Newbie question - bleeding my tank

[cyberfed]

OK, I'm a fairly new diver (3 months certified) I currently own one aluminum 80 tank. I obtained my Nitrox cert not too long ago. Since then I have dove a mixture of air and Nitrox depending, depth, location ect...

Today I decided to bleed my tank completely empty so next time I get it filled I won't have a partial Nitrox blend. I have been letting the air out slowly out of the tank.

I noticed that after a few minutes of the tank emptying it'll stop! I have to then turn the valve even more to the open position to continue emptying the tank. I have done this like 4 times now.

Why does air stop flowing out, while the valve is open?

 

[IceBergSlim]

It's not really a good idea to empty your tanks. Moisture can get in there and in many cases shops will not fill them if they are totally empty.

Don't worry about your mix. The shop will be able to give you exactly what you want.

 

[SteveAD]

I agree with slim, there is no need, and it's not a great idea to completely dump the tank.

But, to answer the question. When the tank is full, the pressure inside helps to force the valve open. As the tank pressure drops, if the valve is barely open, the lack of pressure pushing thru allows it to close.

 

[mwilding]

Unless you are really riding the fine line of MOD and need/want a specific mix, having an odd nitrox mix should not be an issue. Calculating the EAD and MOD is easy and practicing it will let you avoid relying on the standard 32 and 36% tables. If you dive by computer, it should not matter either as it will account for whatever mix you tell it you are diving.

Depending on the shop you go to, you should be able to get whatever mix you ask for unless they are just doing fills from a bank of tanks like they do at my local quarry in which case you are getting a blend of your current mix and 32%.

 

[DA Aquamaster]

The passages in the valve ice up at high flow rates in air where heat transfer is poor.

All that expanding gas needs heat energy to expand and it draws it from the tank and valve which eventually get re-warmed by heat exchanged from the air. In the short term however, the valve gets really cold and the partially open passages get blocked with moisture that freezes onto to very cold internal surfaces of the valve. So the flow slows then stops - until you open the valve some more.

I would only bleed the valve down to about 100 psi or so. There is a very minor risk in bleeding it down all the way as air could potentially be drawn into the tank where the moisture in it will condense inside the tank on the cold tank walls. In reality that risk is extremely remote as the tank is then warming and any air flow will be out the valve. (close the valve on an empty tank and when it warms you will have 10-20 psi in it.)

What you must avoid is bleeding the tank and in the process cooling it and then removing the valve as the warm and comparatively moist air outside the tank can get in it and condense. If I had a dollar for every time I saw a shop do exactly that I could probably take a really cool dive trip to Chuk or some place equally expensive. I make a habit of bleeding my tanks down to 50-100 psi before taking them in for an inspection to prevent exactly that scenario. In a steel tank dumping 1000 psi or so and then removing the valve to inspect the tank can be enough to cause flash rusting.

The real risk you face in draining an AL tank is that your LDS will insist on a VIP of the empty tank to ensure there is no water in since it is totaly empty. If you leave 100 psi in it the LDS is happy and the air in it is small enough in volume that it will not screw up your nitrox mix more than 1 or 2 10ths of a percent.

In many cases the exact mix also does not really matter. If using tables and you have 31% - use a 30% table rather than the 32% table - just be sure to heed the slightly shallower MOD of the 31%.

I have at times switched from one nitrox mix (for example 25%) to another (32%) without dumping anything and over the course of a few fills it finally approximates 32%. Unless you really need an exact mix for some reason it does not really matter so in a quarry or anywhere else where bottom time/NDL's or MOD are not critical I would not worry about it.

Larry

 

[pescador775]

I think Steve is right; the tank valve seat rotates in a brass body around a course screw thread which has a certain amount of "slack" in the mechanism but also particularly in the seat itself which is made of an elastic material. Normally, a closed valve balances the opening and closing forces. Turning the valve knob just a bit removes some of this closing force which will cause high pressure air to press against the seat and lift the mechanism enough to leak. After the tank pressure drops somewhat, the opening force caused by pressurized air against the seat also drops shutting off flow.

While not commenting on whether you should or should not drain the tank, this is the procedure which I use. I open the valve wide and blast the air out. Note that the valve may have to be opened wider once or twice as press drops. This is due to the principle of air flow being proportional to opening size and to the mechanics of the valve discussed above. Ice formation may also figure in to it. When the press drops to about 200 psi, I shut the valve and let the tank warm up. When the tank is warm and relatively dry, the remaining air is drained. Note that some water may blow out of the valve during this last step. This is normal and harmless.

 

[beanojones]

Hey this is an interesting thing here.

This is a perfect example of why you never turn the valve of your tank back closed a bit after you open it. Open it all the way and leave it alone.

Despite what your Open Water instructor tole you.