Question Isobaric counter diffusion in CC

[LFMarm]

Here is how the "theoretical" loop mix looks like by switching from 11/74 to 21/35 to 40/0 to 100/0, with set point 1.3:

​

And here it is how it looks like by keeping 11/74 all the way with setpoint 1.3:

​

 

[flymolo]

I’m confused why dil switching on ascent has any effect on helium in the loop…when is dil being added on ascent or during deco?

I was taught that dil switching was done so that you would no longer have hypoxic gas plugged in to your bov or dil mav where it is no longer safe to add

 

[flymolo]

there have been a lot of threads on this topic. see this comment:

Advanced Gas Switching

Total time on backgas is 2 minutes, 30 seconds of which was moving up to the next stop. If you double the time to stow, a "new diver" would be on backgas for 3 minutes and that really is inconsequential in the grand scheme of things, certainly not enough IMO to cause worry about ICD since you...

scubaboard.com

 

[Alekseolsen]

I’m confused why dil switching on ascent has any effect on helium in the loop…when is dil being added on ascent or during deco?

I was taught that dil switching was done so that you would no longer have hypoxic gas plugged in to your bov or dil mav where it is no longer safe to add

Dil switching is "way above" the dives I'm trained for, but those who advocate it does dil-flush when changing, so that's when the dil is added.

 

[flymolo]

Dil switching is "way above" the dives I'm trained for, but those who advocate it does dil-flush when changing, so that's when the dil is added.

i do not add dil on ascent. if i plumb in another gas i hit the purge on bov to verify gas is flowing

 

[Whitrzac]

Here is how the "theoretical" loop mix looks like by switching from 11/74 to 21/35 to 40/0 to 100/0, with set point 1.3 until 21m and then 0.5:

View attachment 859647
​

And here it is how it looks like by keeping 11/74 all the way with setpoint 1.3 until 21m and then 0.5 up to surface:

View attachment 859644
​

Why tf are you running 0.5 on deco?
Who taught you that?

Your opinions are no longer valid, please listen to experienced people.

 

[LFMarm]

Why tf are you running 0.5 on deco?
Who taught you that?

Your opinions are no longer valid, please listen to experienced people.

SP on 0.5, I manage pO2 manually in the last 15-20m to manage minimum loop. From what I have seen, this is common practice with eCCR. Do you do differently?

 

[LFMarm]

I’m confused why dil switching on ascent has any effect on helium in the loop…when is dil being added on ascent or during deco?

I was taught that dil switching was done so that you would no longer have hypoxic gas plugged in to your bov or dil mav where it is no longer safe to add

Correct but you do a loop flush every time you switch dil

 

[flymolo]

Correct but you do a loop flush every time you switch dil

i dont. that sounds like a waste of gas. for what reason?

 

[LFMarm]

i dont. that sounds like a waste of gas. for what reason?

I am not saying this is the right way; just one of 2 school of thought. The main reasons for those doing dil switches during ascent are these:

• With a BOV+dilout configuration, it yields a consistent BO procedure, regardless of depth.
• Periodic verification of cells at a PO2 that is above the operating point
• Maximally efficient off-gassing of He
• The dil flush may reduce the presence/likelihood of condensation on one or more sensors without wrecking the PO2.