New Shearwater Dive Modes.

[scubadada]

The key is gases that are TURNED ON.

 

[MyBuddy]

FFS, I even circled it red for you.

“INCLUDING NDL”

You can pull that up right from Perdix 2 RECREATIONAL MANUAL, which is separate from the technical manual.

FFS, Those are two different paragraphs.

The portion you circled in the 2nd paragraph states “…time to surface, decompression stops and decompression time…” there’s nothing in the second paragraph about NDL.

The first paragraph is referring to what calculations are based on predictions which include NDL. It is not saying that the programmed, but not selected gas, is playing a part in calculating your NDL.

 

[ReefHound]

If you program 36/32/21 on the computer and have them turned on, the computer will calculate your NDL based on the fact you will use those 3 gasses on the dive.
But if you then do a dive to 130 ft and all you have with you is 21%, your actual NDL will be much shorter than what the computer would be telling you during the dive.

Heres an easier way to visualize it, which would have a longer NDL:
A dive to 90ft with 36%
A dive to 90ft with 21%

Having all 3 gasses on but 21% selected is not the same as telling the computer the only gas you have with you is 21%.

Let's say you have 36/32/21 and you do three 60 minute dives, same profile.

Dive 1, 32% for 40 minutes, 36% for 10 minutes, and 21% for 10 minutes.
Dive 2, 32% for 20 minutes, 21% for 10 minutes, and 36% for 30 minutes.
Dive 3, 32% for 10 minutes, 21% for 40 minutes, and 36% for 10 minutes.

Would it calculate each dive the same? If not, then clearly how long each gas it used (including 0 minutes) and the order each gas is used must be known. Did you input that information before the dive? And when it is calculating, how does it know at what depth you will be at 40 minutes or even if your dive will last 40 minutes? It seems to me the only way to even perform a calculation would be on what you are currently doing in addition to what you have already done.

 

[Bald Wookie]

This is my understanding:
Lets use two different gases, programed "ON" in the computer, but able to use at similar depths - 21% and 50%. You have 21% as your back gas and you're slinging a 50% bottle. Don't fight me on the configuration here.
The computer is not going to calculate your NDL at say, 60 or 70ft (common dive depths at many locations) using the 50% mix if you currently have the 21% selected as your breathing gas. It will likely flash yellow at you telling you you have a better gas mix available though.
When you go past your NDL and into DECO, it will calculate your deco obligation assuming that you have the intention of switching the 50% mix. If you don't it will continue to calculate your deco when you reach the stops based on the 21%, but you will not get accurate stop information. (I may be incorrect about this last sentence, but I would also like clarification if I am wrong).

 

[Pavao]

I was wrong on the NDL, it will not be affected by the gases you have turned on, it will give you an NDL based on the gas you have active.
If you do go into deco for any reason, then the profile will be dependent on the gases that are turned on.

Whether it’s a good idea to leave them turned on is another matter.

I don’t dive my shearwaters on recreational dives, sorry for the confusion

 

[Miyaru]

I was wrong on the NDL, it will not be affected by the gases you have turned on, it will give you an NDL based on the gas you have active.
If you do go into deco for any reason, then the profile will be dependent on the gases that are turned on.

Whether it’s a good idea to leave them turned on is another matter.

I don’t dive my shearwaters on recreational dives, sorry for the confusion

...and there was light.

 

[Scared Silly]

Well, sorry to say but you’re not using your computer properly. When you program those 3 gasses and go do a dive, the computer is calculating your decompression based on having those 3 gasses with you on the dive and that you will make 2 gas switches.

Late to the party but as said, decompression is calculated based on your actual dive profile. It has no idea when or even if a gas switch will happen. That said, in these cases I am planning no-stop dives so I am not concerned.

 

[Capt Jim Wyatt]

decompression is calculated based on your actual dive profile.

That is almost true.

It is in fact based on what it thinks your dive profile is, based on the gas it thinks you are breathing and what GFs you have set.

If you are breathing air and you have EAN32 programmed as the active gas your NDL will be much longer on the computer than it really is.

Decompression is actually based on what gas you are telling it you are breathing and the GF settings.

Maybe that is what you said.

 

[MyBuddy]

Decompression is calculated based on your actual dive profile. It has no idea when or even if a gas switch will happen.

Yes, decompression is calculated based on your actual dive profile BUT your computer’s predicted decompression stops/profile and TTS is based on the assumption that you will switch to a richer gas once you ascend to the richer gas’s MOD for the PPO2 selected for deco. If the rich gas is not utilized for deco or the computer not told of the switch, the actual deco time and TTS will be longer, often significantly, than predicted.

 

[Pavao]

Late to the party but as said, decompression is calculated based on your actual dive profile. It has no idea when or even if a gas switch will happen. That said, in these cases I am planning no-stop dives so I am not concerned.

Well, yes and no. The statement you are quoting me on this response is correct, but it’s not correct in relation to what I was quoting you on, which was about NDL, I was wrong about the NDL predictions.

So the yes and no above, “decompression is calculated based on your dive profile” yes (plus other factors of course)
But, “It has no idea when or even if a gas switch will happen” is incorrect, at least as far as the decompression calculation is concerned, in fact the computer assumes you will make the gas switch within 5 seconds (seems short to me) of reaching that gases MOD.

But the computer can’t know if or when you’ll make the gas switch, I think that’s what you’re trying to say, but has to make the calculations based on the assumption you know what and when to do the switch, after all, you told the computer what your plan is by entering the values you did.