New Shearwater Dive Modes.

[ReefHound]

I think the computer prompts you to change the tank to the tank with the gas mixture based on your depth - it selects from the 3 tanks it thinks you have.

I think it doesn't. And even if it did, the calcs wouldn't change unless you accepted the prompt.

 

[Dsix36]

Surgical tubing also has more stretch than bungee. You can set a length for wet suit and it will still work great with a drysuit. No need for adjustment at all. It stays put better too.

 

[Manatee Diver]

How exactly is it going to do that when it doesn't know WHEN or IF you will switch?

SRI's TTS calculation is based on switching to deco gasses at a PPO2 of 1.6. IIRC stop times are also based on making those switches. It will still only clear stops based on the gas you are breathing. It will prompt you that a "better" gas is available by applying a yellow background to the current gas display.

Also another IIRC but I believe it bases deco gases if it has over 50% oxygen content, so it doesn't apply it to hypoxic deco gasses that would be bottom gasses for other dives.

Thus it is best to turn off gases that you don't have on you. Like I have 18/42 and 50% in my computer based on a recent dive I did. But those are set to turned off, only 30% and 96% are turned on.

It should be in the manual, but I'm too lazy to look it up right now.

 

[Tracy]

How exactly is it going to do that when it doesn't know WHEN or IF you will switch?

The computer assumes some level of competence when in multi gas modes. If you have the gas turned on, it assumes you will use it and bases time to surface estimates on that.
If you have three gases turned on and are only carrying one, your tts will not be accurate.

 

[inquis]

How exactly is it going to do that when it doesn't know WHEN or IF you will switch?

It assumes you WILL switch to the available gas with the highest PO2 that does not exceed 1.6 bar when in deco. The ONLY thing impacted is the Time To Surface (TTS) prediction [ETA: and a prompt to switch]. As such, it IS a convenient way for NDL divers to select their nitrox% (in spite of the blanket admonition in the manual to only enable gases you're actually carrying).

 

[iointerrupt]

But it isn't just TTS that is affected, all the Stop times will be completely wrong too. The blanket admonition is quite critical:

https://www.shearwater.com/wp-content/uploads/2022/06/Perdix-2_RecreationaL_Manual_RevB.pdf

Page 49: "The decompression algorithm assumes that the diver is carrying and plans to use every gas that is currently turned on. Leaving gases that are not expected to be used turned on will result in inaccurate time to surface, decompression stop and decompression time information being displayed".

You will get nicely bent if it thinks you are carrying 100%, gives you a 5 minute stop, and instead you spend that 5 minutes on 21% and then get out.

 

[ReefHound]

The computer assumes some level of competence when in multi gas modes. If you have the gas turned on, it assumes you will use it and bases time to surface estimates on that.
If you have three gases turned on and are only carrying one, your tts will not be accurate.

Estimates. But the point is your actual calculations will only be on the gas you are using or have used.

 

[Tracy]

You will get nicely bent if it thinks you are carrying 100%, gives you a 5 minute stop, and instead you spend that 5 minutes on 21% and then get out.

Yes, if you just do five minutes and get out.
If you don't switch the computer to the new gas, it will calc based on what you tell it you are breathing. It just won't be a 5 minute stop like it originally showed.

 

[L13]

You will get nicely bent if it thinks you are carrying 100%, gives you a 5 minute stop, and instead you spend that 5 minutes on 21% and then get out.

Not really. It will continuously update your stop time, extending it as long as you are on 21% until you have completed an adequate stop for 21%. (Though the continuous update will always be shorter than the actual stop time until the end, since at each update it will assume that you will switch to the recommended gas at that time).

As long as you don't lie to the computer about what you are actually breathing, it won't tell you to go to a shallower stop until it is safe (per your GF selection).

 

[Tracy]

Estimates. But the point is your actual calculations will only be on the gas you are using or have used.

Yes, that is correct. But for myself and many others, TTS is how the dive is turned. If that isn't accurate, you may be doing a much longer dive than you planned.