flow and ambient pressure in SCR

Baron Gautsch

Hello everybody

I'm interesting in knowing the increasing of flow with growing ambient pressure (go deeper! ).

My REB is an Azimuth AF. I've some doubts about the fact it could rise linearly as the growing of Ip / Pa in a balanced first stage (MARES MR12), maybe something lesser.

Do u have some dates about this

I tried to get up the intemediate pressure by acting on the first stage spring but in a dry enviroment (surface ) I hardly could have had scientific sounded results

Thx in advance

Freef

[the red section was added as an edit, see posts below for reason why]

If the jets are set up for sonic flow, no increase in IP will make the flow rates increase-once you hit sonic flow, that's it, as long as the downstream pressure [IE the ambient pressure] stays below a certain value. The difference in pressures is called the Critical Ratio.

If the IP was increased, or depth compensated then you would be able to keep the sonic flow for longer [for example the Dolphin falls below the critical ratio for sonic flow at 81m].

You need to find the minimum pressure for the critical ratio and work on that. The following gives a bit of an insight, but I didn't cover the subject in depth.

http://www.rebreatherworld.com/semi-...ssure-scr.html

__________________
David.

Currently owner of two differently sized ankles.

DanDunfee

David:

Better take a look at the flow analogies again:

Once you're sonic, and stay above the critical pressure ratio, the mass flow doesn't change with the downstream (ambient) pressure, which means that you can confidently set up your diving mass flow on the surface. HOWEVER

Once you're sonic, and stay above the critical pressure ratio, the mass flow does increase or decrease with different upstream (IP) pressure, at any given downstream (ambient) pressure.

With a 'non-compensated' 1st stage, like the Draeger Dolphins, you must keep the MOD pressure less than half the surface-set pressure to avoid a flow reduction. The flow rolloff below the critical pressure ratio starts slowly, but will finally, essentially stop when the downstream (ambient) pressure-at-depth equals the surface-set IP, as you pointed out in your post.

With a 'compensated' first stage, which Azimuth has, the 1st stage reg will maintain the critical ratio down to significantly greater depths.

There are a couple of examples of tests already imbedden in other similar threads. I think Tom Rose documented some results of flow rates vs surface IPs in one of his fairly recent posts. .
==============
PS: It wasn't Tom, but marekm, here's his data.
[Quote: I recently had time to do some flow rate tests with the McMaster Carr 0.004" orifices sent to me courtesy of Tom Rose. The results were as follows Quote]:....................It looks quite linear above about 90 psi.

psi...bar....gas...flow (LPM)
50...3.4....o2...0.38
70...4.8....o2...0.45
90...6.1....o2...0.53
110..7.5...o2...0.58
150..10.2..o2...0.79

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Bosh

Theres a teardown and explanation at Dave suttons website : Diver Daves Rebreather Site

Best regards

Bo

gibbogeo

Ehm, no. It is a general error, but the reality is different.

The pressure that doesn't influence the flow is the pressure _after_ the restriction (until it is less than about half the pressure that feed the restriction).

Here you can find an article explaining how dolphin works and how Ray works.

How Dolphins work

Here a more general explanation of the theory:

Gas flow through an orifice
http://www.therebreathersite.nl/04_L...ads/Choked.pdf

We debate for years in Italy about that matter. Many people who built equipment (and many more that use them) don't understand this principle and spread incorrect information.

It is quite unimportant in a SCR, (ok, you have to take care of it in planning gas consumption, but it is not a big deal until you do sport dives), in CCR it is a more serious matter.

Freef

Thanks for pointing out my mistake, I cut myself off a bit on the sonic flow being affected by the downstream pressure, but that sort of self corrected in the next paragraph on the critical ratio [information from the 'Dow Dolphin's Work' page quoted by the others].

I'll write my hundred lines out now, and I promise not to use CTRL+C CTRL+V.

__________________
David.

Currently owner of two differently sized ankles.