Question Perdix Experts - Question for You...

[Belzelbub]

Are you saying that when the Perdix was first introduced there was no single gas "Air" mode or single gas "Nitrox" mode?

Yes. That is correct. There was just tech mode and rec mode. Rec mode is identical to what is now called 3 gas Nx.

Why would any non-tech diver need a 3-gas mode since gas switching (for whatever purpose, e.g., accelerated deco) is strictly a tech diving practice?

For me, I have no real use for 3 gas mixes. Two at most, and only in certain situations. Mainly if I’m carrying a pony that happens to have a different mix than my main tank. I might see that if the shop in the Keys that filled my tanks is out of O2. Not critical.

 

[arkstorm]

There are probably more people doing switching gases without tech c-cards than the ones with tech c-cards around the world especially with one deco bottle.

Maybe, but you don't need a tech certification to use OC Tech mode on your computer.

 

[inquis]

Multiple gas slots are useful for single-gas users as a quick way to pick from their most often used mixes. 21, 32, and 36% will cover most areas. Just turn the old off / new on.

At the risk of derailing the thread, for NDL dives I leave them all enabled and just activate the one being used. No, this does not invalidate the NDL time. Search for past discussions if you disagree or just ignore this tip.

 

[arkstorm]

Multiple gas slots are useful for single-gas users as a quick way to pick from their most often used mixes. 21, 32, and 36% will cover most areas. Just turn the old off / new on.

At the risk of derailing the thread, for NDL dives I leave them all enabled and just activate the one being used. No, this does not invalidate the NDL time. Search for past discussions if you disagree or just ignore this tip.

From the Shearwater Perdix 2 Recreational Manual chapter 7.2 Multi-Gas Example Dive, at pg. 32:

This is an example of displays that might be seen on a
multi-gas decompression dive in 3GasNX Mode.
Max Depth: 40 meters Bottom Gas: 28% O2
Bottom Time: 20 minutes Deco Gas: 50% O2

1. Gas Setup - Best practices include checking your
gas list before each dive. This screen is available in the
Nitrox Gases section of the System Setup menu. All
gases that are turned on will be used to calculate the
decompression schedule. Turn off gases you are not
carrying. Note that the MOD displayed on this screen
will only impact the bottom gas (28% O2). Deco gases
are governed by Deco PPO2.

8. Gas Change - All decompression predictions are
made assuming you will switch to the best available
gas on ascent. At 21m, the breathing gas turns yellow
indicating that a better breathing gas is available.
If the switch is not made, deco stop and time
information will be inaccurate.

(Emphasis provided.)

 

[arkstorm]

Multiple gas slots are useful for single-gas users as a quick way to pick from their most often used mixes. 21, 32, and 36% will cover most areas. Just turn the old off / new on.

...So if you set up your three favorite mixes, e.g., 21, 32 and 36, the computer will always be calculating your nitrogen loading limits, i.e., NDL, as though you are going to use 36 whenever you are within the MOD for 36.

This is only a problem if you leave all three gases set to on.

 

[arkstorm]

Slight thread creep as this still doesn't answer my original question.

 

[Belzelbub]

...So if you set up your three favorite mixes, e.g., 21, 32 and 36, the computer will always be calculating your nitrogen loading limits, i.e., NDL, as though you are going to use 36 whenever you are within the MOD for 36.

This is a problem.

He said NDL dives. Not deco dives. The computer will handle NDL calculations using the gas selected.

For deco dives, only the gasses with you should be on.

 

[arkstorm]

He said NDL dives. Not deco dives. The computer will handle NDL calculations using the gas selected.

For deco dives, only the gasses with you should be on.

Yes, you are correct. I only noticed at second glance that he said he sets the gases he's not using to off.

 

[inquis]

for NDL dives I leave them all enabled and just activate the one being used.

(Emphasis added)
They are all On/Enabled/Available (same thing). It only assumes you will switch to a richer mix -- and even prompts you to do so -- when a deco ceiling is present.

Slight thread creep as this still doesn't answer my original question.

I was addressing your secondary question:

Why would any non-tech diver need a 3-gas mode

based on my experience that it is useful in an NDL context.

 

[arkstorm]

based on my experience that it is useful in an NDL context.

I'm not sure its any more difficult to just change the FO2 in single gas nitrox mode than it is to pre set your favs in 3-gas mode.

In any event, based on the Shearwater manual, it is clear that the intended purpose of 3-gas mode is for deco dives so why wouldn't it just be best practice to use OC Tech mode?

I just don't think its only clever marketing. Usually, Shearwater is pretty on point when it comes to function and feature but I just can't figure this one out.

Like I said in my original post, when in 3-gas mode there are certain things you can't do that you could do in OC Tech, e.g., change GF during the dive. But why limit out that feature and maybe other in 3-gas mode if its intended for deco dives?

Just very curious to me.