Fundamentals Of Constant Mass Flow (CMF) Systems

DanDunfee

CMF basics re-visited...this will make do till the original article submitted to Rebreather World gets archived in the RBW Library. Hope its worthwhile to the newcomers to, or those less seasoned in, mCCRs and SCRs....... Dan

CMF fundamentals by Dan Dunfee

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Beanie

The Article is now in the Libary

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jorgey

Hi,

I have read the article, a very interesting thema.- i would have some questions.-

the advantage of a compensated 1st. stage would be, that there is no depth limit for it? and the disadvantage the gas comsuption? and the no truly constant flow?

if the upstream should be at least twice the downstream, taking for a example a kiss classic (i think, it has a non compensated 1st stage), that it's recommended for no deeper as 75m, means that, that the 1st stage at them is setted to 17bar <(1+7,5)x2>? or more, because safer would be to take 2,5x ambient pressure, i think.- it would mean over 20bar.- is this right?

j o r g e

DanDunfee

Quote--Baz from private e-mail

G'Day .... would you mind if I tyrned the article into a short Powerpoint presentation ???

I would like to use it when I teach MCCR and SCR courses.

regards Baz
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Baz

Thanks for the interest and inquiry.

You, and any other certified instructor, are more than welcome to use all, or parts of, the article specifically in your course content.... provided that:
* The context and technical integrity of the content are preserved, and
* Due credit is given to the article and author.

Best Regards

Dan

__________________
There are 'Bold' divers and old divers, but NO old, ' Bold ' divers

onetime

Thanks for the article. Well written and to the point.

DanDunfee

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jeorgey

It is true that, at a given surface IP and SEV flow setting, a compensated 1st stage allows a greater diving depth at the desired mass flow than an un-compensated 1st stage. However, there are also depth limitations, with uncompensated 1st stages that need to be observed. Take this simplified, idealized example:

1st Stage IP at the surface= 160 psig (175 psia) / 2 X critical CMF pressure ratio (based on abs. press. )= 88 psia, 165 fsw, 50 msw,
Below this critical 2X pressure ratio, the orifice starts to go sub-sonic, and the mass flow begins slowly to decrease, until at 175 psia, , 360 fsw, 110 msw, the upstream and downstream pressures are equal, and the mass flow stops entirely.

As indicated in the Article, equal MOD performance is achieved, by running the uncompensated systems at higher initial surface IPs to achieve the same SEV flow values as a compensated stage.

Do I understand correctly that the data you wanted to see illustrated graphically is how fast the mass flow falls off (rolls off) after the orifice critical pressure ratio falls below 2X and the mass flow rate becomes progressively lower. ??

__________________
There are 'Bold' divers and old divers, but NO old, ' Bold ' divers

jorgey

hi Dan,

thank you for your answer.- i have seen such information somewhere already.- what i wanted to know, was, what for a IP has uncompensated 1st. stage in a CMF Rebreather, that goes f.ex. 75m like the kiss.- i mean, have they a IP near to 20bar (290psi, if i convert right) or they accept the flow decay and the diver needs to feed more often?

thanks

j o r g e

DanDunfee

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Jorge

The KISS 1st stages (Apeks DS4) are typically set for a surface IP of ~ 135 psig=9.2 bar (atm)=10.2 ata.
The IP preload springs on these regulators reportedly 'bottoms out' at a maximum of about 13.5 bar=14.5 ata. 14.5 ata / 2 (for critical sonic pressure ratio) = 6.75 atm or ~68 msw. Deeper than this, the orifice begins to go sub-sonic and the flow rate begins to drop off. This, apparently, is the main basis for Jetsams recommended MOD of 75 msw. Remember that altho they are excellent rigs, the KISSes are sold primarily for recreational and shallower technical diving..... not for deep-throat technical, caving, or wrecking.

In the Rebreather World Homebuilt Rebreather Forum, I started a Thread entitled 'Max IP for DS4 ??'. You may find it interesting. It was reported in the thread that a special spring is available (NOT from Jetsam or Apeks) to provide DS4 IPs up to ~ 20 bar. I personally have no more details on the spring or sources for it, but one of the posters claims to .

Freef (Rebreather World) has posted a lot of good modelling work on this subject, altho mostly for SCRs. Below is a URL to a good, comprehensive-but-concise, Article, courtesy of Rebreathers Worldwide- Jan Jahns & JW Beck. It contains some graphics on sub-sonic MF rolloff rates.

Gas flow through an orifice

I hope this provides the info you were seeking.

__________________
There are 'Bold' divers and old divers, but NO old, ' Bold ' divers

jorgey

Hi Dan,
yeap, very interesting.- but of course i was not saying, that the kiss was a bad rig.- i took it only as example and because i wanted to know about the design premises of such a system.- you put it very clear, thank you.-

the graphics i told about before were the one of "gas flow through an orifice".- i will be reading the other sources, too.-

ciao
j o r g e

Doppler

Interesting and good information. Thanks for taking the trouble to post this