The misunderstood mCCR explained

[Divetech Cayman]

Here's an article I wrote to better explain how the mCCR works. I've heard lots of mis-information about these rebreathers spread around because there can be a lack of understanding by some about how they work. The "M" in mCCR has traditionally stood for 'manual' closed circuit rebreather. We feel 'mechanical' closed circuit rebreather is a much better description. Hope this helps!

https://www.divetech.com/post/the-mccr-explained

Tony

 

[fsardone]

Here's an article I wrote to better explain how the mCCR works. I've heard lots of mis-information about these rebreathers spread around because there can be a lack of understanding by some about how they work. The "M" in mCCR has traditionally stood for 'manual' closed circuit rebreather. We feel 'mechanical' closed circuit rebreather is a much better description. Hope this helps!

https://www.divetech.com/post/the-mccr-explained

Tony

Hello Tony,

I might have read it wrong but I beg to differ in acouple of point which are fundamental:

1st the constant mass flow orifice works by establishing a sonic flow in a convergent divergent orifice (there is a shock wave in the narrowest section) there are no sonic pressure waves ...

2nd you say that while increasing depth we need less oxygen. This is, in my understanding of human physiology, wrong. We need the same amount of oxygen with a constant metabolic load. Also increasing pressure past the orifice (as long as the ratio is above 1:2 and therefore the flow is sonic) will not change the mass flow of the orifice ... hence the constant mass flow name of it.

You might want to address these two flaws. Did not read any further.

Cheers

Fabio

 

[Divetech Cayman]

Hello Tony,

I might have read it wrong but I beg to differ in acouple of point which are fundamental:

1st the constant mass flow orifice works by establishing a sonic flow in a convergent divergent orifice (there is a shock wave in the narrowest section) there are no sonic pressure waves ...

2nd you say that while increasing depth we need less oxygen. This is, in my understanding of human physiology, wrong. We need the same amount of oxygen with a constant metabolic load. Also increasing pressure past the orifice (as long as the ratio is above 1:2 and therefore the flow is sonic) will not change the mass flow of the orifice ... hence the constant mass flow name of it.

You might want to address these two flaws. Did not read any further.

Cheers

Fabio

Hi Fabio.

The term sonic was used in the article because in some rebreather circles was historically known as the 'sonic orifice.' You are correct, it is a shock wave that actually regulates the flow, however a 'shock wave' is relative to the speed of sound, hence the term 'sonic.' Many people use the terms interchangeably. It's not the main point of the article, which is why I don't expand on it. When the article was being peer reviewed, someone brought up that it might be relevant to include this term, since some may still be familiar with this term. The article was peer reviewed by an aerospace engineer, a rebreather manufacturer, and a rebreather instructor trainer before it was published by the way.

For the second issue, I think you might have read that wrong? My quote was "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."
Never did I say the human requires less oxygen.

Please read the rest! I have some nice illustrations in there.

Thanks!
Tony

 

[bletso]

Well I dive an MCCR, eg DOCCR and I have never noticed an increase in my PO2 at depths to 67msw with the sonic orifice. Remember we metabolise the same 'number of molecules of O2' regardless of depth. It is based on your metabolic needs. (VO2) rebreather support page and http://airheadscuba.com/bletsop.html

 

[Divetech Cayman]

I'm starting to think you guys have a bone to pick with me.

Well I dive an MCCR, eg DOCCR and I have never noticed an increase in my PO2 at depths to 67msw with the sonic orifice. Remember we metabolise the same 'number of molecules of O2' regardless of depth. It is based on your metabolic needs. (VO2) rebreather support page and http://airheadscuba.com/bletsop.html

That's good, you shouldn't notice an increase! Which is why I describe how the mass flow orifice automatically decreases it's flow relative to your depth in this paragraph:

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. Unlike almost every other 1st stage regulator in the world, these regs do not compensate for the water pressure change. The simple version is, as we go deeper the trickle of oxygen gradually decreases - a perfect balance of what we need and what it provides.

Please read the article again, You'll observe my only references to metabolic changes, are respective to workload, not with depth.

Thanks!
Tony

 

[bletso]

Actually, according to sonic orifice functionality, a constant IP down to the magic ratio will not change the mass of O2 molecules leaving the orifice. Once past that ratio, (in the down ratio) it will decrease. If you use a standard reg it will increase with depth as the absolute IP will be increasing. In fact a way to tune the flow is to increase or decrease the IP. That is the reason one uses a absolute pressure regulator in diving an orifice.

Flow (lpm) = 11Pbar * D^2
D diam of orifice in mm

 

[Divetech Cayman]

Actually, according to sonic orifice functionality, a constant IP down to the magic ratio will not change the mass of O2 molecules leaving the orifice. Once past that ratio, (in the down ratio) it will decrease. If you use a standard reg it will increase with depth as the absolute IP will be increasing. In fact a way to tune the flow is to increase or decrease the IP. That is the reason one uses a absolute pressure regulator in diving an orifice.

Flow (lpm) = 11Pbar * D2

Pretty sure we are saying the exact same thing, so I'm not quite sure where the confusion is. ;-)

Tony

 

[fsardone]

I'm starting to think you guys have a bone to pick with me.

No bone to pick or willing to nitpick but if you want to divulge you need to be precise and understandable.
The bit I am questioning is:

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.

which means we need to add less oxygen into the loop This is incorrect
the ‘trickle’ of oxygen is automatically limited based on the increase in water pressure This is incorrect

The amount of oxygen (amount is the number of molecules, moles, grams) needs to be constant. [EDIT] the number of moles, grams of molecules of the other gases will be higher ... Just to clarify further[/EDIT]
With a sonic orifice as long as the pressure upstream is constant and at least twice as downstream the amount of gas going through is constant (ignoring the effect of temperature ... which in our range can be done).

Cheers

 

[bletso]

we may be.

 

[fsardone]

Then I’ll stop.