question re. Oceanic ProPlus 3 gas switch (use of QD while submerged)

[rmssetc]

I've got a new-to-me Oceanic ProPlus 3 and I'm curious about one feature.

This is a hosed (console) AI computer, with a quick-disconnect fitting.

The computer allows you to define up to 3 gases, each with their own % O2 and FO2 setting. The 3 set gases are supposed to be used during the same dive, and the computer has a "switch gas" sequence of commands.

While I'm not planning to dive multiple tanks or use this feature, I'm trying to understand the actual process and what it implies about the computer design. As I understand, the gas switch would involve removing the computer from one tank (via the quick-disconnect fitting) and connecting it to the next tank in the sequence, while submerged.

1. I thought that the quick disconnect fitting could not be disconnected while under pressure. Is the intended process to switch to breathing from then next tank, shutdown the first tank, release residual pressure by purging a regulator on the first tank, then remove the PDC and attach it to the hose (with QD fitting) on the 2nd tank?
2. This use case implies that the computer-side of the QD fitting is watertight -- that moving the computer to a different high pressure gas line while submerged (when there's a little water inside the mechanical QD fitting of the new tank) won't "inject" a few CCs of [salty] water into the PDC. Is this accurate? If so, it implies that the computer can be thoroughly soaked for cleaning while disconnected from the hose.

Thoughts? Corrections? Suggestions to read a thread from 2004 about knowing your computer?

 

[tbone1004]

the gas switch is to allow for accurate tracking of decompression if you are using different mixes. You can't disconnect/reconnect that fitting when it is under pressure, and if you depressurized the entire system and disconnected it, you would push water into the computer which is not great.
The computers AI function tracks consumption of the bottle it is plugged into, but since gas consumption is irrelevant to decompression calculations, the AI and decompression tracking functions of the computer are independent. It doesn't know or frankly care which gas mix it is connected to, it has no bearing on your deco schedule

 

[tursiops]

As I understand, the gas switch would involve removing the computer from one tank (via the quick-disconnect fitting) and connecting it to the next tank in the sequence, while submerged

No. The gas switch is only to tell the computer which gas you are using so deco is correct. You'll need an SPG on each tank to monitor pressures, and AI will be useless. The proplus is a terrible deco computer anyway.

 

[scubadada]

Hi @rmssetc

As per @tbone1004 The window displaying cylinder pressure will switch to displaying SPG. Another poster recently brought up the same topic, among other things Oceanic ProPlus 4 - better explanation for the "complications" section?

 

[rmssetc]

No. The gas switch is only to tell the computer which gas you are using so deco is correct. You'll need an SPG on each tank to monitor pressures, and AI will be useless. The proplus is a terrible deco computer anyway.

Ah, that makes much more sense. I fixated on the word "switch", thinking of it as a physical action with the PDC, rather than meaning to change tracking N2 and dive time/depth using the PDC without the AI.