The misunderstood mCCR explained

[rsingler]

This layperson newbie didn't get confused until he read the thread after reading the article. I thought the article was very well written and made an unfamiliar subject easy to understand.

I read the article too, and it read easily. If you knew nothing about CCR, and little about physics, it was an easy introduction to mCCR.
But if you know "a little", the article raises a few questions. This was the paragraph that did it to me:
"Any automated flow of oxygen must be adjustable though. As we descend in the water column, diving physics increases our partial pressure of the oxygen, which means we need to add less oxygen into the loop. For the eCCR diver, the computer simply opens the solenoid valve less often. For the mCCR diver, the ‘trickle’ of oxygen is automatically limited based on the increase in water pressure. This is because the oxygen first stage regulator is a fixed intermediate pressure."
The problem this raises is that "adding less oxygen" just isn't correct, though opening the solenoid less frequently is correct.
And then (if you know a little) an "automatically limited" trickle of oxygen from a CMF due to increasing ambient pressure issues raises a red flag. The whole point of the CMF is constant mass flow despite changing pressure up to the point that ambient is greater than half of IP. Yes, an unblocked first stage would deliver more oxygen molecules through a CMF as you descend, but an mCCR uses a blocked regulator. So the article must have been talking about that portion of the dive at depths where sonic flow no longer occurs, because below 6m, oxygen flow doesn't increase, and holds constant all the way to the depth at which ambient is half of IP, at which point it decreases below metabolic requirements, and completely stops at its limiting depth.

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.

If you author an article for lay people, it doesn't require you to be the world's expert. Otherwise we'd have no Millennial copywriters, lol! And he admitted it wasn't intended to be a scientific treatise. But when the questioning got more detailed here on this forum, the frank errors in his understanding of mCCR function at the extremes of its depth limit were a problem. That's why things got a little hot and heavy, IMO.
I notice he hasn't returned since oxygen delivery at 90m was questioned. If that means he's gone back to the books a little bit, that's a good thing. The questioning was tough on him, for sure. But he appears to run a tec diving shop, so I think it's fair that the standard of knowledge demanded was higher.

 

[doctormike]

This layperson newbie didn't get confused until he read the thread after reading the article. I thought the article was very well written and made an unfamiliar subject easy to understand.

Right, but that's the point. If you are a layperson newbie, how would you possibly know if there were inaccuracies in the article? It's not enough for something to be well written and easy to understand if there are content issues.

That's what everybody here was trying to help with. I'm sorry that the OP seems to have taken that personally. I know that a second set of eyes on whatever I write is often helpful.

 

[rjack321]

The problem this raises is that "adding less oxygen" just isn't correct, though opening the solenoid less frequently is correct.
And then (if you know a little) an "automatically limited" trickle of oxygen from a CMF due to increasing ambient pressure issues raises a red flag. The whole point of the CMF is constant mass flow despite changing pressure up to the point that ambient is greater than half of IP. Yes, an unblocked first stage would deliver more oxygen molecules through a CMF as you descend, but an mCCR uses a blocked regulator. So the article must have been talking about that portion of the dive at depths where sonic flow no longer occurs, because below 6m, oxygen flow doesn't increase, and holds constant all the way to the depth at which ambient is half of IP, at which point it decreases below metabolic requirements, and completely stops at its limiting depth.

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.

The IP on my kiss is set to 200psi and I have had it down to about 260ft. If the 1/2 IP were correct I should have experienced a slowing of the flow past 220ft which I did not (in any profound way, but maybe if my IP with only 170psi I would have).

 

[HKGuns]

I took most of the comments as offering constructive criticism, even though they were confusing to someone with zero experience with re-breathers.

Was not trying to cast aspersions because of my own confusion.

 

[rsingler]

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.

This article is a reasonably clear summary of things from a scuba point of view. Basically, the pressure at the inflow of the mass volume orifice has to be twice the outflow.
Understanding Constant Mass Flow • ADVANCED DIVER MAGAZINE • By Paul Raymaekers

The relevant passage is this:
Constant Volume Flow
When we discuss CMF, there is one law in physics we are specifically focusing on:

When a gas is pushed through a small hole, also called orifice, or nozzle, the speed of that gas is limited and can never be higher than a certain maximum speed, also known as the sonic speed (Vmax)

When the conditions to reach maximum or sonic speed are achieved, increasing or decreasing the pressure at the entrance of the hole or even vacuuming on the exit side of the hole, the speed of the gas-travelling through the hole will not change, but stay constant at Vmax.

This means, because the speed of the gas is limited, for an orifice with a fixed diameter, the flow (l/min) of gas through the orifice is also limited, and can never increase once sonic speed is achieved: The result is termed “Constant Volume Flow” (flow = speed of the gas X surface of the opening in the orifice)

Now when do we reach the maximum speed, or sonic speed? We can apply a simple rule: Sonic speed is reached when the inlet pressure P1 is at least twice the outlet pressure P2 or P1 >= 2 x P2." [my emphasis]

Now, constant volume flow become constant mass flow when you have a constant IP, or a blocked first stage.

The 'two atmospheres above IP", I believe comes from a depth which still allows the MAV to be effective. But it's no longer sonic flow, and thus the CMF orifice is no longer delivering the metabolic amount you planned for.

I am NO EXPERT, so I'm fully prepared to be told I've got this wrong.

 

[rjack321]

This article is s reasonably clear summary of things from a scuba point of view. Basically, the pressure at the inflow is the mass volume orifice has to be twice the outflow.
Understanding Constant Mass Flow • ADVANCED DIVER MAGAZINE • By Paul Raymaekers

The relevant passage is this:
Constant Volume Flow
When we discuss CMF, there is one law in physics we are specifically focusing on:

When a gas is pushed through a small hole, also called orifice, or nozzle, the speed of that gas is limited and can never be higher than a certain maximum speed, also known as the sonic speed (Vmax)


When the conditions to reach maximum or sonic speed are achieved, increasing or decreasing the pressure at the entrance of the hole or even vacuuming on the exit side of the hole, the speed of the gas-travelling through the hole will not change, but stay constant at Vmax.

This means, because the speed of the gas is limited, for an orifice with a fixed diameter, the flow (l/min) of gas through the orifice is also limited, and can never increase once sonic speed is achieved: The result is termed “Constant Volume Flow” (flow = speed of the gas X surface of the opening in the orifice)

Now when do we reach the maximum speed, or sonic speed? We can apply a simple rule: Sonic speed is reached when the inlet pressure P1 is at least twice the outlet pressure P2 or P1 >= 2 x P2."

Now, constant volume flow become constant mass flow when you have a constant IP, or a blocked first stage.

This is going the other direction.

Flow can never be MORE the mass with sonic speed no matter how high the IP. But a 2x pressure differential isn't the minimum pressure required for a CMF to work. (edit I should have said "'substantively change" above). This is in part because of what was elaborated on by dsix above.

Which is consistent with this "For the rEvo in mCCR or hCCR mode, we even limit the maximum operation depth to 20m less then the depth where the IP equals the water pressure: as the graph shows. At that depth there is still a reasonable mass flow through the orifice, and still enough differential pressure over the MAV to inject extra oxygen." So there's the 2ATA rule.

 

[rsingler]

Fair enough.
So @Divetech Cayman may be effectively correct as far as actual use of the mCCR is concerned, at least down to 70m. And deeper than that, the decay in metabolic oxygen may be slow enough (due to the sheer number of O2 molecules in the loop) that unless a long time is spent at 90m, the diver won't notice. Have I got that right?

Just as long as we're not expecting O2 from the MAV at 90m, and we agree that it's not the full metabolic amount of oxygen at 89m.

 

[rjack321]

Fair enough.
So @Divetech Cayman may be effectively correct as far as actual use of the mCCR is concerned, at least down to 70m. And deeper than that, the decay in metabolic oxygen may be slow enough (due to the sheer number of O2 molecules in the loop) that unless a long time is spent at 90m, the diver won't notice. Have I got that right?

Just as long as we're not expecting O2 from the MAV at 90m, and we agree that it's not the full metabolic amount of oxygen at 89m.

Yeah I asked this exact question a few years ago on CCRx because everyone cites the "flow stops at the IP" narrative. But how far above the IP do you need to be for the IP vs water pressure differential for the CMF to basically work even if its not precisely the same all the way down? That's where Paul talked about the 2ATA rule of thumb which seems to work in practice. Even 1ATA above the IP works because the ppO2 decay is so darn slow. Its just when you do finally have to hit the MAV you might press it for 2 seconds instead of for the fraction of a second you might press it shallower. To end up delivering the same mass of O2 that is 'slow'.

 

[rsingler]

Can one of the math guys tell me how abrupt the falloff in flow is below p1/p2=2?
If it's linear then things are okay.
If it's

then things are great!
If it's

then it's not so good.

 

[rjack321]

Can one of the math guys tell me how abrupt the falloff in flow is below p1/p2=2?
If it's linear then things are okay.
If it's
View attachment 601274
then things are great!
If it's
View attachment 601275
then it's not so good.

Minus the math, strictly from a practical dive perspective, I can say its more like the top curve. When I did a 90m dive with @nadwidny and he was on his Kiss and I was using my Meg he stopped at 85m when his MAV stopped working. It worked "good enough" just a few meters above that and was not a big deal.