Free flow at depth?

[perpet1]

you know I asked myself the same question. I found the a worse case free flow in the first stage. Then I got to think about it more and took you the mechanical drawings of a regulater (first stage and second stage). Even if the second stage freeflows the first stage will open wide to keep up with the demand so in a sense the entire system is freeflowing directly from the tank. I agree the head loss will be great and the pressure at the end of the system (coming out of the second stage) will not be 3000psi but the entire system will be in a state of free flow subject to the laws of fluids.

It still comes down to Delta P as head loss will be consistant (if you are really free flowing)

 

[DA Aquamaster]

Charlie99 once bubbled...
There was a post in another thread about how you can't really breathe off the octo very easily when there is a freeflow on the primary, because the IP goes very low.

This is true with a low performing first stage where it has a flow capacity that is inadequate to meet the demands of two seconds stages flowing at the same time (the one freeflowing and the one you are breathign from. In that case you are better off breathing from the free flowing reg (they do still teach that don't they?).

With a high performance first stage and a modern high capacity tank valve it should not be an issue and IP should remain adequately high.

 

[perpet1]

Wood once bubbled...
Could be a bit off here; but I think a balanced regulator will keep it's internal pressure ratio to ambient pressure pretty even until you start going really deep. At those depths the "drag" that divers feel is the thickness (density) of the gas itself. At 4 ATA the gas would be 4x as dense (not that this is really deep…)

A free flowing regulator should flow just as much gas at depth as it would at the surface, maybe just a tiny bit more or less based on some of the physics presented here. For the sake or keeping it very simple, you could probably assume that you would blow through 4x as much gas free-flowing at 99fsw as you would at the surface due to the gas density on a balanced regulator.

Should this happen I'm with the rest, grab your buddy and get the hell out of there.

This is correct if you are not in a state of freeflow. The general idea of a first stage regulator is to maintain a constant IP regardless of demand (and I would say ambient pressure hence balanced regs). With ANY type of first stage, the first stage will open to maintain a constant IP but it will have to stay "wide open" if the second stage is freeflowing. In this state there is an open path (although a high head loss path) to the 3000psi in the tank. ===>fluid laws apply

 

[perpet1]

You all know this is an utterly useless thread but it has made me review regulator operation and dig way back to when I used to have to care about fluid dynamics......

to that I say THANKS to the ZenSquirrel for posing the question

 

[Diver0001]

Charlie99 once bubbled...
Have you actually done a test on this ..... such as measured how many bar you lose from depressing the purge for 15 seconds at 30 meters vs 15 seconds of purge at surface? Which reg?

If the orifice of the demand valve were the only limiting factor, and the IP stayed close to rated value even during freeflow, then you statement is probably true. If on the other hand, the main flow restriction is in the first stage, then variation in freeflow rate vs. depth would much less.

Not having done this test, I had just assumed that it would be a bit faster, but nowhere near the 4 times you describe.

Thanks in advance,

Charlie

Depends on a few factors. A typical K valve will deliver about 1100 litres per minute wide open without a reg on it. Some first stages will deliver up to 5000 but the pilar valve is still teh bottle neck in most cases. A typical 10 litre tank will have about 2200 litres in it when full. So without the reg it takes about 2 minutes to be totally empty. I've seen a tank with a reg on it freeflow until empty in about 3 1/2 give or take. A better reg might have gone a little faster, i don't know. In reality, most freeflows aren't problaby "wide open" anyway so although not accurate behind the decimal I said 4 minutes because it was easier to calculate with to get the concept across. In fact the 30 seconds that DA Aquamaster reported at 140ft rings true with me especially knowing that the tank wouldn't have been entirely full when it started.

R..

 

[Diver0001]

norcaldiver once bubbled...
SO now this is really buging me. I understand the resistance issue. I understand that compression due to presure also.
I'm thinking that the air is at 3000psi in the tank. That doesn't change until it's out of the tank. Because the tank volume doesn't "really" change at depth due to the tank strength, the air doesn't become MORE compressed inside the tank. Am I right? So wouldnt' that leave just the resistance?
You can't think "balloon" science here because the water presure istn' pushing the air out of the tank like it would a balloon.

Entirely correct.

R..

 

[perpet1]

Diver0001,

I agree with you and both of your last posts as the tank is in fact compressed and the concept of a boloon is incorrect.

I also agree that a regulator flow rate is an critical factor for this discussuion. and regulators are desined to maintain a specific "max flowrate" under "normal conditions".

That said, i in in your camp that it will not drain the tank 4 times faster as was initially discussed in this thread. BUT it will NOT be consistant with the surface either.

You implied that a Typical K value is 1100 l/min which I am in asgreement with give or take.

In the example you provide it states you can drain 2200l in 3.5 min. In that case the flowrate was 2200l/3.5min=628.57l/min. I am saying her and now that this flowrate will be slightly less at depth due to the fave that the driving force is the difference in pressure between the pressure in the tank and the external pressure. In a freeflow contition the regulator is open constantly (not cycling) so flow based on the difference in pressure interal to external.

Pete

 

[Diver0001]

perpet1 once bubbled...
Diver0001,

I agree with you and both of your last posts as the tank is in fact compressed and the concept of a boloon is incorrect.

I also agree that a regulator flow rate is an critical factor for this discussuion. and regulators are desined to maintain a specific "max flowrate" under "normal conditions".

That said, i in in your camp that it will not drain the tank 4 times faster as was initially discussed in this thread. BUT it will NOT be consistant with the surface either.

You implied that a Typical K value is 1100 l/min which I am in asgreement with give or take.

In the example you provide it states you can drain 2200l in 3.5 min. In that case the flowrate was 2200l/3.5min=628.57l/min. I am saying her and now that this flowrate will be slightly less at depth due to the fave that the driving force is the difference in pressure between the pressure in the tank and the external pressure. In a freeflow contition the regulator is open constantly (not cycling) so flow based on the difference in pressure interal to external.

Pete

Hi Pete,

Fair enough, thanks for the clarification. It's comforting to know that one would get a few extra seconds at depth to deal with the problem.

I'd like to add one more thing to this discussino. Several people have mentioned "getting out of there" or "get the surface" or suchlike in the event of a freeflow at a deep depth. What I'd like to add is that you shouldn't really be doing deep dives unless you either have a buddy you can trust to be there for you and/or you have a pony or suchlike to avoid losing all of your air if a regulator malfunctions. I realise the difficulties of kitting up for every eventuality when dive-boat diving in rented gear on vacation etc but even when the water is warm and clear deep is still deep and rented gear is also well.....broken in.....and you should take that into consideration when you prepare for the dive. If you *do* dive deep and you have a problem with a regulator your first instinct, in my opinion, should be to run to your buddy not to flee to the surface. I personally believe it's better to try solving manageable problems like a freeflowing reg under water to avoid a possible DCS or other injury from a too-fast ascent. Also when fleeing to the surface you are also in many/most cases fleeing further from your buddy who is the one person down there ready and able to help you.

Just another 2cents worth.

R..

 

[perpet1]

I totally agree and I too was guilty of it. Although this would be a stressful event there are a few things you have to think about.

1. you have some time so NO need to ditch the weights and bail. I have actually seen this happen..... To many instructors teaching the ditch and bail method to solving problems.

2. if your buddy is accessable there is perfectly good (safe) source of air. Do you have time to get to them? This is a decission you need to make quickly. In fact you should pretty much know the availability of your bubby at all times anyway. If you were planning an activity that could result in buddy seperation and that was acceptable to you then you should be hypervigilant regarding the availability of your buddy. I would think this is one on the skills required for doing such a dive like fishing or photog.

3. even if you are deep (say 100') and your buddy is not within reach, you should have enough time to get to where you can safely make it to the surface. I watched a diver (navy diver at that) make an emergency swimming ascent from 80' without exceeding any ascent rates. The trick is NOT to panic.

4. propper dive planning and redundancy if you are planning activities that will potentially result in buddy seperation would increase your safety margin.

I agree we/I missed the point ot the thread but it was a lot of fun (yes I am a true geek).

Thanks,
Pete

 

[DA Aquamaster]

Ditto what Diver0001 said. If you are deep you need a redundant air source, whether it be properly configured doubles, a adequately sized pony bottle or an attentive and aware buddy.

All free flows are not created equal.

A freeflow where a mechanical failure has ocurred in the first stage would most likely be a leaking high pressure seat and the second stage then freeflows as the IP is excessive. These range from a second stage that starts freeflowing slightly several minutes after the last breath was taken to a steady flow of bubbles that would be annoying but would allow a normal ascent as air loss is still within reasonable limits. A failure of the second stage seat is similar with a slight freeflow that may not even be noticeable in a heavy breathing diver to one that produces a steady stream of small bubbles after the seat has had a few more dives to wear itself out to a greater degree. I have never seen a failure of this type go from slow leak to a massive loss of air in the confines of a single dive. With prompt maintainence a relatively slow leak is all you will encounter and a normal ascent is possible.

A freeflow induced by the first stage freezing is another matter entirely. This will cause the first stage piston or diaphragm to freeze open and will almost always result in a maximum or near maximum rate freeflow, which once it starts, cools the reg enough to ensure it will not stop until the tank is empty, the valve is turned off, or in very rare cases, where substantially warmer water is reached (usually warmer water is not enough is not enough).

The required action here is to switch to another air source (buddy or otherwise) and if possible turn the tank valve off to save the air. The reg will thaw in a couple minutes and air should again be ready for use.

With most doubles arrangements you will need to turn off the appropriate valve to prevent both tanks from being emptied in short order.

It is important to note though that in cold water a small failure and freeflow as indicated in the first case can quickly lead to a frozen reg and a the resulting catastophic freeflow in the second case. The additional air flowing due to the slight freeflow and the lack of any period where no air is flowing through the first stage (during the pause between inhale and exhale and during the exhale itself) is ften enough to exceed the first stages ability to absorb enough heat from the surrounding water to prevent the formation of ice crystals in the reg.