CMF provides a solid mass at any depth ?

anarchista

anarchista

New Member
Is constant supply pressure nozzle CMF provides a solid mass at any depth, for a constant pressure regulator?

NO

At what depth is still solid dosage,
for oxygen and pressure of 6, 7, 8 bar.

What type of regulator ensures constant dosing at any depth?

Often there is information about the depth to which it works, the theme is a little more complicated.

rc greet
 
CMF theory is fairly simple.

Fixed Dose routine from Orifice
The orifice allows for a fixed volume/mass of gas to flow through at a given drive pressure. So as long as the pressure optput of the regulator is fixed the flow of O2 is fixed. Say at 0.8l/min.. so all you need to do is make sure the regulator pressure stays constant

Fixed Regulator Output pressure
A standard scuba regulator is pressure compensating. That is to say as you go deeper and the ambient pressure increases so does the output pressure from the regulator.

i.e. If the regulator has an intermediate pressure (IP) of 9 bar at the surface - the pressure in the hose is 10 bar absolute at the surface, 11 bar absolute at 10 metres depth etc.

This would mean that the mass/volume of oxygen would increase with depth with a standard regulator. So to prevent this the regulator is made non-compensating (they block the compensation mechanism) ensuring that the output pressure is independent of depth.

Issue of depth on a non-compensating regulator
Because there is no increase in pressure as you descend, there will come a point where the ambient pressure exceeds the fixed IP of the regulator. At this point the orifice will no longer deliver O2 to the loop.

Therefore is the IP is set to 8.5 bar.. it stops delivering O2 at 75 metres depth.

I hope this explains the theory - though there may be several practical issues that it overlooks. Or are you asking a different question that I misunderstood?

EDIT - because the flow through an orifice is quite low (0.7 - 1.0 l/min) it only requires a small pressure differential over ambient to provide the dosage. There could be a curve of decrease in delivery of O2 - and that would be an interesting research topic (testing pressure differentials against ambient) but the results are probably not relevant to day-to-day diving practice in the water as monitoring variations are probably larger than the deviation.
 
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Can you explain better what you are asking?
It is nearly impossible to understand what you are asking.



Best,

Igor P

Sent ftom my GT-I5800 using Tapatalk.
 
anarchista said:
You have a very superficial understanding of the problem.

Security question, for which pressure ratio is supercritical dosing, subcritical for what?

rc greet
Click to expand...

There is a language issue here - I feel we are not communicating effectively.

I did ask if I had misinterpreted the question. The meaning of "Solid" is not clear to me, I attempted to interpret the question and made an effort at response based on that interpretation.

I understood the question as applying to CMF in normal diving operation, and a question about the functioning of diving regulators within operational ranges. I am sorry of your question was aimed at designing or building. I should have looked at the forum section I suppose - I see you were after a "more advanced" answer.

The calculations for design are obviously more involved - the maths is quite elegantly explained on other sites. It's not worth me attempting to re-hash it here. Indeed - I am not sure that I completely understand it well enough to explain it. "If you can't explain it simply, you don't understand it well enough" is often quoted.

I am afraid I cannot interpret the question you ask here either. It does not parse in English, so I will not attempt to be of assistance as I may get my interpretation wrong again and disappoint you..

Best of luck with your search for information.
 
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AFAIK, there's no ratio involved; as long as the opening is narrow and long enough, and the pressure difference between the two sides is enough, the molecule count (and therefore mass) flow through the opening will be proportional to the surface of the hole and the higher pressure.

So the answer would be p1 >= p2 + 2 bar.

To answer the original question "what type of regulator... at any depth", well none. The point of the regulator in this case is to make the "higher pressure" bit, and so the mass flow, constant. But once you get within 2 bar of that, the "pressure difference" bit won't be valid anymore.

Again, AFAIK.

Cheers,

Matthieu
 
emmbee said:
AFAIK, there's no ratio involved; as long as the opening is narrow and long enough, and the pressure difference between the two sides is enough, the molecule count (and therefore mass) flow through the opening will be proportional to the surface of the hole and the higher pressure.

So the answer would be p1 >= p2 + 2 bar.

To answer the original question "what type of regulator... at any depth", well none. The point of the regulator in this case is to make the "higher pressure" bit, and so the mass flow, constant. But once you get within 2 bar of that, the "pressure difference" bit won't be valid anymore.

Again, AFAIK.

Cheers,

Matthieu
Click to expand...

+ or * ?
 
anarchista said:
More interesting is the next step, the value (beta), used for mixtures?

rc greet
Click to expand...

Why do we care about mixtures? Only pure O2, no?

I thought the formula for critical-ratio (P1/P2) was:

[ 2/(y+1) ] ^ [ y / (y-1) ]

where y is the ratio-of-heats for oxygen (1.4):

0.833. ^ 3.5 = 0.5282... (as you state).

Not sure what the question is!

Matt.
 
So p1>=p2*2 implies that with an IP (p1) of 8.5 bar the O2 delivery will start to fall off at an ambient (p2) of 4.25 bar around 32m???
 
SimonK said:
So p1>=p2*2 implies that with an IP (p1) of 8.5 bar the O2 delivery will start to fall off at an ambient (p2) of 4.25 bar around 32m???
Click to expand...

I'd guess you'd be running the IP nearer 10 or 12 bar, but yes, with a butchered 1st stage (that does not sense water pressure) then for 8.5/2=4.25=32.5m, at this point the sonic-flow condition is not met the mass is no longer constant.

That calculation actually says the ratio is 1.89 for O2 so stall-depth maybe nearer 35m in your example (I'm no expert, by the way, just like sums/own calculator/biro).

Matt.
 
matthewoutram said:
I'd guess you'd be running the IP nearer 10 or 12 bar, but yes, with a butchered 1st stage (that does not sense water pressure) then for 8.5/2=4.25=32.5m, at this point the sonic-flow condition is not met the mass is no longer constant.

That calculation actually says the ratio is 1.89 for O2 so stall-depth maybe nearer 35m in your example (I'm no expert, by the way, just like sums/own calculator/biro).

Matt.
Click to expand...

so if I understand correct, the delivery of O2 at 70 m would not be 0.8l/min (if set at surface to be o.8l/min at 9 bar IP) but would fall to less. What would it be than?
 
matthewoutram said:
Why do we care about mixtures? Only pure O2, no?

I thought the formula for critical-ratio (P1/P2) was:

[ 2/(y+1) ] ^ [ y / (y-1) ]

where y is the ratio-of-heats for oxygen (1.4):

0.833. ^ 3.5 = 0.5282... (as you state).

Not sure what the question is!
Click to expand...
What is the calculation for example, mixtures of 45% helium, 55% of Air?

Main question.
CMF dosing system, without any restriction depth, looks like.

Igor P said:
so if I understand correct, the delivery of O2 at 70 m would not be 0.8l/min (if set at surface to be o.8l/min at 9 bar IP) but would fall to less. What would it be than?
Click to expand...

If you want to have a solid dose to a depth of 70m, where there is a total pressure of 8 at. This pressure before the nozzle must be greater than 8 at / 0.528 = 15.15 at

rc greet
 
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One wonders if any of you have dove CMF rebreathers or taken a course? ;-)

They work exactly as you would expect, the theory is as stated but not needed for usage (only design- which someone already did)

Plenty of online nerd-gasims here but you're not helping anyone IMO :-)
 
Ben Field said:
One wonders if any of you have dove CMF rebreathers or taken a course? ;-)

They work exactly as you would expect, the theory is as stated but not needed for usage (only design- which someone already did)

Plenty of online nerd-gasims here but you're not helping anyone IMO :-)
Click to expand...

Not sure what your point is, Ben.
 
Igor P said:
so if I understand correct, the delivery of O2 at 70 m would not be 0.8l/min (if set at surface to be o.8l/min at 9 bar IP) but would fall to less. What would it be than?
Click to expand...

Yes, it would eventually stop when the water pressure equals the IP.

I'm not sure that the reduction is linear - you could probably work it out from here if you knew the orifice Cv: http://www.idealvalve.com/pdf/Flow-Calculation-for-Gases.pdf (could be 0.00022).

Matt.
 
matthewoutram said:
Not sure what your point is, Ben.
Click to expand...

Anarchista is posing and answering question- why? A skeptic might say its an attempt to look clever but that would be totally pointless, I can't see how this thread is of any benefit to anyone.... my point was- if you have a point go ahead and make it, rather than asking questions then being negative about the answers your given.

To be fair it could be a language barrier- no offence to anarchista if thats the case but its coming across as plain confusing and missleading- whats the purpose of the thread OP?
 
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