Tank Moist Air Intrusion during venting

A dive shop in Florida has a habit of letting the remaining (up to about 50-100 psi) air out of my tank whenever I request a Nitrox fill. They like to start with a fresh tank. My concern is this- They put the tank outside to do this.

While letting the air out of the tank, the tank gets cold, the surrounding temperature is hot and humid. Condensation builds up both on the tank and valve.

In addition to the condensation buildup on the tank and valve there is another more pressing concern-

The partial pressure of the moist air outside is higher than the partial pressure of the moist air in my tank- assuming there is no moist air in my tank to begin with.

I believe the difference in partial pressure would allow for moist air to migrate into my tank even though the tank is venting and has a positive pressure.

I have experienced this situation while working in an environmental test lab using thermal chambers to test aerospace components. The chamber was pressurized with nitrogen gas, but it would still snow inside the chamber at cold temperature due to leaks and moisture intrusion from the surrounding air in the test lab. The fact that the chamber was pressurized did not keep the moisture out.

Any comments or experience with this would be appreciated.

Putting aside the wisdom of the shop's approach to tank draining, as well as just how simple a gas blender needs to be in order to have to start with an empty tank, why not just slowly drain the tanks at home to whatever their desired pressure is? That way you control the condensation issue, if there is one, and you don't have to fight with the shop about their blending procedures. Or maybe do the gas blending calculations yourself and deliver them along with the tank, because math is hard for them.

But I'd find a different shop myself.

How does water vapor move into a tank against a pressure gradient?

I assume he means it goes in after the tank is empty and then condenses on the cold valve?

---------- Post added September 3rd, 2013 at 11:59 PM ----------

Just because a room is pressurized does not mean it is dry.

Unless they are blending in the tank, there is no reason to completely drain the tank below a 4 or 5 hundred PSI, which is usually what you encounter. Draining the tank by leaving the valve open for that short time should not allow any moisture into the tank. Because the tank, valve and air inside the tank is colder (denser) than the outside air.

Now if you left that valve open for a couple of days with the tank in the sun, the expansion caused by heating would raise the pressure inside the tank and spill air... And at night when it cools down and the air is usually more moist, that air would enter the tank... that cycle repeating itself will rust it out, if steel or accelerate the oxidation rate if Aluminum.

If I want to go NITROX in that tank, I check the pressure and the tank volume, look up the mix I want from my chart and dump the banked 32% on top of it to a pressure and finish with air. After 30 minutes or so, the mix check in that tank shows a close desired mix. The dive shops can do the same thing, but they would have to have some better training for the valve turning guy. I know I save a bunch of filter use and compressor run time by not blowing the remaining air and then having to put it right back in.

The only time I drain a tank down close to empty is when I do a TRIMIX fill where I dump the Helium first to a pressure, then with NITROX 32% premix from the bank to a pressure and then top air.

Find a new shop. That one is either lazy or worse, can't be trusted if they can't figure out the blend.

I had a highly recommend (on Scubaboard) shop in FL do the same to me when I was there trying to get fills. No biggee, really, assuming you're emptying the tanks immediately before refilling them. If you let the tanks sit open in the environment for a day or two you may have cause for alarm, but in the 10 minutes it takes to go from empty to full again, I don't think you're getting any moisture in your tank.

As long as air is coming out of the tank, nothing is going in. However if they let it drain down to 0PSI and leave the valve open, it's entirely possible for the outside air to enter the tank.

I'd find another shop. Aside from the contamination issue, draining the tank then refilling it stresses the tank and shortens it's life.

Either the gas blender is lazy or incompentent or the shop wants to make more money by selling more O2. Do they charge by the CuFt?

> letting the remaining (up to about 50-100 psi) air out of my tank

Do you mean you're bringing back a tank that has 50PSI in it? They may be draining it so they can take the valve off and see if it's contaminated.

flots.

find a new shop. There is never a reason to drain the tank when filling with the same mix. Even then it is not necessary to drain all of the gas unless changing from some mix to 100% O2.

I would imagine this. You are underwater breathing your tank. Outside the tank there is pure water. As you breath, you are "venting" your tank (the air moves from the interior of your tank to an open air space - your lungs).
In this situation, a normal situation for a scuba tank, there is no condensation migration from the outside (water) to the interior.
As has already been said, only if the valve is left open after the tank equalizes to 0 psi relative, humidity could go inside, but still, very few.