CCR First Stage Regs - IP Stability

[fsardone]

Well, in that case, assuming the environmental seal stayed intact and deformed with depth, the relative IP provided by the unit probably dropped 5-12 psi for every 33fsw, because the environmental seal was preventing transmission of ambient pressure to the main diaphragm. That would make it a lot easier on the solenoid, lol. But I would expect the first stage to potentially provide inadequate O2 below 250 ft or even less, both via a solenoid or an orifice. I'm not too sure of CMF physics at low pressures, but it seems likely.

The CMF physics requires two things:
- constant density (hence constant pressure)
- pressure at least twice than ambient.

The second is necessary to have sonic flow (and therefore constant speed since the speed of sound is only function of the (absolute) temperature which in the range we can dive we can assume constant 273-300K ).
The first one gives you the constant mass flow=density*speed*surface of the orifice.
From the gas state: PV=nRT for density we can simplify in n/V and so P/RT so if pressure goes down at a certain point you loose sonic flow and always you loose mass. If pressure increases you increase mass flow and never loose sonic flow.

Cheers

 

[rjack321]

- pressure at least twice than ambient.

No it doesnt need to be twice.
At 100m you would need a huge IP which isnt true. ~14bar is enough to maintain the planned CMF rate at 100m

 

[fsardone]

No it doesnt need to be twice.
At 100m you would need a huge IP which isnt true. ~14bar is enough to maintain the planned CMF rate at 100m

You are probably right. Twice was not precise enough. You need 2,05338 down to 1,70358 times the dowstream (ambient pressure) if you use air, changing gas might change. Oxygen is quite close to air since is a bi-atomic gas

The reason why 14 bar might be sufficient is because while not achieving sonic flow (choked flow is sonic) you are increasing density. But you are out of the design point so the system is not behaving with a constant flow mass.

From wiki ...

The choked flow of gases is useful in many engineering applications because the mass flow rate is independent of the downstream pressure, and depends only on the temperature and pressure and hence the density of the gas on the upstream side of the restriction. Under choked conditions, valves and calibrated orifice plates can be used to produce a desired mass flow rate. Choked flow usually occurs when the downstream static pressure drops to below 0.487 to 0.587 times the absolute pressure in stagnant upstream source vessel.


Choked flow - Wikipedia

 

[Bigbella]

yeah don't do that, your repair tech will hate you and it's pointless. If you're going to fill it with something, since it is normally air sealed, I'd fill it with alcohol a la Poseidon used to

Oh, but they still do, in my book . . .

 

[tbone1004]

Oh, but they still do, in my book . . .

some of mine have been known to do it, though I use scotch for mine, not a big vodka drinker...