Valve Condensation

[markyd]

I recently purchased a used steel tank. It was shipped with the valve removed, but with a plastic plug installed (not sure if it was actually sealed or not though).

When it arrived, I did a basic visual check inside and it looked good: a little bit of surface flash rust but nothing major (just some discoloration, no pitting was evident, and certainly no water). So I spun a valve onto the tank temporarily until I had a chance to get it re-filled.

Last night I removed the valve again in preparation to get it filled (I just wanted a different valve than the one I had installed temporarily), and I found condensation on the valve itself, inside the tank. I ran a finger in the neck of the tank and there wasn't any condensation where I could reach inside the tank itself.

I know what causes condensation to form on the valve (it was relatively cold, maybe 45-50 degrees), but my question is whether there is anything I should do to 'dry' the tank before I have it filled. I'm assuming a couple of dives and refills will effectively purge the remaining humidity from inside the tank, but I don't know how long it will be before I can get some dives in.

I'm also thinking that every time you take a valve off a tank to do a VIP, you can have the exact same problem (humid air getting in the tank that can later condense), so maybe there is nothing to worry about.

Any opinions?

marky-d

 

[H2O 70]

For drying out tanks after cleaning, I've got a piece of 3'x1/2" clear PVC hooked up to a hair dryer. I turn the tank upside down and turn the hair dryer on low and keep it on till the tank feels warm. That way I know it is nice and dry on the inside and hopefully the warm air has further dried out the air. I then throw the valve onto and keep it closed. As the air inside cools down it sort of creates a vacuum. When you take it to the shop to get filled, they will hook up the tank. They should then open the bank and then the tank. Only clean dry air should enter your tank!

Does this help?

 

[oxyhacker]

PVC can outgas if it gets too hot, and the fumes are not good for. Personally, I'd rather have moisture in the tank!

Re the original question, temperature swings can produce visible condensed moisture in the tank, for example, if you seal the tank on a hot humid day and open it again on a cold one. Put the the valve back on and warm the tank up, and it'll vanish again, re-absorbed by the air. All the same I don't like the idea of resealing a tank with visible moiture inside, so drying it out by blowing some warm air through the tank is a good idea. Still, if there is no rusting a lot of divers wouldn't worry about it for exactly the reason you give - the super-dry scuba air when the tank is filled should absorb up any moisture unless maybe you are diving in very cold water.

For drying out tanks after cleaning, I've got a piece of 3'x1/2" clear PVC hooked up to a hair dryer. I turn the tank upside down and turn the hair dryer on low and keep it on till the tank feels warm. That way I know it is nice and dry on the inside and hopefully the warm air has further dried out the air. I then throw the valve onto and keep it closed. As the air inside cools down it sort of creates a vacuum. When you take it to the shop to get filled, they will hook up the tank. They should then open the bank and then the tank. Only clean dry air should enter your tank!

Does this help?

 

[H2O 70]

PVC can outgas if it gets too hot, and the fumes are not good for. Personally, I'd rather have moisture in the tank!

I have to be real mindful with the PVC. The hair dryer on low doesn't appear to get it warm enough to any off gassing. Any other suggestions? PEX?

 

[markyd]

Thanks guys. I had thought I might try warming the tank or blowing hot air inside, but it got me thinking that hot air would actually hold more moisture than cold air, so although the visible condensation may 'go away', it's really just going into the air that you are trapping inside the tank when you put the valve on, and so you're right back to where you started once the tank/air cools down.

But balance that against the hot air taking up more volume, and I don't know if you end up with more or less moisture inside when you're done! I guess I'm going to have to dig out my physics books and do the math if I want to be sure....

But I think maybe I'll just go have the tanks filled and use them instead.
The next VIP is due in a few months anyway, so I guess I'll find out then if they'll need a tumble.

marky-d

 

[duckbill]

I assume the air you will be heating to blow into the tank will just be the natural surrounding air wherever you are, right? Well, heating it will enable that air to absorb more moisture, like you said, and that is the point of introducing warm air- to absorb the moisture that is in the tank right now. The air you heat will not magically create more moisture in itself just because you heated it, but it will be capable of absorbing more moisture than had you not heated it.

Anyhow, if I install a valve during a time of relatively high humidity, I make it a practice to fill, let cool, drain slowly, refill...to purge most of the ambient moisture out with the dry, compressed air. Just one refill will purge almost all of the excess moisture.

Say you have three parts water in the tank to every 100 parts air (3%) at ambient pressure (1 atmosphere- "empty"). 3000psi is 214 atmospheres (3000/14.7). Filling it with dry air (we'll assume it's as dry as can be) will give you about three parts water for 21400 parts dry air. That means there is less than 14/1000 of 1% (.014%) water now in your tank. Purging and refilling, though not necessary, will purge even more moisture out.

Just quick calculations. If I'm wrong, fire away.

 

[Puffer Fish]

I assume the air you will be heating to blow into the tank will just be the natural surrounding air wherever you are, right? Well, heating it will enable that air to absorb more moisture, like you said, and that is the point of introducing warm air- to absorb the moisture that is in the tank right now. The air you heat will not magically create more moisture in itself just because you heated it, but it will be capable of absorbing more moisture than had you not heated it.

Anyhow, if I install a valve during a time of relatively high humidity, I make it a practice to fill, let cool, drain slowly, refill...to purge most of the ambient moisture out with the dry, compressed air. Just one refill will purge almost all of the excess moisture.

Say you have one part water in the tank to every 100 parts air (3%) at ambient pressure (1 atmosphere- "empty"). 3000psi is 214 atmospheres (3000/14.7). Filling it with dry air (we'll assume it's as dry as can be) will give you about one part water for 21400 parts dry air. That means there is less than 14/1000 of 1% (.014%) water now in your tank. Purging and refilling, though not necessary, will purge even more moisture out.

Just quick calculations. If I'm wrong, fire away.

Duck you are absolutely correct. The thing to avoid is taking a cold tank into a warm environment and leaving the tank open...as you will get condensation in side the tank, which can be more water, but even then, if filled with dry air, you now have a dry tank with 1/242th of the water that was in the ambient air(plus whatever was in the compressed air)..

 

[markyd]

I understand what you are saying...mostly.
As you said, if you heat up the tank, the warmer air will absorb more moisture. So now what? You have warm, moist air inside the tank. Either you put the valve on and trap that moist air inside the tank and you're basically where you started, or you leave the tank open while it cools and maybe it escapes into the open air and maybe it recondenses inside.

I think you are mistaken about the last part. Yes, if you have one part water to 100 parts air (not sure how you get 3% from that instead of 1%), then fill with dry air, you will have one part water to 21400 parts air. But the point is you still have the same amount of water (one part), so it can potentially do the same amount of damage to the tank. You do not get 1/242th (or even 1/214th) the water, as Puffer Fish says. It's the purge that gets rid of it, so if you don't dump a tank full, you're not helping anything.

marky-d

 

[duckbill]

I understand what you are saying...mostly.
As you said, if you heat up the tank, the warmer air will absorb more moisture. So now what? You have warm, moist air inside the tank.

We were discussing blowing warm air into the tank. That warm air will also exit the tank. The circulating air will take the moisture out with it.

(not sure how you get 3% from that instead of 1%)

Make sure to read my post directly, not the quoted text in Puffer Fish' post. I had corrected it yesterday.

I think you are mistaken about the last part. Yes, if you have one part water to 100 parts air (not sure how you get 3% from that instead of 1%), then fill with dry air, you will have one part water to 21400 parts air. But the point is you still have the same amount of water (one part), so it can potentially do the same amount of damage to the tank. You do not get 1/242th (or even 1/214th) the water, as Puffer Fish says. It's the purge that gets rid of it, so if you don't dump a tank full, you're not helping anything.

marky-d

You are correct. The same amount of water is in the tank after having filled it initially, but percentage-wise there will be far more molecules of dry air to every molecule of water. To be honest, I've forgotten how compression, partial pressures and vapor pressures work into this or how it would help, or not; But, a purge and second fill should do the trick.