The misunderstood mCCR explained

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

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 ....

Fabio,

You've clearly not read the article. If you did, you'd have seen the paragraph I put in about what happens when you workload (and metabolism) increased.

You've just PM'ed me this note "Tech divers need to know about tech." This is true, to a degree. As I've said multiple times, this thread is for the layperson. Someone who is perhaps interested in CCR, but not a CCR diver. So their level of knowledge at a casual informational stage of research isn't required, nor is an in depth technical explanation.

Furthermore, a tech divers doesn't need to know the chemical properties of sorb. They just need to know that it absorbs CO2, how to use it safely not not to get it wet. A tech diver doesn't need to know the amount of chromium in the stainless steel, the exact composition of their batteries, etc.

You're being argumentative for the sake of being argumentative at this point. We get it, you're a really smart guy who knows lots of stuff. You're confusing everyone else. Your english is a bit confusing and that's not helping matters.

This is an article for people to learn a bit more about the the concept of the mCCR, nothing more.
 
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.

That is correct. An mCCR is limited to half the pressure of the sealed first stage intermediate pressure (the flow stops being sonic and you would loose mass rate )or worst case to the pressure of the sealed first stage (you loose ability of adding oxygen even manually).

In an eCCR you always have the preset intermediate pressure above ambient (in my case is 7.5 bar, above that you risk seizing the solenoid) and you are correct the solenoid fires less and less frequently with increasing depth. Sometimes it make me wonder and I recheck ppO2 just to make sure ...

Cheers
 
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.
Hi Bernie,

Correct, eCCR's do have a traditional depth compensated 1st stage. mCCR's do not, they have fixed IP first stages. In that paragraph I'm making the comparison between how an eCCR works: (less or shorter solenoid actuations) vs a mCCR (less flow from the orifice. However that section comparing the eCCR is limited to that paragraph. The remainder of the article I'm speaking about the mCCR.

When I say "add less oxygen" here essentially is what I am saying - I'm diving my eCCR. At 20 feet to effect a PO2 change, I might need to push my manual O2 button for 1 second. At 200 feet to effect the same PO2 change, I might need to push the button for just a brief fraction of a second. I need to add less volume because of the depth increase.

It doesn't last longer because the gas density has increased.
 
Fabio,

You've clearly not read the article. If you did, you'd have seen the paragraph I put in about what happens when you workload (and metabolism) increased.

You've just PM'ed me this note "Tech divers need to know about tech." This is true, to a degree. As I've said multiple times, this thread is for the layperson. Someone who is perhaps interested in CCR, but not a CCR diver. So their level of knowledge at a casual informational stage of research isn't required, nor is an in depth technical explanation.

Furthermore, a tech divers doesn't need to know the chemical properties of sorb. They just need to know that it absorbs CO2, how to use it safely not not to get it wet. A tech diver doesn't need to know the amount of chromium in the stainless steel, the exact composition of their batteries, etc.

You're being argumentative for the sake of being argumentative at this point. We get it, you're a really smart guy who knows lots of stuff. You're confusing everyone else. Your english is a bit confusing and that's not helping matters.

This is an article for people to learn a bit more about the the concept of the mCCR, nothing more.

I believe you are right.
The article is perfect for the stated purpose.

Good night
 
Now I got it, thanks! The terms more and less are subject to the time you push the button of the MAV.
 
That is correct. An mCCR is limited to half the pressure of the sealed first stage intermediate pressure (the flow stops being sonic and you would loose mass rate )or worst case to the pressure of the sealed first stage (you loose ability of adding oxygen even manually).
Then how are we able to get O2 flow at depth deeper than this? By your example, my max depth limit then would be 40m. How then does my orifice keep up with my metabolic rate until 90m?
 
If you have truly matched your CMF to your metabolic rate, with an O2 first stage IP of 10atm then things begin to change deeper than 5 atm.

And at 90m, your ambient equals your IP. How then, do you even add O2 via the MAV, much less depend upon the CMF?

I was enjoying your two guys' debate, since you both seemed to be talking past each other. Your article made sense, even if it was phrased somewhat unscientifically. But this last does not make sense to me...
 
Something to ponder in this deep stuff with an mccr. Lets say 600' or so, the gas is so darned dense and the actual O2 molecules are so jam packed into the same volume in the loop that it literally will take you 20 minutes or more to even drop the po2 by .02 (that is not a typo either). The flow can be completely shut down from the ambient vs. cmf and you would not notice it for a very long time.

I know this is an extreme example but it works along the same lines even when still above the cmf threshold for depth. Gas density is what drives this whole "needs less at depth" topic.
 
https://www.shearwater.com/products/swift/

Back
Top Bottom