Air Integration & NDL Accuracy?

[Tribal]

Hi KenGordon,
what would be your answer to the following statement:
- Perdix AI in OC REC, Transmitter, Singe Tank Nitrox 32%
- Dive planned with a stop of 3min at 40m to look at a nice cave entrance
- NDL is reached after 3min, as planned, so deco stop mandatory. GTR is OFF (as deco stop)
- my buddy is having an issue with his equipment OR make up another fact that makes me stay longer than 3min at -40m
- what data should I monitor to "estimate" how long more I could "reasonably" stay at 40m ?

Nothing sarcastic here, I hope you understand my point and the fact that I want to learn something from experienced divers.

Let’s assume something happens at 40m, your buddy lost his gas.
You will use more air in that situation, let’s say 30l/min x 5bar so 150l/min. Before you are ready to ascent you need 2 minutes to get ready, so 300liter.
Since you have no deco gas you will have to make it to the surface. If you don’t stop for deco you will need 4 minutes on a average depth of 20m. That’s 30l/min x 3bar x 4min, so another 360liter. 660liter x 2 for your buddy, that’s 1320 liter or close to 120bar on a 80cft.
Don’t forget you have to solve you problem in a smooth way, don’t panic, have a smooth ascent and skip deco to make it with this amount of air.

Long story short, don’t do those kind of dives without the right training and gear for it.

 

[Jan_oi]

Thank you all for your replies.
I'm quiet astonish (and worried to be honest) about my last 130 dives that I've done within the past 3 years mainly in the red see (Egypt and Sudan): single tank, 12L, 220 bar, nitrox (28%-31%), always on a live-aboard.
The first dive in the morning very often hits between 35m and 40m (sometimes more). The second and third dives of the day are usually shallower. Of course the dives are planned, one DM for 3/4 divers max. It seemed to me to be the "normal" diving there. And yes it's supposed to be no deco dives but yes, it happens to see the NDL at 0 and thus go through deco stops very carefully.
And this is exactly the reason why I just got the Perdix AI to try to learn more, study more, thus dive in a safer way (even though I and thousand of daily divers in the Red Sea don't feel in danger, I guess!).
So I'm trying to understand all the data that the Perdix AI in REC, and even more in TEC mode, can provide. On top of the dive's planning of course.

 

[doctormike]

And yes it's supposed to be no deco dives but yes, it happens to see the NDL at 0 and thus go through deco stops very carefully.
And this is exactly the reason why I just got the Perdix AI to try to learn more, study more, thus dive in a safer way (even though I and thousand of daily divers in the Red Sea don't feel in danger, I guess!).

You can do what you want, but I feel an obligation to respond for the benefit of any recreational divers reading this thread. This is the very definition of normalization of deviance.

To do deco safely you need the appropriate training, planning and equipment. You need to be able to complete your decompression obligation even if you lose your primary gas supply. CESA is no longer a viable option. There is nothing that the Perdix or any other dive computer can give you that will make up for the basics of technical diving. Even if you go through your stops "very carefully".

 

[CaveSloth]

You can do what you want, but I feel an obligation to respond for the benefit of any recreational divers reading this thread. This is the very definition of normalization of deviance.

To do deco safely you need the appropriate training, planning and equipment. You need to be able to complete your decompression obligation even if you lose your primary gas supply. CESA is no longer a viable option. There is nothing that the Perdix or any other dive computer can give you that will make up the basics of technical diving. Even if you go through your stops "very carefully".

It's comments like this that make me confident in my decision to completely avoid deco until I have training in deco. I just don't know what I don't know. There are procedures and planning around it and I would want to know those before I just "go for it".

 

[Belzelbub]

And yes it's supposed to be no deco dives but yes, it happens to see the NDL at 0 and thus go through deco stops very carefully.
And this is exactly the reason why I just got the Perdix AI to try to learn more, study more, thus dive in a safer way (even though I and thousand of daily divers in the Red Sea don't feel in danger, I guess!).
So I'm trying to understand all the data that the Perdix AI in REC, and even more in TEC mode, can provide. On top of the dive's planning of course.

Good move in trying to learn more. I recently got a Perdix as well. Probably overkill for how I'm diving at the moment, but all my research was leading me to it.

The benefit of the DC is in providing real-time information during your dive. Without one, if you find out on the surface that you skipped a deco obligation, it's kind of too late. At least with a DC, if you find you have an obligation, you can fulfill it while still in the water. That's much better than the alternative. However, it should be recognized that this was a failure somewhere. Your DC will also alert you to time left before going into deco. That's the point to make adjustments, not after.

The way you originally asked seemed somewhat of a cavalier attitude, as if you had planned a no-deco dive, found yourself in deco, but still wanted to hang out for a bit. That's just asking for trouble. Even staying within the NDL is no guarantee that you won't come up with DCI. I've seen it happen to another diver diving the same profile. I still had about 15 minutes left at that depth, but another diver came up with an issue and had to go to the chamber.

 

[doctormike]

The benefit of the DC is in providing real-time information during your dive. Without one, if you find out on the surface that you skipped a deco obligation, it's kind of too late. At least with a DC, if you find you have an obligation, you can fulfill it while still in the water. That's much better than the alternative.

Again, not sure I understand. There are advantages of a dive computer, but I'm assuming that anyone who isn't using one is doing some sort of planning to know their NDL for the presumed square profile, right?

I guess a DC is good for that if your alternative is "swim around until you run out of gas, then ascend".

 

[Fastmarc]

Again, not sure I understand. There are advantages of a dive computer, but I'm assuming that anyone who isn't using one is doing some sort of planning to know their NDL for the presumed square profile, right?

Unfortunately the reality I experience is those who do not have a dive computer swims around until they see everyone ascending or are told to ascend. That is of course they do not run low on air and have to be assisted to the surface.

 

[doctormike]

Unfortunately the reality I experience is those who do not have a dive computer swims around until they see everyone ascending or are told to ascend. That is of course they do not run low on air and have to be assisted to the surface.

Hah! And even more unfortunate is that some people who DO have a dive computer don't really understand what it does. I remember being on one dive boat where someone asked the DM if he could fix her new computer, because it kept beeping...

 

[scubadada]

...I guess a DC is good for that if your alternative is "swim around until you run out of gas, then ascend".

Back in the day, we did OK with our J-valves. It was 2 years before I had a SPG

 

[doctormike]

Back in the day, we did OK with our J-valves. It was 2 years before I had a SPG

The point wasn't about gas volume, it was about nitrogen loading and NDLs.

I'm assuming that back in the day, you had a plan for that, right? You didn't just swim around until you ran out of gas...