The misunderstood mCCR explained

[Divetech Cayman]

Hi Fabio,

To begin with, the article was written for laypersons, so perhaps the language used isn't as precise as what you may like. However when an article for this audience gets too technical, then people fail to understand it.

Nothing in this article is inaccurate. Like I said, it was vetted by several professionals with vast expertise in these fields.

which means we need to add less oxygen into the loop This is incorrect

That is not incorrect. If we were to add the same volume of oxygen to the loop at depth as we did at the surface, we would certainly be hyperoxic. We do need to physically add less volume of oxygen as we get deeper. On an eCCR this means the solenoid opens less often or for shorter durations. On the mCCR the flow is limited by the first stage regulator.

the ‘trickle’ of oxygen is automatically limited based on the increase in water pressure This is incorrect

Again I''m not sure how you can claim this isn't the case. Do you dive an mCCR by chance? As we get deeper, the volume of oxygen coming out of the orifice gradually decreases relative to depth. Again if this did not happen, we'd be hyperoxic at any depth below 6m.

The 1st stage regulator on these is fixed, meaning it does not compensate for increases in ambient pressure. It will always be, for arguments sake 10 bar. The actual pressure of the 1st stage will be the difference between the 1st stage pressure and ambient pressure. If we were at 4 bar, then the IP coming out of the reg would be 6 bar, which of course is less than it is on the surface at 10 bar.

I'm very curious how you can disagree with this, and if you still do, I'm curious as to your explanation of how an mCCR works when one is at depth?

Thanks!
Tony

 

[fsardone]

I did say I would stop I will say only

PV=nRT amount is n not V

 

[Divetech Cayman]

I did say I would stop I will say only

PV=nRT amount is n not V

Hi Fabio,

I've not asked you to stop. I enjoy a good discussion, and if I am wrong I certainly want to know about it. And for the benefit of the readers who might not understand the discussion, it would be beneficial if we clarified out points.

I understand the ideal gas law as you've quoted. But you've not explained then, how this works in a practical setting on an mCCR, and how in fact one does work then.

• In the article, there was no statement about any relationship between depth and the human metabolic rate as you claim in your first post.
• It is a fact, that as a rebreather diver descends in the water column, the amount (volume) of oxygen one needs to add from their tank to the loop is less than what it is at shallower depths.
• It is a fact that all rebreathers, eCCR or mCCR add less oxygen to compensate for this. mCCR's do so by means of the fixed IP regulator.

So I'm very confused as to where the disconnect is. Could you kindly explain then, what would happen if the same volume of oxygen were added during the deep phase of our dive as we added during the shallow phase, what would occur?

Can you also tell me then, how an eCCR or mCCR limits the amount of oxygen added to the loop as we descend?

Thanks!
Tony

 

[davehicks]

I found this article to be very interesting and informative. I'm not understanding all the nit picking and people swinging around their big equations. It's a scuba forum not arXiv after all.

I've been diving mCCR since 2007 on Kiss units but still learned and clarified a few things I didn't know or had not retained from training.

Thanks!

 

[fsardone]

Hi Fabio,

I've not asked you to stop. I enjoy a good discussion, and if I am wrong I certainly want to know about it. And for the benefit of the readers who might not understand the discussion, it would be beneficial if we clarified out points.

I understand the ideal gas law as you've quoted. But you've not explained then, how this works in a practical setting on an mCCR, and how in fact one does work then.

• In the article, there was no statement about any relationship between depth and the human metabolic rate as you claim in your first post.
• It is a fact, that as a rebreather diver descends in the water column, the amount (volume) of oxygen one needs to add from their tank to the loop is less than what it is at shallower depths.
• It is a fact that all rebreathers, eCCR or mCCR add less oxygen to compensate for this. mCCR's do so by means of the fixed IP regulator.

So I'm very confused as to where the disconnect is. Could you kindly explain then, what would happen if the same volume of oxygen were added during the deep phase of our dive as we added during the shallow phase, what would occur?

Can you also tell me then, how an eCCR or mCCR limits the amount of oxygen added to the loop as we descend?

Thanks!
Tony

Ok I'll bite ....

The amount of oxygen that you add to the loop needs to be constant because, save the amount you vent to clear your mask or otherwise, you use up a constant amount: the metabolised amount.

So no matter depth, you need to add the same amount of moles. The n in the PV=nRT. Please note that the point is the amount of oxygen that is exactly what the CMF orifice does (Constant Mass Flow).

Now since nRT is constant when you dive down P increases V goes down. That is volume not amount. Sorry but even for me that are a non native speaker amount has a well defined meaning. According to the oxford dictionary: a collection or mass, especially of something that cannot be counted AMOUNT | meaning in the Cambridge English Dictionary

If we accept this, meaning volume does not equate amount. Physics and oxford dictionary are quite in agreement. The fixed IP regulator is needed to keep density constant upstream of the CMF.
CMF works keeping the speed constant (speed of sound) in the orifice. Now the volume of gas going through is speed (of sound) by surface of the orifice. So the mass is density (upstream) x volume. If you were to change the pressure of the regulator you would be supplying more mass with increased depth.
If you IP is set at 10 bar, the CMF will stop be sonic at around 40 meters (5 bar) and then you would loose constant mass.

The point you make about the volume of the added gas after the orifice is irrelevant because it is after expansion. The orifice (while it is sonic) will always add the same volume (at the ustream pressure) as it does on the surface and any intermediate depth up to half of the intermediate pressure of the reg.

Choked flow - Wikipedia

Cheers

 

[rddvet]

The article was pretty darn good, especially compared to most dive articles. I think you guys are really nitpicking on some of the very finite points that most people don't care about. I understand the whole "sonic flow" thing but don't think this article needs to be that precise in word choice based on the target audience. It's not a scientific paper.
Good article, people shouldn't be on your case about minutia.

 

[fsardone]

The article was pretty darn good, especially compared to most dive articles. I think you guys are really nitpicking on some of the very finite points that most people don't care about. I understand the whole "sonic flow" thing but don't think this article needs to be that precise in word choice based on the target audience. It's not a scientific paper.
Good article, people shouldn't be on your case about minutia.

I said I would stop, he asked to explain ... I did.

 

[Divetech Cayman]

Hi Fabio,

You're really nitpicking here, but I'll address your points for the benefit of others reading.

The amount of oxygen that you add to the loop needs to be constant because, save the amount you vent to clear your mask or otherwise, you use up a constant amount: the metabolised amount.

So no matter depth, you need to add the same amount of moles. The n in the PV=nRT. Please note that the point is the amount of oxygen that is exactly what the CMF orifice does (Constant Mass Flow).

Ok so far we are in total agreement, the amount of moles we need to add back to the loop is the same regardless of depth.

Now since nRT is constant when you dive down P increases V goes down. That is volume not amount. Sorry but even for me that are a non native speaker amount has a well defined meaning. According to the oxford dictionary: a collection or mass, especially of something that cannot be counted AMOUNT | meaning in the Cambridge English Dictionary

If we accept this, meaning volume does not equate amount. Physics and oxford dictionary are quite in agreement. The fixed IP regulator is needed to keep density constant upstream of the CMF.

Ah, ok we are making progress. You are correct again. In scientific terms, using the terms amount in place of volume is not correct. But this isn't a scientific paper. As I said earlier, this article is authored for those who don't know about the mCCR to learn about the topic. As such I've written it in differently that I would for a scientific review. You're obviously scientifically literate and I don't need to teach you this stuff. My target audience would probably include those who aren't scientifically literate. If i were submitting this for a doctoral dissertation, I'd probably word it differently.

By your own dictionary link, the definition of 'amount' includes: "a quantity of something." I don't feel it's a huge stretch to use the word 'amount' in this context do you? And I did clarify in post 13 by using the term 'volume.'

CMF works keeping the speed constant (speed of sound) in the orifice.

Ironically your statement here backs up my position in post #3. You for some reason took issue with my use of the term sonic, which in fact you've confirmed, it is in fact sonic, as in related to the speed of sound.

Now the volume of gas going through is speed (of sound) by surface of the orifice. So the mass is density (upstream) x volume. If you were to change the pressure of the regulator you would be supplying more mass with increased depth.

The point you make about the volume of the added gas after the orifice is irrelevant because it is after expansion. The orifice (while it is sonic) will always add the same volume (at the ustream pressure) as it does on the surface and any intermediate depth up to half of the intermediate pressure of the reg.

Cheers

What you're saying in a roundabout way, is the amount of moles coming out of the orifice is proportional to the pressure. Which is what I've been saying in simpler terms.

I will readily admit I am not as versed in the intricacies of the mass orifice flow as you are, so I will ask a legitimate question for my own knowledge, how is it then, that the flow rate from the orifice perfectly matches a diver's metabolic needs, at every given depth? It would seem by your claim, that the flowrate would provide too much oxygen at certain depths, and too little at other depths, but that isn't the case.

Thanks!
Tony

 

[fsardone]

Ironically your statement here backs up my position in post #3. You for some reason took issue with my use of the term sonic, which in fact you've confirmed, it is in fact sonic, as in related to the speed of sound.

Sorry I took issue to sonic pressure waves, there is no wave and no pressure only a sonic flow.

What you're saying in a roundabout way, is the amount of moles coming out of the orifice is proportional to the pressure. Which is what I've been saying in simpler terms.

No. The number of moles is constant no matter what the ambient pressure does as long as it stays below half of the intermediate pressure of the firsta stage that in a mCCR is sealed and, therefore, not sensing ambient pressure. After that point the mass flow diminishes and becomes zero when the reg IP is equal to ambient pressure.

I will readily admit I am not as versed in the intricacies of the mass orifice flow as you are, so I will ask a legitimate question for my own knowledge, how is it then, that the flow rate from the orifice perfectly matches a diver's metabolic needs, at every given depth? It would seem by your claim, that the flowrate would provide too much oxygen at certain depths, and too little at other depths, but that isn't the case.

Thanks!
Tony

No, again. Since the CMF orifice provides exactly the same amount of mass of oxygen which is needed to oxidate the same amount of glucose in the human metabolism, with constant muscle work rate, no matter which dept (within the CMF range effectivenes) you would add by cmf and subtract by metabolism approximately the same amount (mass) of oxygen. Chemistry works with masses not volumes. CMF works with masses not volumes.
Finally it does not perfectly matches. Strong current or any other stresses will increase metabolic use and therefore you will need to manually add oxygen. CMF usually is se for steady state resting metabolism, otherwise you would need to dilute ad waste gas ....

 

[Bernie_U]

To be honest, to understand your article, I had to read Paul Raymaekers' article on constant mass flow again.

Still struggling with two statements from your text:

1. 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.
2. After all, when your depths approach 100 meters, you need so little oxygen to sustain you.

If it is intended for a layman, one could easily think: "The deeper I go, the less oxygen I need. My tank and my scrubber will last twice as long."

From Paul Ramaeker's article I took the message that eCCRs don't have fixed IP, but a constant over-pressure related to ambient pressure (like OC 1st stages). That means, at depth the IP pressure rises, and so does the oxygen density upstream the solenoid. Hence, the amount of oxygen (expressed in units of mass) per shot increases with pressure. Finally, the solenoid triggers less often at depth to deliver the same amount of oxygen.