The misunderstood mCCR explained

[fsardone]

For me, the author's credibility problem began with his responses to @fsardone . The two of them may have some history, but I don't know anything about that. If he'd admitted that flow is less than metabolic requirement at depths with pressure greater than 1/2 of IP, but can be made up with the MAV, then he would have been fine.
But when he suggested he's good all the way to 90m, at which depth his MAV won't even work any more, much less have a CMF keeping up with metabolic requirement, then we have a problem.

Hello there,
I have no prior knowledge or exchange of messages with the author. I even contributed my knowledge by PM (acknowledged by the OP in the thread) in order to avoid appearing as one trying to demolish a good piece for minor imprecisions or as a know it all. I consider those imprecisions acceptable for a layman or even for somebody trying to make specialistic knowledge available to divers completely unaware of xCCR (fill the x at your will), but not to specialised dive center personnel.

No bone to pick or willing to nitpick but if you want to divulge you need to be precise and understandable.

we may be.

Then I’ll stop.

After it was apparent that I was coming out as somebody that was pedantic an excessivelt "scientific" I decided to stop, but the OP invited me to continue and I made the mistake to do so.

Hi Fabio,
I've not asked you to stop. I enjoy a good discussion,

I have no interest in showcasing what I know nor to win the argument. I was only trying to avoid dangerous misconceptions to gain a foothold in the community. I am not an instructor, I have no responsibility toward others other than be a good buddy when I dive in a team. I was trying to be a good buddy to the scubaboard team pointing out inconsistencies. When I write a paper I welcome critical comments, in order to improve it. I need to understand that people may differ on this.
Apologies to those whom might have been bored by the discussion.

 

[doctormike]

When I write a paper I welcome critical comments, in order to improve it.

^^ This

 

[Bernie_U]

Where does this 1/2 the IP calculation come from? My understanding (from Paul R. at Revo and others) is that the molar mass flow (see what I did there?) does not change from the surface down to about 2ata (~30psi) above the IP. Then it starts to drop off and obviously becomes zero at the point where the ATAs match the IP.

Had a look into my fluid dynamics text book. The "1/2" come from an rule of thumb based on an equation that was derived on 19th century physics. The equation result is about 0.49 for helium an 0.53 in air. So, yeah, 0.5 is quite okay.

Two more comments:

• @rsingler, Thursday at 2:05 AM: supersonic flow can occur, e.g. in a De-Laval nozzle. Consequently volume flow can increase (and it does in that nozzle, since both cross-sectional area increase and velocity increase). However, density goes down, leaving mass flow constant (straightforward, the mass cannot go elsewhere).
• @rsingler, Thursday at 2:43 AM: The shape of the curve is similar to your sketch #1, pretty flat at the beginning. You can get an idea from here, just bear in mind that the right portion (M > 1) has to be ignored here. When p2 goes from 0.5 p1 to 1.0 p1, you go back the M axis from 1.0 to 0.0. Then, the vertical axis shows the fraction of mass flow coming through your orifice, subject to designed / intended amount.