is it bad to empty a tank?

[BelikinBob]

I was playing with my new gear in a pool, I dont have my computer yet and no guages. The tank I was testing out with my new gear was just about empty so I took it out of the pool. I was concerned that water or moisture could enter the tank. there is some pressure but not enough to breath off of.

so, what happens if I let the tank go down to 0 psi?

Bob

 

[The Kraken]

To empty it, no.
To leave it empty, yes.

If an LDS is doing a nitrox from a bank, the filling operator should empty the tank and then fill it to insure a proper % of O2.

I know of no rule of thumb for leaving a tank completely empty, but I wouldn't want to do if for more than 2 or 3 weeks.

Perhaps others have more experience in this matter and will chime in.

Time is of the essence.

the K

 

[truck1]

If the tank was emptied in the pool,(or anywhere under water for that matter) there is a chance that you could possibly get water into the cylinder.Over time this could be very detrimental to the cylinder.I recently just finished one of Netdocs classes and he was telling the class a story about a woman who lent her tanks to her friend to dive with.Her friend sucked the bottom out of a tank and water was introduced, then the tank was refilled.(No one knew that there was water in the tank.)In less than a year, the tank was up for VIP.It failed VIP even though the tank was still almost new.Bottom line.If it where me, any time my tank was empty for what ever the reason, I would have it VIP to be safe.I would rather spend 10 bucks or so once in a while then have a cylinder fail and have to replace it.Also, most compressed air cylinders, either scuba or scba or what ever, recommend that anytime a tank is empty to have it VIP.

 

[Walter]

As long as the valve is closed, it's not an issue. Tank valves are designed to hold high pressure, they aren't going to let in moist, low pressure air.

 

[Walter]

If the tank was emptied in the pool,(or anywhere under water for that matter) there is a chance that you could possibly get water into the cylinder.Over time this could be very detrimental to the cylinder.I recently just finished one of Netdocs classes and he was telling the class a story about a woman who lent her tanks to her friend to dive with.Her friend sucked the bottom out of a tank and water was introduced, then the tank was refilled.(No one knew that there was water in the tank.)In less than a year, the tank was up for VIP.It failed VIP even though the tank was still almost new.Bottom line.If it where me, any time my tank was empty for what ever the reason, I would have it VIP to be safe.I would rather spend 10 bucks or so once in a while then have a cylinder fail and have to replace it.Also, most compressed air cylinders, either scuba or scba or what ever, recommend that anytime a tank is empty to have it VIP.

Actually, water is put into more tanks than you might imagine. Spray hits the tank valve, water gets on the valve when we rinse the tank, The tank is not under cover during the rain. There are lots of ways water can be positioned where it can get into a tank. The valve will keep it out, but we need to help keep it out. Open the valve and blow the water from the valve before it's filled. If you have it filled with the water at the orifice, the air with which we fill the tank will force that water into the tank. Blow out your valves before having them filled.

 

[TSandM]

Some shops will insist on a VIP before they will fill an empty tank.

 

[JeffG]

Some shops will insist on a VIP before they will fill an empty tank.

Thats why I own a transfill whip.

 

[SDAnderson]

so, what happens if I let the tank go down to 0 psi?

Probably, nothing. Zero pressure in the tank can easily become a little bit negative due to temperature changes, which can introduce contaminants, including atmospheric moisture. That's not likely to cause big trouble, though in extreme cases it can destroy a cylinder in just a few months. More to the point, however, is that a completely drained tank tells the fill station operator that you didn't/couldn't follow the rather simple protocol to leave a little positive pressure in the cylinder which raises questions about what else you didn't do properly. Around here, zero pressure in a tank usually means the fill operator is going to want a new visual inspection, a minor penalty for breaking a rule.

Remember, the kinetic energy in a tank is equivalent to a smallish bomb. If the tank fails an inspection, you're going to be out the value of the cylinder. If a tank fails, however, the guy who usually takes it in the shorts isn't the diver - it's the fill station operator. Forgive them if they get kind of hinky about even minor protocol violations.

 

[dannobee]

Like others have said, if the cylinder was in the water when the pressure went to zero, there's a chance that water got in through the regulator, and some shops will want to take the valve off and take a peek inside if there's no hissing when the valve is cracked. If the tank valve was left open while on dry land, there's no chance that water got into the cylinder (unless by some chance you stored it IN a bucket of water or something. The shops that require a vis on empty tanks are justified as who knows what could be in there. Interestingly, oxygen tanks (the welding kind, but not always the scuba kind) are vacuumed out to 29 in Hg just to be sure that everything is out before filling (any moisture will boil out at that vacuum/low pressure).

If there's pressure in the tank, there's no way that water will get into the cylinder without refilling (assuming that all of the previous fills were with dry air), no matter how much water is around the valve-not to say it's not a good thing to keep water away from the valve as it could get into your regulator or spg. There's a slight chance that *some* water can be introduced into the cylinder if water is around the valve at the time the cylinder is refilled (in the tank valve or whip hose before the valves are opened), but I'd hope that most fill operators are careful and aware of this if they put the cylinder in a water tank while filling.

 

[truck1]

As long as the valve is closed, it's not an issue. Tank valves are designed to hold high pressure, they aren't going to let in moist, low pressure air.

This is true.
I do think that he was referring to emptying a tank underwater where there woud be no pressure to hold the water out.Like you I do beleive that more water/contaminents than we think gets in our tanks, for one reason or another.Where I work, anytime one of our scba bottles are emptied, even by accident, we have to take it out of service to be VIP just in case.And our cylinders are composite/fiberglass wrapped.