Rock bottom, 500 PSI, or something else?

[Lobzilla]

I guess I must have misunderstood. I got the idea that for calculating rock bottom, the assumption was that in an emergency, the divers would go through the entire min deco profile, which you also said was only done as a training strategy for future decompression diving. I would not think that an OOA emergency would be the best time to work on skill acquisition for future decompression diving.

I also don't understand why 10 FPM is used for an OOA emergency.

Let's not get hung up on what certain agencies practice in certain classes.

What matters is that any diver is able to calculate the critical gas reserve based on expected consumption at depth and can make an informed decision when to ascend/turn.

In reality, the turn pressure depends entirely on the conditions of the dive and we need to get a grip on the math to be able to make a responsible decision. In a team of three, I cut it closer than as a pair. For entering a familiar cave against the flow, turning at 1/3 is conservative. With the flow, even 1/6 can get really tight. There is no one right "canned" answer. You need to do the analysis for every dive and for that you need some tools.

The tools taught by GUE or UTD (shown above) make this a whole lot simpler than for example this article. Use the simplest, correct tools with whatever numbers your team feels comfortable with. If you want to do a 30ft/min ascent with safety stop, the tools (CAT and Tank Factor) will still give you the right numbers.

 

[boat sju]

Maybe tech divers and instructors shouldn't be allowed to post in the "basic" forum. Oops, did I type that out loud.

 

[Lobzilla]

Maybe tech divers and instructors shouldn't be allowed to post in the "basic" forum. Oops, did I type that out loud.

Strongly disagree with that.

Critical life support topics have been dumbed-down in the main-stream open water world to the point of getting dangerous. On the other side, tech divers have simple and easy to understand tools to get a grip on critical issues. Unless you think that drowning is somehow more pleasant for an open water diver than for a cave or technical diver, you would be a fool not to learn and use these tools.

Even as we digressed into deco procedures it was IMO valuable for the OW divers to realize that even for them the maximum ascent speed has a physiological limit. The OW diver is actually more at risk because you can fix decompression sickness but you cannot fix an arterial gas embolism resulting from a totally botched ascent.

 

[PfcAJ]

Maybe tech divers and instructors shouldn't be allowed to post in the "basic" forum. Oops, did I type that out loud.

Oh yeah, sorry. Let’s keep it basic.

Back on the boat with 500psi and ascend no faster than yer smaller bubbles.

Ok good luck have fun!

 

[boulderjohn]

Let's not get hung up on what certain agencies practice in certain classes.

What matters is that any diver is able to calculate the critical gas reserve based on expected consumption at depth and can make an informed decision when to ascend/turn.

Maybe I'm mistaken, but I thought the topic this thread was planning minimum reserve for an NDL dive. If agency #1 is planning minimum reserve based on an ascent at 30 FPM, and agency #2 is panning minimum reserve based on an ascent of 10 FPM, that is going to make a significant difference in the amount of gas reserved. If my agency is telling me that I need to ascend at 10 FPM in an OOA emergency and therefore need to plan a reserve that will accomplish that, then I will want to know why they picked that number.

 

[PfcAJ]

Maybe I'm mistaken, but I thought the topic this thread was planning minimum reserve for an NDL dive. If agency #1 is planning minimum reserve based on an ascent at 30 FPM, and agency #2 is panning minimum reserve based on an ascent of 10 FPM, that is going to make a significant difference in the amount of gas reserved. If my agency is telling me that I need to ascend at 10 FPM in an OOA emergency and therefore need to plan a reserve that will accomplish that, then I will want to know why they picked that number.

If agency 1 and agency 2 assume different SAC rates for that ascent it might not be all that different.

 

[2airishuman]

It seems odd to use a sac and acsent rate that are both unrealistic just because they balance out and produce a reasonable answer.

 

[Lorenzoid]

It shouldn’t be. It should be 30fpm to your first stop.

GUEs calculation method uses 10fpm and a .75 sac rate. It just so happens to give (close too, at least) the same value at the end of the day as a 1.0 sac rate with a 30fpm rate to the first stop and 10fpm thereafter.

I don't mean to add to the confusion between the theoretical 10fpm ascent rate for gas calculation purposes and actual ascent rate between stops, but I think I was taught to ascend at 30fpm to the first 30-second stop (at half maximum depth) and take 30 seconds to get from each 30-second stop to the next. That's not 10fpm between successive stops, but it evens out to 10fpm over multiple 30-second stops. That's for recreational dives for which most non-GUE people would say to just do a continuous 30fpm ascent to the safety stop, etc.

 

[stevensamler]

Ascent speed of 10ft/min seems excessively slow, especially in an emergency situation on a no stop dive.

30 second stop 30 second move to the next 10' shallower stop , it's pragmatic and controlled.

 

[boulderjohn]

If agency 1 and agency 2 assume different SAC rates for that ascent it might not be all that different.

SAC rates and ascent rates are entirely different things. I would think that the best idea would be to select the SAC rate which is most likely to produce a safe result, and an ascent rate which is most likely to produce a safe result. Then put the two together to see what happens. I do see the point in simplification--if someone were to magically determine that 0.83 is the best planning SAC rate, I would not be in favor for that.