[MattyBoyUK]

So if a cylinder valve was opened on the surface and then replicated in exactly same way underwater, would the cylinder empty at the same rate or more quickly or slowly under water?

 

[David Pye]

I can't see why the rate would be any different.

The amount of gas left in the cylinder would be different, though, by 1 bar every ten metres (assuming the cylinder was upside down to stop it filling with water). So, on the surface, it would have 1 bar left in it when 'empty'.

At 10m, it would have 2 bar left in it when at ambient pressure.

David

 

[nigelH]

Very slightly more slowly as more external pressure means less net force to drive the gas.

It's a pretty trivial difference at normal diving depths but, of course, at 3kilometers deep it wouldn't bubble at all.

 

[MattyBoyUK]

Very slightly more slowly as more external pressure means less net force to drive the gas.

It's a pretty trivial difference at normal diving depths but, of course, at 3kilometers deep it wouldn't bubble at all.

ok thanks that's what I thought. would there be a different answer is say a second stage was purged ie the loss would be after the first stage pressure adjustment?

 

[nigelH]

would there be a different answer is say a second stage was purged ie the loss would be after the first stage pressure adjustment?

The effects would be very different. The first stage tries to regulate so about 10bar above ambient so theoretically the flow should be roughly constant instead of falling off as the cylinder pressure dropped. Naturally with a hose open it isn't going to be able to regulate but it's going to try but to what level it does is going to be very much dependant of the geometry of the system and I wouldn't like to even guess. Let's just say that with more clutter in the way the gas will flow a bit slower.

Divers are very blasé about pressure. Just because we say things like 300 bar to one another doesn't mean we have a clue what it means.
1 bar is 1Kgm per square cm. In believable units a postage stamp is 4sq cms so 300bar on that is about 1200kgs which is an old Ford Escort with the driver sat in.

One tenth of a bar on your chest (say 30x30cms) is the equivalent of having somebody sit on you, one bar is a car, a nice 50bar reserve is a tank, the military version.

 

[scubabob]

The rate of flow of any fluid between two points, is dependant on the pressure difference between those points. If the ambient pressure around the cylinder is increased, then the flow from an orifice in the container will decrease in direct proportion. If the ambient pressure is the same as the container's contents then there will be no flow. I think both previous posts made this point. I just thought I would add my twopennorth.

Actually I think the relationship of decrease is probably exponential, sorry it's been a long time since my last class.

 

[A. Berk]

The effects would be very different. The first stage tries to regulate so about 10bar above ambient so theoretically the flow should be roughly constant instead of falling off as the cylinder pressure dropped. Naturally with a hose open it isn't going to be able to regulate but it's going to try but to what level it does is going to be very much dependant of the geometry of the system and I wouldn't like to even guess. Let's just say that with more clutter in the way the gas will flow a bit slower.

Divers are very blasé about pressure. Just because we say things like 300 bar to one another doesn't mean we have a clue what it means.
1 bar is 1Kgm per square cm. In believable units a postage stamp is 4sq cms so 300bar on that is about 1200kgs which is an old Ford Escort with the driver sat in.

One tenth of a bar on your chest (say 30x30cms) is the equivalent of having somebody sit on you, one bar is a car, a nice 50bar reserve is a tank, the military version.

Very good Nigel... reminds me of last year when an engineer (don't ya love 'em?) asked me why the diver couldn't pull off a cap at -70m. I took the top off his stainless thermal coffee cup, put it on the deck and asked him if he could prise it off with me standing on it... with two other guys standing on my shoulders? He kind of got the message :wink:

Berok :wink:

 

[Lazlo]

Very slightly more slowly as more external pressure means less net force to drive the gas.

The rate of flow of any fluid between two points, is dependant on the pressure difference between those points.

Is there not a maximum flowrate depending on the design of the orifice? Or is that maximum too high to consider?

Consider a HP hose versus a LP hose. A hole in a LP hose will empty the tank quicker - presumably because the flowrate is limited at some point.

If there were a maximum flowrate, then the tank would empty at the same rate for most of the time, but the surface tank would still empty a little quicker towards the end.

 

[nigelH]

Is there not a maximum flowrate depending on the design of the orifice? Or is that maximum too high to consider?

No. You're probably thinking of the trick called the 'sonic' orifice that they use on manual rebreathers to get a constant flow. It makes the flow independent of the pressure it is flowing into.
Put enough pressure behind something and gases and liquids flow harder.
It's not linear but it goes up.
Google for Reynolds numbers if you want a laugh.

 

[Lazlo]

Fair enough. I think I've identified my confusion - although there is a max flowrate for a given DP, that maximum will be increased as the DP rises.

Funnily enough I was coding a Reynolds Number calculation a couple of weeks ago.

 

[RichardC]

When the pressure differential across an orifice is high enough to push the velocity through the orifice up to the speed of sound then the flow is called choked flow. At upstream pressures higher than this the flow becomes independent of the downstream pressure and the volume flow rate becomes constant. This doesn't mean that the mass flow is constant though because the density of the gas applied to the orifice is dependent on the upstream pressure and the mass flow is linearly dependent on the density.

In the example by the op the external pressure would only affect the flow when the cylinder was within a few bar of being equalised.