the need for speed... I recommend taking the classes, getting the right equipment and experience, and using the right gases. Read the disclaimer in your computer manual that says

"WARNING!
THERE IS ALWAYS A RISK OF DECOMPRESSION ILLNESS (DCI) FOR
ANY DIVE PROFILE EVEN IF YOU FOLLOW THE DIVE PLAN PRESCRIBED
BY DIVE TABLES OR A DIVE COMPUTER. NO PROCEDURE,
DIVE COMPUTER OR DIVE TABLE WILL PREVENT THE POSSIBILITY
OF DCI OR OXYGEN TOXICITY! An individual’s physiological
make up can vary from day to day. The dive computer cannot account for
these variations. You are strongly advised to remain well within the exposure
limits provided by the instrument to minimize the risk of DCI. As an added
measure of safety, you should consult a physician regarding your fitness before
diving."

Nuff said.

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Re: Decompression and dive computer

Post by lamont » Mon Aug 02, 2010 10:21 pm

I'd add to bob's article a few of the "failure modes" of recreational divers stumbling into decompression limits accidentally:

1. Mistake: Vapor locking on the bottom.

The computer is telling you that you need to ascend. If you signal a DM or buddy thumbs, and you get back an OK and then you swim along for 5 minutes, you're just getting further into deco, swimming along as both of you are narc'd and not communicating well. Sometimes narc'd divers will 'stall out' waiting for each other to ascend with nobody taking any initiative. You need to initiate the process of getting off the bottom with some urgency to stop the decompression clock, and you need to effectively communicate to people that you need to end your dive.

2. Mistake: Panic and bolting from the bottom.

You need to leave the bottom with some urgency, but not by abandoning your buddies and shooting to the surface in a panic. In all likelihood you are still at a very low risk of DCS, even if you add a minute or two more time on the bottom. You still want to calmly signal your buddies and abort the dive, and probably ascend upslope if the dive plan calls for that. If there's an upline you probably want to swim back to that and not do a free ascent. If you have visual contact with the upline and you don't have blasting currents you probably want to swim up at the same time as you are swimming back to the upline (try to meet the upline at about 1/2 your max depth).

3. Mistake: Losing control on the ascent

If you ascend rapidly you might find yourself ascending out of control. That's part of the reason why bob recommends doing a stop at 1/2 max depth. You need to leave the bottom with some urgency, but not so hastily that you lose buoyancy control and pop to the surface. And once you have hit 1/2 max depth you can stay there for 20-30+ minutes before beginning to rack up additional decompression.

4. Mistake: Ascending in a hurry to exactly your ceiling.

Your computer may be saying 10 feet, but you can decompress from a recreational dive just fine at 30 feet. A common recreational dive profile on 32% is 30 mins @ 100 feet, 15 mins at 50->30, and 15 minutes from 30 to the surface. Even if you go a couple mins into deco at 100 feet, the slow ~2 fpm ascent from 50 feet will clear the deco obligation after a few minutes at even the intermediate depths.

While a novice diver should probably end their dive after going into deco, to a more experienced diver a couple minutes of decompression on a recreational computer is just a warning that the deep portion of the dive has hit its end -- the dive proper may not even be 1/2 done with (of course, gas supplies permitting).

Ascending in a hurry may just annoy your dive buddy who hadn't gone into deco yet on their computer and since you've both got tons of gas less they weren't done with the dive -- or, an ascent directly from 100 feet targeting precisely the 10 foot ceiling may encounter buoyancy problems and result in a direct ascent to the surface.

5. Mistake: Re-descending after a blow-up to the surface

If you do make a mistake and wind up on the surface with a pissed-off computer and you've blown off your stops you've very likely shifted your DCS risk from something like 1-in-1,000 to 1-in-50 and there's some slight risk of skin bends, or a shoulder hit. You might feel sleepy. There's no reason to panic that you're going to die because you blew off a couple minutes of recreational deco. Do not attempt to re-descend and do in-water-recompression just because you pissed off your computer. You could panic, lose buoyancy control, descend back down to 100+ feet, rack up more deco, could shunt bubbles and give yourself a type 2 hit, or bounce around enough to give yourself an air embolism. Just stay on the surface and exit the water.

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Re: Decompression and dive computer

Post by Joshua Smith » Mon Aug 02, 2010 11:34 pm

Tubesnout23 wrote:I have read Bob's article. It is straight to the point, simple and clear. Thanks Bob! I am still pondering about this, though and I haven't made up my mind yet.


Plenty of good advice so far. The bottom line is that you really shouldn't play around with decompression without the proper training and equipment. It can happen by accident sometimes, though. And if it does, you should understand what your computer is telling you to do. How you accomplish that goal is up to you, but please don't walk into the bear's den and poke it with a stick to see what it does, if you get my drift. I screwed this up with my old suunto once, not too long after I started diving. That's when I found out that suunto's solution for violating the NDLs is basically to get you into 10' of water and have you wait there until your air is almost gone. That was a very cold 30 minute safety stop, and I was diving wet at the time.
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Re: Decompression and dive computer

Post by Kirby » Tue Aug 03, 2010 7:15 am

Which is why I now dive a 600$ computer as a bottom timer LOL.

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Re: Decompression and dive computer

Post by BillZ » Tue Aug 03, 2010 7:18 am

Hi Betty,
Here's the link to the free Suunto Dive Manager software. It's primarally used for downloading your dive data (the Gekko doesn't have the PC interface capability) but you can also create simulations and watch the readout on the Suunto computer interface.
http://www.suunto.com/en/Products/Softw ... e-Manager/

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Re: Decompression and dive computer

Post by Tangfish » Tue Aug 03, 2010 9:45 am

+1 to what all these guys said here. It's good that you're asking the question though, rather than thinking about the manual once you've already accidentally gone into deco. One of the common mistakes is that people see the display telling you 10ft for X amount of time - and they go to exactly 10ft. Usually the computer is telling you NOT to ascend beyond a certain depth when in deco - so going right to 10ft. will mean that you'll actually probably be breaking that plane. For each second you break that plane of 10ft. the computer will give you a corresponding amount of "penalty time". Anyway, you'll learn all of this by understanding how your computer works and by getting the proper decompression diving training.

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Re: Decompression and dive computer

Post by whatevah » Tue Aug 03, 2010 10:59 am

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?


I like your thinking Betty. Looking ahead and becoming informed on this is a great idea. With experience, divers learn to see how depth and time work against them on both the nitrogen loading clock and gas remaining, and they also develop a feel for the way that they get time back as they ascend. The NDL is not some fixed, binary situation - it is in fact a sliding point on a scale - it delineates two areas but the transition is very fuzzy. One thing I have learned from my own diving experience is that letting your computer go into decompression mode is not the end of the world. There is some risk of DCS associated with diving until you are _almost_ at the NDL, and there is slightly more risk associated with diving until you're just barely past the NDL. My computer's manual has a statement that simply recommends a diver always try to ensure their nitrogen load bar graph is in the green zone when reaching the surface and I think that's really the best advice there is. Reading your computer's manual, and using simulators etc is great and you should be familiar with the way your computer behaves in decompression mode. But I think perhaps there is also something valuable to gather from a dive where you barely go into decompression and then work within your computer's plan - you will see that if you calmly, steadily ascend and enjoy the rest of your dive at lesser depths you will generally come out of decompression mode long before you even have to do your first obligation. I am not necessarily recommending this to you - it's a personal decision and only you know where you are in terms of readiness or motivation. I'm just saying that an experienced diver might learn something valuable from a brief experimental foray into the twilight zone - but I would be really sure to make it a controlled experiment - make sure you're not working with a bunch of other unknowns - do it somewhere where there is structure all the way to the surface, minimal current and be certain that you have plenty of breathing gas available - be sure that your equipment is all working well and that you've got your buoyancy control nailed. Be ready to change your dive plan and give up the experiment for another day. Consider making a deep stop in addition to a nice long safety stop at your regular 15' to 20' - a couple of minutes around half your maximum depth. I think that perhaps experience like this can turn the NDL from a scary unknown into a controlled known, and I really like calm divers. I feel the same way about solo diving, FWIW. I've known of a lot of divers who behaved irrationally when they lost their buddy in the murk because their instructors put far too much emphasis on that safety blanket.
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Re: Decompression and dive computer

Post by dwashbur » Tue Aug 03, 2010 11:05 am

My wife and I recently exceeded our computers' NDL's by a few minutes on purpose, for just this reason (well, that and we wanted time to really explore the bottle field at Redondo). We were careful about our air etc. and checked both our computers to see what they wanted in the way of stops. I have a Sherwood Wisdom 2, she has an Aladin Prime. Both computers were calling for a four minute stop at 10 feet. So when it was turnaround time on our air, we came back up the slope like we usually do. My computer starts counting down the routine safety stop at 20 feet rather than the 15 feet we're all taught, but that doesn't seem like a big deal. When we got to 20 fsw, mine started counting down the decompression stop. So we stayed around that depth until the four minutes had counted off, then headed on in. Within minutes of hitting shore, her computer had gone into SOS mode, which is basically "you skipped a decompression stop so you can't use me for the next 24 hours." It wasn't until we got home and I studied both manuals that I realized a major difference between our two computers: my Sherwood starts counting the deco stop when I get within 10 feet of the prescribed depth. Hers doesn't; if it wants you at 10 feet, it starts counting at 10 feet and not an inch sooner. That's why it locked up on her: we did our deco stop by my computer, around 15-18 feet, so as far as hers knew we never actually performed a stop. So next time we do that, we'll want to go by her computer, since it's clearly MUCH more conservative than mine is.

That's one way to find out what your computer does; just fudge by a little, enough to throw the computer into that mode. If you're experienced enough and have enough air and are comfortable with such a scenario. If even the least little thing seems "off" about the idea, don't do it. Standard disclaimers apply.
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Re: Decompression and dive computer

Post by CaptnJack » Tue Aug 03, 2010 11:36 am

dwashbur wrote:So next time we do that, we'll want to go by her computer, since it's clearly MUCH more conservative than mine is.


Actually you could view this both ways:
Your computer gave you "credit" for time deeper.
Her computer wanted to get her to 10ft asap and wasn't going to count any time deeper for anything.

If you play with actual bubble deco software (like V-planner) you'll see that actual stop depth isn't too critical. As long as you "do the time" somewhere reasonable (within the offgassing zone) you'll clear the obligation.

In a pure dissolved model such as Buhlmann there's not enough pressure gradient to offgas until you are shallow.

Most people consider this latter profile type <more> aggressive since its hustling to get you as shallow as possible where yes your dissolved offgassing is maximal but any small bubbles you do form deeper will also grow the most on this kind of profile.

The "conservative" profile would combine the 2 outputs and do enough time at intermediate depths (~50-20ft) and would clear your computer (lowering the risk of bubble growth) and then do enough time shallow to clear her's (maximizing dissolved offgassing).
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Re: Decompression and dive computer

Post by Tubesnout23 » Tue Aug 03, 2010 2:59 pm

BillZ wrote:Hi Betty,
Here's the link to the free Suunto Dive Manager software. It's primarally used for downloading your dive data (the Gekko doesn't have the PC interface capability) but you can also create simulations and watch the readout on the Suunto computer interface.
http://www.suunto.com/en/Products/Softw ... e-Manager/


Thanks Billz! I will look at it when I don't have to wash the dishes and pack scuba gear to go diving in a couple of hours.
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Re: Decompression and dive computer

Post by Tubesnout23 » Tue Aug 03, 2010 3:17 pm

Hello folks! Thank you very much for your ‘dense’ replies. The conversation that has sprouted from my initial post is very interesting. Surely the amount of info is too much to chew and digest during one meal alone!

I am not planning to do any 'experiments' with computer-deco any time soon. My reason to ask that question in the first place is to understand my computer better. The manufacturer ‘drops’ the chapter about deco mode in the manual without adding much background info and I have gathered limited knowledge about it during my various scuba classes and past conversations with Sambolino44 (my primary buddy).

I guess that's why do agencies offer a class on computers?

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Re: Decompression and dive computer

Post by dwashbur » Tue Aug 03, 2010 3:20 pm

CaptnJack wrote:The "conservative" profile would combine the 2 outputs and do enough time at intermediate depths (~50-20ft) and would clear your computer (lowering the risk of bubble growth) and then do enough time shallow to clear her's (maximizing dissolved offgassing).


Excellent point, I hadn't really thought of that. Sounds like the best plan to me.

And anybody feel free to correct me if I'm wrong, but I get the impression that most of these dive computer algorithms are built more around boat diving, where you would be coming up a line at a fairly steady rate, rather than around shore diving where most people work their way back up a slope at a much slower rate. Hence, the ones that give us credit for slow ascent are fairly inconsistent about it. Am I off base in that notion?
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Re: Decompression and dive computer

Post by Joshua Smith » Tue Aug 03, 2010 3:54 pm

dwashbur wrote:
CaptnJack wrote:The "conservative" profile would combine the 2 outputs and do enough time at intermediate depths (~50-20ft) and would clear your computer (lowering the risk of bubble growth) and then do enough time shallow to clear her's (maximizing dissolved offgassing).


Excellent point, I hadn't really thought of that. Sounds like the best plan to me.

And anybody feel free to correct me if I'm wrong, but I get the impression that most of these dive computer algorithms are built more around boat diving, where you would be coming up a line at a fairly steady rate, rather than around shore diving where most people work their way back up a slope at a much slower rate. Hence, the ones that give us credit for slow ascent are fairly inconsistent about it. Am I off base in that notion?



Decompression algorithms are not built around the type of diving one does, they're built around physics, human physiology, and witchcraft. They might work better for line ascents, but they are not shaped by them.
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Re: Decompression and dive computer

Post by Raydar » Tue Aug 03, 2010 5:29 pm

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.

Yeah, your computer might show you in deco at the end of your deepest part, but by the time you wind your way up to the shallows for your safety stop, it's clean.

The most important thing to remember about your computer is that it's stupid and doesn't know anything about you. It's not a magical device that will keep you from a chamber ride. It doesn't know how much sleep you had last night, how much water/soda/beer you've drank, or when you last ate. It doesn't know if you're a 100 lb toothpick or a 300 lb butterball. All it knows is that based on these circumstances in a dive, this is what's likely the outcome in X% of people. (Then the manufacturers make it more conservative.)

The other most important thing to remember is that deco isn't a hard line in the sand. It's very, very, very fuzzy. I've bent the crap out of my Vyper and felt fine. I've been well within deco limits and felt like hammered whale dung.

*shrug* YMMV, TANSTAAFL, CKA, etc.
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Re: Decompression and dive computer

Post by BDub » Tue Aug 03, 2010 5:39 pm

To add to Ray's point, and in addition to going into deco without really understanding decompression, keep in mind that going into decompression changes your entire gas plan.

For those that dive Rock Bottom, Min Gas, etc, your ascent time has now increased and more gas is needed to complete the ascent properly.

So, that Min Gas you needed to get you to the surface in a no deco dive is now not enough to cover your decompression stops.
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Re: Decompression and dive computer

Post by lamont » Tue Aug 03, 2010 5:42 pm

dwashbur wrote: It wasn't until we got home and I studied both manuals that I realized a major difference between our two computers: my Sherwood starts counting the deco stop when I get within 10 feet of the prescribed depth. Hers doesn't; if it wants you at 10 feet, it starts counting at 10 feet and not an inch sooner. That's why it locked up on her: we did our deco stop by my computer, around 15-18 feet, so as far as hers knew we never actually performed a stop. So next time we do that, we'll want to go by her computer, since it's clearly MUCH more conservative than mine is.


I'd toss both of those computers. Those are examples of decompression algorithm silliness which has more to do with lawyers and less to do with physiology. If you go a few minutes into decompression and then float for 20 mins @ 30 feet and your computer hasn't cleared your deco obligation and demands a 4 minute stop at 10 feet, your computer has an issue with matching reality.

IMO that's actually bad since if you have more significant deco obligations and you're not credited for intermediate depth time then you're burning needless gas hanging out trying to satisfy the computer when you could be getting out of the water. Longer time in the water is not necessarily better -- not when you are low on gas, or need to communicate an emergency to the boat, or you're cold, or whatever. Its better to have accurate deco hang times than just the longest.

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Re: Decompression and dive computer

Post by lamont » Tue Aug 03, 2010 5:54 pm

BDub wrote:To add to Ray's point, and in addition to going into deco without really understanding decompression, keep in mind that going into decompression changes your entire gas plan.

For those that dive Rock Bottom, Min Gas, etc, your ascent time has now increased and more gas is needed to complete the ascent properly.

So, that Min Gas you needed to get you to the surface in a no deco dive is now not enough to cover your decompression stops.


assuming 2.0 cuft/min combined SAC rate and Al80s, you'd need 1250 psi (on top of your rockbottom) to complete 10 mins of deco at 20 feet.

( ( 20 / 33 + 1 ) * 2 * 10 ) * 3000 / 77.4 = 1245 psi

assuming 1.2 cuft/min combined SAC rate and HP130s, you'd need 500 psi to complete 10 mins of deco at 20 feet

( ( 20 / 33 + 1 ) * 1.2 * 10 ) * 3500 / 130 = 518 psi

of course you also need to be able to solve problems underwater and if you have an e.g. free-flow issue at the worst possible time you need to be able to manage that and get the affected diver out of the water while not violating your deco ceiling.

really, i'm descending into the area of the discussion where you need to get training to do deco dives...

however, long before i took any technical/cave training, i was working out how much gas it would take to deco out me and a buddy for 10 minutes on backgas, so that i knew what that value was. i never used that value in anger, but since i was pushing the NDL limits, i did work out the +10 minute contingency plan in case of an emergency.

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Re: Decompression and dive computer

Post by CaptnJack » Tue Aug 03, 2010 8:14 pm

Joshua Smith wrote:Decompression algorithms are not built around the type of diving one does, they're built around physics, human physiology, and witchcraft. They might work better for line ascents, but they are not shaped by them.


Yeah some computers I'd probably feel ok about blowing off 5 mins of "deco" - some computers might bend me in the "yellow".

Personally I feel that a slow ascent at no greater than 10ft/min from 50% of my recreational depth works for me and I feel better (and I have not used an actual computer in 7 yrs). Since buhlmann thinks such an ascent is far "too slow" and wants to race me up to 10ft, I've concluded buhlmann is hooey.
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Re: Decompression and dive computer

Post by loanwolf » Tue Aug 03, 2010 8:25 pm

BDub wrote:To add to Ray's point, and in addition to going into deco without really understanding decompression, keep in mind that going into decompression changes your entire gas plan.

For those that dive Rock Bottom, Min Gas, etc, your ascent time has now increased and more gas is needed to complete the ascent properly.

So, that Min Gas you needed to get you to the surface in a no deco dive is now not enough to cover your decompression stops.


Everyone has put out very good points.

Here is one that I feel needs to be added. No one should consider pushing into deco without a redundant system (aka pony bottle). That just in case, especially if you are one of those that pushes the gas management.
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Re: Decompression and dive computer

Post by lamont » Tue Aug 03, 2010 8:37 pm

CaptnJack wrote: 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?

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