Decompression Stop Guidelines - What we have to do if got deco alert?

[The Chairman]

We agree 1000%.

We are... as I posted, I was just adding a caveat, lest divers misconstrue what you posted.

But, I wouldn't dismiss the value of having the GTR info.

but at least they can make an informed choice based on real information.

This is perhaps the one thing we disagree on. I would use this info to verify my gas supply during normal diving and would not trust it should things go sour. These PDC calculated reserves don't, er can't adjust their numbers in lieu of possible panic and/or having to share your air with a panicked diver. I mean what do you do if you cut it so close with your deco only to find out your buddy blew it and now you're both on your gas? Ooopsie. Not only will you double your consumption by having two divers, but your OOA buddy is probably hyperventilating from his recent scare of sucking down that last PSI. He might be breathing four to five times normal and you watch in horror as your reserve simply goes up in bubbles. I can guarantee your PDC can't anticipate or keep up with those calculations.

@Doppler once opined that tech dives should be approached as a formal event. There are protocols and strategies that need to be observed and much of those are to prevent the diver from under estimating how much gas they really need. Many divers approach 5 minutes of deco with a fairly cavalier attitude in regards to what tech divers go through. They get away with it, so they do it over and over, thinking that they are gaining valuable experience. All they are really accomplishing is to create some bad habits.

 

[halocline]

Source: IDC Internship Blog - Blue Season


Be sure to participate in the right Divemaster Internship program while you’re in Bali, to learn how to face and excel in any underwater condition. I

Was this whole nutty post basically an advertisement for your DM internship program? Not a very good one if you ask me.

 

[dmaziuk]

I mean what do you do if you cut it so close with your deco only to find out your buddy blew it and now you're both on your gas? Ooopsie. Not only will you double your consumption by having two divers, but your OOA buddy is probably hyperventilating from his recent scare of sucking down that last PSI. He might be breathing four to five times normal and you watch in horror as your reserve simply goes up in bubbles. I can guarantee your PDC can't anticipate or keep up with those calculations.

If this was an unplanned contact of fecal matter with air displacement device, I don't see how not having an air-integrated computer would make it any less bad. If, OTOH, you did plan for just such contingency and have ample gas to get you both out of trouble, then all the PDC can tell you is you have enough gas.

This whole thing reminds me of the stories of people driving into rivers off demolished bridges because "the GPS shows there's a road here". Like, d'uh. Ever tried opening your eyes? "Computers make very fast very accurate misteaks", that's something they should teach outside tertiary Comp. Sci. curriculum.

 

[scubadada]

@guyharrisonphoto made some good comments regarding GTR and about knowing your own computer.

I have no experience with Scubapro computers, but, after more than 1250 dives with Oceanic AI computers, I can attest to the general accuracy of the GTR calculation. Your chosen safety stop (none, 3 or 5 minutes) or your deco obligation is taken into account in the calculation. Starting your normal ascent at a GTR of 0 will get you on the surface with near the end pressure you have chosen (300-1500 psi in 100 psi increments). If you happen to have GTR available, it's just one more piece of information you can use at your discretion.

Though Scubapro and Oceanic handle GTR identically, Shearwater handles it differently. Using a Perdix AI, a GTR of 0 means making a direct ascent to the surface will end the dive near your chosen end pressure, a safety stop is not included in the calculation. When there is a deco obligation, GTR is no longer displayed. If one was switching from a Scubapro or Oceanic computer to a Perdix AI, it would be good to know these differences.

 

[The Chairman]

I attended a recent Cave Diving conference, where the biggest take away was the concept that we tend to plan for when the dive goes RIGHT, whereas we should be planning for when the dive goes all WRONG!

 

[dmaziuk]

I attended a recent Cave Diving conference, where the biggest take away was the concept that we tend to plan for when the dive goes RIGHT, whereas we should be planning for when the dive goes all WRONG!

Agree, but in my work cost-benefit tends to be a log curve. First few simple steps double your chances of getting out safely, but very soon you get to the point where double the costs improve your chances by mere 0.x1%. I have hard time seeing an open water dive on, say, El Aguila in Roatan, planned using thirds on the off-chance that a big grouper might give somebody in the group a panic attack while I spend an extra minute photographing garden eels at the bottom. If you're going into a cave 60 metres down, the numbers are obviously different, but still: at some point you have to say "if this many things go wrong at the same time, I guess we're screwed".

 

[guyharrisonphoto]

I think everything said is valid, but off the point of the thread. The thread was not about planning a deco dive, or planning gas sharing, or whether GTR works in other circumstance such as panic, air sharing, etc, or any of the other parade of horribles that are possible in any worst case dive scenario.

The thread was about what a recreational (ie not technically trained or equipped) diver can do when they themselves when they are faced with an unplanned deco obligation, for whatever reason. For this, the GTR is valuable, in fact, very valuable, to avoid panic and allow reasoned decision making n balancing the deco obligation against available gas.

But, just to say, GTR does work in an air share situation, as it calculates the GTR based on the volume of gas being drawn from the tank over time, and so is self-regulating for depth, increased breathing rate, and even the number of divers using the tank. And, yes, it will keep up with the situation (maybe a few seconds time lag to catch up with the new consumption rate), far more quickly and accurately than trying to estimate. So, if you hit unplanned deco, do need to air share, and a stressed buddy is hoovering up your shared gas, the GTR will account for that, so you know if or when you have to abort your deco stops and head for the surface.

Nothing beats planning and following the plan. But when the unplanned is upon you, having accurate real time information to rely on is better than any other alternatives. That's all I meant to say.

 

[lermontov]

@guyharrisonphoto made some good comments regarding GTR and about knowing your own computer.

I have no experience with Scubapro computers, but, after more than 1250 dives with Oceanic AI computers, I can attest to the general accuracy of the GTR calculation. Your chosen safety stop (none, 3 or 5 minutes) or your deco obligation is taken into account in the calculation. Starting your normal ascent at a GTR of 0 will get you on the surface with near the end pressure you have chosen (300-1500 psi in 100 psi increments). If you happen to have GTR available, it's just one more piece of information you can use at your discretion.

Though Scubapro and Oceanic handle GTR identically, Shearwater handles it differently. Using a Perdix AI, a GTR of 0 means making a direct ascent to the surface will end the dive near your chosen end pressure, a safety stop is not included in the calculation. When there is a deco obligation, GTR is no longer displayed. If one was switching from a Scubapro or Oceanic computer to a Perdix AI, it would be good to know these differences.

which DC doesnt display the GTR when having a deco obligation? this surprised me it cant be difficult for the DC to compute this, how then if you've got different f02 in 2-3 bottles ( assuming you need a receiver per bottle?) does it calculate the GTR allowing for the gas switches

 

[boulderjohn]

which DC doesnt display the GTR when having a deco obligation? this surprised me it cant be difficult for the DC to compute this, how then if you've got different f02 in 2-3 bottles ( assuming you need a receiver per bottle?) does it calculate the GTR allowing for the gas switches

This thread is about unplanned decompression stops on a recreational dive.

 

[The Chairman]

I have hard time seeing an open water dive on, say, El Aguila in Roatan, planned using thirds

I believe I said I would use @ two thirds in those circumstances. For a rec dive, I find 10psi/foot is adequate for my thumb pressure, with 600psi min for 60 ft and shallower.