Don't breathe tanks to zero?

[FinnMom]

delete this

 

[Panama Jones]

I feel that "period" actually has a question mark following...

Yes you can breath it down to ambient pressure.
Most of 1st stages are normally open so they will allow the air to pass through at any pressure.

Hmmmmm. I know we're kind of on to a new topic but can you clarify for me? My take, and honestly I may be wrong and not just arguing, is that most first stages will not allow air though at any pressure. In fact, ambient pressure is enough to close the 1st stage thereby keeping air in the tank at thousands of PSI. You actually have to lower the pressure on the 1st stage to below ambient by sucking on the demand valve (2nd stage) in order to get the 1st stage to release air on the tank. The deeper you go, the higher the ambient pressure on the 1st stage which is why you have to suck harder on the second stage at depth. Am I wrong? I may also be misinterpreting your response.

 

[captain]

Not connected to a tank the 1st stage is normally open. If it wasn't you would not be able to pressure up the 2nd stage. When the tank valve is opened the air flows through the 1 stage until the pressure the IP (135-140 psi) is set at is reached and it then closes. Each time you take a breath you reduce the IP by some amount and the 1 stage again opens to bring the IP back up to its set pressure.
The 1st stage is exposed to ambient pressure. Because it is it doesn't make any difference what the ambient pressure is the IP will always be ambient pressure plus IP. It does not make any difference if you are at the surface or 100 feet Therefore you do not have to suck harder the deeper you go because the pressure to the 2nd stage demand valve is always what ever the IP is set at above ambient pressure. At 100 feet the ambient pressure is 44 psi and if you could connect an IP gauge located on the surface to the regulator it would read 44 psi + 135-140 = 179 to 184.

 

[elan]

Basically captain has explained it already.
The point where you are not accurate is that "you have to lower the pressure on the 1st stage to below ambient" The 1st stage will maintain an intermediate pressure of say 140psi above ambient. Once you drop it by sucking on the second stage it will try to recover it back to the preset value.

As an example, if you unscrew any of the LP plugs on the 1st stage and open the tank valve the air will just drain the tank completely until it's empty because the 1st stage will try to maintain an IP and will not be able to as the plug is open; so it will simply drain the tank.

With depth the IP stays the same relatively to the ambient pressure. You do in fact need to suck a bit harder with depth, not because of the IP but because the gas becomes more dense. With a good reg you will unlikely notice it in recreational depth range. Some regs will overcompensate the IP with depth to minimize this effect.

Hmmmmm. I know we're kind of on to a new topic but can you clarify for me? My take, and honestly I may be wrong and not just arguing, is that most first stages will not allow air though at any pressure. In fact, ambient pressure is enough to close the 1st stage thereby keeping air in the tank at thousands of PSI. You actually have to lower the pressure on the 1st stage to below ambient by sucking on the demand valve (2nd stage) in order to get the 1st stage to release air on the tank. The deeper you go, the higher the ambient pressure on the 1st stage which is why you have to suck harder on the second stage at depth. Am I wrong? I may also be misinterpreting your response.

 

[The Chairman]

It's easy to test. Try to suck on a reg with the dust cover off. Unless it's one of those "dry valves", you'll be able to get air right through the reg. No, it's not near as easy as it is with pressure pushing it through, but it demonstrates that you can breathe a tank down to ambient, and perhaps a psi or two lower!

 

[Panama Jones]

It's easy to test. Try to suck on a reg with the dust cover off. Unless it's one of those "dry valves", you'll be able to get air right through the reg. No, it's not near as easy as it is with pressure pushing it through, but it demonstrates that you can breathe a tank down to ambient, and perhaps a psi or two lower!

The two previous posts make sense (Elan /Captain). But was about to argue the point of the 1st stage bleeding the tank to ambient if disconnected from second stage because I did try it. In a previous post on this thread i tried to breathe a tank down with two different regs and found that the tanks had a fair bit of pressure even after the regs stopped breathing. In other words, if I disconnected the first stage, i could crack the valve and get a good blow. Not sure at what pressure because it was low on the spg. But I stopped when the regs stopped breathing normally and a normal breath got nothing. So I don't actually know if I could have forcibly sucked them to zero. either way, I stand corrected

 

[OkByMe]

I'm not talking about monkeying with the ports. The original question was if you could contaminate a tank with moist air by breathing it down completely. You can't breath a tank dry -PERIOD- The seat will lock with spring pressure to at least the IP setting. Now if you want to hang around under water sucking on the regulator until you are gasping for air to disprove my point, knock yourself out.

If you want to check this out without turning blue...

install a set on a nearly empty tank. Push in the purge valve on the second stage until no more air comes out. You can suck on it until you turn blue, but at least you won't drown. Now, without closing the tank valve try to remove the first stage from the tank. Just joking, and I don't do jokes. Close the valve, remove the set. Take a plastic shopping bag and put it over the tank valve, pull it down to the neck and tape it closed over the valve. Open the tank valve through the bag. Did it pop? Did you jump? You judge.

 

[lamarpaulski]

I participated in some of these exercises years ago in a pool and they were pretty valuable to get the 'feel' of running low.


This excersise is intended to show to the students how does it feel an empty tank and how a second stage behaves when low on air.
A tank in this condition, still needs some minutes to completelly vent the air inside until zero relative pressure.[/QUOTE]

 

[elan]

I'm not talking about monkeying with the ports. The original question was if you could contaminate a tank with moist air by breathing it down completely. You can't breath a tank dry -PERIOD- The seat will lock with spring pressure to at least the IP setting. Now if you want to hang around under water sucking on the regulator until you are gasping for air to disprove my point, knock yourself out.

If you want to check this out without turning blue...

install a set on a nearly empty tank. Push in the purge valve on the second stage until no more air comes out. You can suck on it until you turn blue, but at least you won't drown. Now, without closing the tank valve try to remove the first stage from the tank. Just joking, and I don't do jokes. Close the valve, remove the set. Take a plastic shopping bag and put it over the tank valve, pull it down to the neck and tape it closed over the valve. Open the tank valve through the bag. Did it pop? Did you jump? You judge.

Which seat will lock the pressure to at least the IP setting?

I routinely drain one of my tanks dry when I need to dry some parts with air. I have an old mk10 with a air gun on it and run the tank empty. You can unscrew the reg without closing the valve once it is empty and no air comes out of the tank.

Your case holds the ground if you use an unbalanced second or a first stage with some kind of ACD on it. In these cases you will still have some gas in the tank

 

[captain]

Which seat will lock the pressure to at least the IP setting?

I routinely drain one of my tanks dry when I need to dry some parts with air. I have an old mk10 with a air gun on it and run the tank empty. You can unscrew the reg without closing the valve once it is empty and no air comes out of the tank.

Your case holds the ground if you use an unbalanced second or a first stage with some kind of ACD on it. In these cases you will still have some gas in the tank

His case doesn't hold water even with an unbalanced 2nd stage. If the purge is held open once the IP drops below set point the 1st stage is open and will stay open until the tank is completely empty ( ambient pressure)