Air consumption?

[mts0628]

The average here is 14.7 psi, or 1 ATM, or 1 BAR with slight variations for barrometric pressure caused by weather, the 1200 foot above sea level altitude I am at, the air handeling system kicking on, etc......

Am I that slow at typing? Look at your time and then mine for posting. Whew (again)! To all reading, I promise I did not copy muddiver's answer. I knew it on my own, really!

 

[The Kraken]

OK.
Now I'm running really, really fast and hard.
What psi am I consuming now?

the K

 

[Seamaster6]

It depends on your tidal volume.

 

[The Kraken]

Well, it's still 14.7 psi, but . . .

. . . the cubic foot per minute VOLUME has changed.

the K

 

[Teamcasa]

You guys still at it over the difference and best use of SAC vs RMV?

Reread the original question.

How much air would you use on a dive at say 30' for 40 mins?

To which I said,

I would use about 60% of the air in my HP100 tank.

And I stick by it.

 

[mts0628]

Well, it's still 14.7 psi, but . . .

. . . the cubic foot per minute VOLUME has changed.

the K

Kraken I agree. You are breathing at 14.7 psi but the volume of gas (air) has changed to suit your body's demands. I think I know where your going with this...

SAC= (((psi_final-psi_initial)/time)/((depth in feet+33)/33))​

=(((14.7-14.7)/(any time [doesn't matter because the numerator is zero])/((0+33/33))=0 psi/minute​

then using the RMV SAC*k, we could use a k of any positive number and our RMV would still be zero, even though we have changed the volume of gas per time requirements. Am I warm so far?

The problem is that when breathing from a compressed gas source, when we exhale, our expired breath is not going back to the tank where our gauge measurements are from. So there will be a difference in the delta psi.

Or did I put the cart before the horse?

 

[The Kraken]

It really doesn't matter to me . . . I just like to argue sometimes!

 

[mkutyna]

Well damn, the last time I went diving I was breathing about 50 psi per minute. Since I'm currently at 14.7 psi, that means that I have 17.64 seconds before I run out of air!

 

[Doc Harry]

SAC and RMV are not related, at least not by the simple formula you quote in the NOAA manual. (Just because you read it in a book doesn't mean it's true.) You don't need a text book for this, it's simple math and scientific deduction.

Since SAC may or may not incorparate gas consumption from non-respiratory activities, a proper formula relating SAC and RMV would have be something along the lines of:

SAC = (RMV + X )* K

Where X accounts for the gas consumption from non-respiratory activities (e.g., drysuit inflation)

I rather enjoy this discourse, don't think it's pointless bickering. If I have offended anyone with "attitude," I apologize.

 

[Montana Diver]

Well, it's still 14.7 psi, but . . .

. . . the cubic foot per minute VOLUME has changed.

the K

But what is my pressure at 3300'?

Is 12.7 psi close? I don't remember the calculations, just remember to add depth to the table for altitude diving.