MCCR in-depth

Janwillem Bech

http://www.therebreathersite.nl/mccr_in_depth.htm

enjoy;

Janwillem Bech
http://www.therebreathersite.nl

RebreatherNut

Janwillem,
Nice Article, thank you!

RBN

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decoweenie

Jan,

Just out of curiousity, any chance you could snap a pic of your Travel-Frame rig sitting next to a Meg just for comparison ?

solocavediver

Thanks for a really great article. One small point, you wrote how flow would stop at 130m deep in your CMF valve if you had a 1st stage valve delivering intermediate pressure of 14 bar. Now that is absolutely true if the 1st stage valve is an "unbalanced" valve. But if it is a "balanced" valve - which many are - it will deliver an IP of 14 bar more than ambient, at any reasonable depth. Or some unreasonable depths - I suspect that a decent "balanced piston" valve would go on pumping gas at the same relative pressure right down to the spine-chilling depths where external pressure was 14 bar less than in the tanks. (i.e. 1850 meters if your tank is pumped to 200 bar). For me this is all hypothetical - my personal max depth is about 55m and I have not yet felt what people tell me, that the 2nd stage of a open circuit rig becomes harder to breathe at such depths if the 1st stage reg. is unbalanced. But if you want to feed a CMF valve at serious depth, get hold of a balanced piston reg. IN THEORY - as I say I've never tried it nor am likely to for a good while to come.

Cheers,

Charles.

decoweenie

It is usually "commonly understood" that mCCR works with either a) uncompensated (i.e. "unbalanced") first stage and orifice, or b) compensated first stage and adjustable metering valve.

Plus, you don't want a compensated reg with IP of 13 bar delivering O2 at 140m...

jradomski

Balanced or unbalanced first stage has nothing to do with it.. You can use EITHER with a CMF system. That function is to compensate for changing cylinder supply pressures ( a cmf will function more smoothly with a Balanced design).. What matters in a CMF system is depth compensation or not... This keeps the IP relative to ambient pressure.

Fixed orifice works best with a non depth COMPENSATED 1st stage
Needle valves work well with either, but is usually used with a depth compensated 1st stage.

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

solocavediver

OK folks so let's express this mathematically (excuse me I'm a mathmo). What I call an "unbalanced" regulator and Joe calls an "uncompensated" regulator produces a roughly constant second stage pressure of 14 bar. So, below 130m nothing will flow out of it - not usually a problem with the recreational divers who buy this kind of reg, since they stop somewhere around 40m. A balanced one produces a variable pressure, if the ambient pressure is X you get an output pressure of X+14, to suit equipment like 2nd stage o/c regs which like a constant pressure difference. But I guess the trick with CMF valves is that you only really get constant mass flow if the intermediate pressure is enough for the physics of it to give that constant mass effect, which if I remember rightly is when the IP is around twice ambient. So you'd need to modify the 1st stage so that at ambient pressure X it produced an output of (maybe) 2X+14. Obviously no o/c reg is made that way because it would freeflow once the IP got significantly above the value X+14 which 2nd stages are designed for (that little spring inside your 2nd stage valve would be pushed back, resulting in a freeflow). If Janwillem has done such an adaptation himself I'd love to see the details of this little wonder of DIY engineering..... But maybe it's just easier to adjust an uncompensated reg to produce a constant 50 bar (e.g. stronger spring inside the 1st stage). That gives at least 2X until you get down to 240m deep. Not many of use dive below 240.

Cheers,

Charles.

jradomski

there are two types of 1st stage regulators, balanced and unbalanced.. the balancing allows supply pressure to get on the other side of the chamber to insure that varying supply pressure does not effect the output pressure.
An unbalanced piston, the IP will generally decrease as cylinder pressure drops.
An unbalanced diaphram, the IP rises as tank pressure decreases.

Compensated or non compensated regulators (can be incorprated in either of the above types)..

COMPENSATED
This typep of reg adjusts the output pressure based on ambient pressure.. all standard scuba regs are of this design.. ambient pressure is transmitted into a chamber (whether directly or indirectly) that is added to the force supplied by a spring to vary the output pressure.. if the pressure goes up so does the output pressure, if the pressure goes down so does the output pressure..

Non-Compensated - No off the shelf scuba reglators, These types of regs are only available as rebreather parts through some manufacturers, or by modifyiong a standard reg..

CMF designs usually (not all of them do) use a non-compensated design because flow rate through an orifice is controlled by 2 factors.. The most important being supply pressure.. the ultimate flow rate is determined by the suppply pressure.. The higher the pressure the higher the maximum flow that can be achieved..
The second factor is the outlet pressure.. as long as inlet pressure is about 2X that of the outlet pressure the mass flow remains constant ( a greater ratio does not increase flow further its goverend by the max flow for a specific inlet pressure) until the pressure ratio drops below that then the flow will vary based on the ration of inlet to outlet pressure and the driving inlet pressure..

if you use an unbalanced and uncompensated regulator, the output pressure still varies based on tank supply pressure, but is generally uneffected by variations in depth.


There are now "balanced" second stages, these balance the cracking pressure with the rise in IP and Ambient pressure.. This is necessary because on some first stages the ambient pressure compensation is greater than the change in ambient pressure.. This means the cracking pressure would have to be set higher than optimal without otherwise it would freeflow at depth..

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Padowan

As far as I am aware, you can't buy (what I call) a non-depth compensating first stage - this functionality is achieved by doing a home made modification to the first stage to fix the IP and prevent it increasing (absolutely) with depth.

All first stages that you pick up off the shelf AFAIK are designed such that the surrounding water pressure changes the (absolute) pressure passed through the reg such that there will always be a considerable pressure above ambient. Whether the gubbins inside is a piston, a diaphragm, sealed, exposed the concept is the same. I agree that different reg designs have slightly different characteristics regarding the linearity of the pressure increase over ambient that they generate, but the result in a normal OC world is the same (ignoring things like flow, density and sonic limitations) - gas comes out at pretty much any depth.

You don't want this in an MCCR, unless you have a depth compensating orifice to maintain a constant molecular flow of oxygen into the loop. IMO it's easier to fix the IP to not compensate with depth, then fix the size of the orifice and accept that there is a limitation on it's use...

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solocavediver

OK Joe, thanks for clearing up my confusion over the notation:


>there are two types of 1st stage regulators, balanced and unbalanced.. the balancing >allows supply pressure to get on the other side of the chamber to insure that varying >supply pressure does not effect the output pressure.
>An unbalanced piston, the IP will generally decrease as cylinder pressure drops.
>An unbalanced diaphram, the IP rises as tank pressure decreases.

>Compensated or non compensated regulators (can be incorprated in either of the >above types)..

>COMPENSATED
>This typep of reg adjusts the output pressure based on ambient pressure.. all standard >scuba regs are of this design.. ambient pressure is transmitted into a chamber >(whether directly or indirectly) that is added to the force supplied by a spring to vary >the output pressure.. if the pressure goes up so does the output pressure, if the >pressure goes down so does the output pressure..

>Non-Compensated - No off the shelf scuba reglators, These types of regs are only >available as rebreather parts through some manufacturers, or by modifyiong a >standard reg..

I am still slightly surprised that "Fixed orifice works best with a non depth COMPENSATED 1st stage". On the basis that "as long as inlet pressure is about 2X that of the outlet pressure the mass flow remains constant", I would have expected you guys to be tweaking your regs so the linearity between ambient and IP was X -> 2X rather than X -> (fairly large) constant.

Live and learn though!

Cheers,

Charles.