Overshooting NDL and mandatory deco stops
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I must have misunderstood completely, then. As far as I could make it out, @PfcAJ 's post which I quoted was answering my post about minimum tank pressure for a rec diver doing emergency deco. And that was the basis for my slightly philosophical post about min pressure recommended in imperial and metric country, respectively.
I'd really appreciate if you could be more specific about which point I was missing here.
@ScubadriverDale As usual, this thread has veered into the weeds. To address your original question, I'm attaching the profile of a dive through the "Devil's Throat" in Cozumel a few years ago. (It was my 25th dive and a total trust-me dive - I don't recommend it). I didn't have my Perdix yet, but was using an Amphos Air PDC (Pelagic Systems - type). The red region is when I slipped into "light" deco. The little boxes show what my computer would have shown at the 4 points I highlighted with arrows. Note that my deco obligation (mandatory - 3min at 10ft) was cleared by the time we had (slowly) worked back up to 40 ft. After that, it showed NDL time, which continued to increase during the safety stop. The only time I've had the Perdix in deco was in tech mode, so I can't show you exactly what it would say in rec-mode for "light deco". Probably quite similar to the Amphos.
And yes, I know I got back on the boat with less than 500 psi... so sue me!
And yes, I know I got back on the boat with less than 500 psi... so sue me!
PfcAJ
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The other thing to remember with "going into a little deco" is that " a little" can be a LOT for one person. In deco training, there are additional factors to consider.
On the Perdix in Rec mode, I set it for high conservatism when diving with students who are using it as a loaner.
I have my OW students using my Perdixes as loaners if they don't want to buy one right away and now that I am solely teaching SDI, computers are required. I use my Predator and either the old Oceanic Veo, Cosmiq, or bottom timer and tables.
We also plan and track training dives on the SDI version of the Navy tables. Usually, we are diving cool water with low vis so there is an added bit of stress. I've also been getting older students and with age comes additional risk factors.
Less fit than they were in their 20's, there may be circulatory issues for example that will effect off gassing, and these need to be taken into consideration.
Gradient Factors affect NDL's and SI times. Usually not an issue unless someone is inattentive enough to not pay attention to the dive plan and get close to the NDL's.
Then it's a good idea to understand why the different modes are used and when. As well as what those GF's mean.
People can choose not to. That's up to them.
I don't go into the amount of detail that I do in an AN/DP class with OW students. It's not necessary. But we do discuss different models and the reasons they are used. We also stress that these are models. Mathematical models. Some with a great deal of human research while at the same time realizing that everyone is different. Which is why 100 divers will do the exact same dive and some will get bent and others won't.
You can't rely on the computer to keep you safe. As Pete said, you need to do that. Doing that means understanding what you are doing and what you are using. Whether it's a computer, sealed or unsealed reg, or a left-handed smoke shifter vs a right-handed one.
If a diver can't tell me why they are using a piece of gear in a class, they are not using it. Because it's easy to use is not good enough. Explain to me why it's easy to use and that you understand why that's so.
Buying a computer like the Shearwater and not understanding why the defaults are what they are and how to verify they are what they should be is not using the computer. It's blind trust.
I would highly doubt one would come from them with a setting not what it should be, but it could happen. Which is why they say, in plain English in the manual, it is not a matter of if this computer will fail, but when.
That "when" could be the first time you turn it on.
And again, that default mode may not be right for you. You don't know. It may or may not be conservative enough for you.
Understanding the algorithm on a computer like this is not going back to college. It's educating oneself to get the most out of the computer and not get hurt.
On the Perdix in Rec mode, I set it for high conservatism when diving with students who are using it as a loaner.
I have my OW students using my Perdixes as loaners if they don't want to buy one right away and now that I am solely teaching SDI, computers are required. I use my Predator and either the old Oceanic Veo, Cosmiq, or bottom timer and tables.
We also plan and track training dives on the SDI version of the Navy tables. Usually, we are diving cool water with low vis so there is an added bit of stress. I've also been getting older students and with age comes additional risk factors.
Less fit than they were in their 20's, there may be circulatory issues for example that will effect off gassing, and these need to be taken into consideration.
Gradient Factors affect NDL's and SI times. Usually not an issue unless someone is inattentive enough to not pay attention to the dive plan and get close to the NDL's.
Then it's a good idea to understand why the different modes are used and when. As well as what those GF's mean.
People can choose not to. That's up to them.
I don't go into the amount of detail that I do in an AN/DP class with OW students. It's not necessary. But we do discuss different models and the reasons they are used. We also stress that these are models. Mathematical models. Some with a great deal of human research while at the same time realizing that everyone is different. Which is why 100 divers will do the exact same dive and some will get bent and others won't.
You can't rely on the computer to keep you safe. As Pete said, you need to do that. Doing that means understanding what you are doing and what you are using. Whether it's a computer, sealed or unsealed reg, or a left-handed smoke shifter vs a right-handed one.
If a diver can't tell me why they are using a piece of gear in a class, they are not using it. Because it's easy to use is not good enough. Explain to me why it's easy to use and that you understand why that's so.
Buying a computer like the Shearwater and not understanding why the defaults are what they are and how to verify they are what they should be is not using the computer. It's blind trust.
I would highly doubt one would come from them with a setting not what it should be, but it could happen. Which is why they say, in plain English in the manual, it is not a matter of if this computer will fail, but when.
That "when" could be the first time you turn it on.
And again, that default mode may not be right for you. You don't know. It may or may not be conservative enough for you.
Understanding the algorithm on a computer like this is not going back to college. It's educating oneself to get the most out of the computer and not get hurt.
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What a thread! My strong bias is for divers to know how their computers work and what their computers are telling them. I have posted on many threads attempting to help divers understand this, several have replied to me that they are simply not interested. The Shearwater rec mode default setting of 40/85 would generally be considered moderate in the conservative to liberal spectrum and would generally keep the most ignorant diver relatively safe while doing no stop diving.
johndiver999
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@ScubadriverDale As usual, this thread has veered into the weeds. To address your original question, I'm attaching the profile of a dive through the "Devil's Throat" in Cozumel a few years ago. (It was my 25th dive and a total trust-me dive - I don't recommend it). I didn't have my Perdix yet, but was using an Amphos Air PDC (Pelagic Systems - type). The red region is when I slipped into "light" deco. The little boxes show what my computer would have shown at the 4 points I highlighted with arrows. Note that my deco obligation (mandatory - 3min at 10ft) was cleared by the time we had (slowly) worked back up to 40 ft. After that, it showed NDL time, which continued to increase during the safety stop. The only time I've had the Perdix in deco was in tech mode, so I can't show you exactly what it would say in rec-mode for "light deco". Probably quite similar to the Amphos.
And yes, I know I got back on the boat with less than 500 psi... so sue me!
That is a very useful and interesting profile you posted and it seems to be a very valid example of how a super slow ascent (after a diver reaches required deco) can actually clear the deco well before the deco stop is reached.
On the other hand, that looks like a pretty aggressive profile for a recreational diver. A recreational diver can come up slow, but that ascent shows a recreational diver staying between 80 and 50 feet for like 12 minutes? (my estimate from a quick look at the profile). I don't think that kind of ascent is at all recommended for a recreational diver, and might not be giving the OP the answer he is looking for.
I agree - the Devil's Throat isn't a great dive for recreational divers. That was almost 4 years and about 150 dives ago, so what was "fun" at the time strikes me as reckless now. You enter a swim-thru at about 85 ft, follow it and pop out around 130 ft, immediately go up to ANOTHER swim-thru and back down to 120. THAT deep dive playing follow-the-leader with a guide now scares the crap out of me. But the long, slow ascent after the deep stuff was a relaxing drift to where the boat would pick us up. What about the ascent strikes you as aggressive for rec divers?
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