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Greg - 11/11/2013 2:08 PM 
If you store them in a cool, dry place for a few days/weeks, that shouldn’t be a problem. I’ve never done that though so I can’t say for sure what the affects will be. It’s always best to keep a little air (~200psi) inside the tanks to ensure there is pressure to keep moisture out.
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As Greg said its always best if theres a minimum of 200 psi in the tanks to deter rust/corrosion, dive classes usually state this as its your life equipment, personally i wouldnt chance it
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Ya I’ve never stored mine away for any periods of time without any air in them. I always leave around 200-500 psi in them to store them over the winter.
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Depends on what you define as empty? 0 psi or a low enough pressure to allow water and or moisture can be a problem causing corrosion. Rust corrosion ruins your tank and depletes the oxygen in your tank. Moist enviroment is a incubator for mold mildew and other nasty stuff not good to breathe. If the tank allways has more pressure than the pressure on the outside of the tank I do not see how there would be a problem. But if it is empty or very low you might have to pay for a new viz due to contamination and safety concerns.
Just re read your post and it sounds like you just had your tanks hydoed and or viz with a place that does fire extinguishers. I have done the same thing to save money. The problem is if you do not get a fill right away there is the possibility of moisture that has not been completely removed or moisture and or corosion quickly accuring. Different dive shops may be more cautious then others some do not accept or like the viz from others particular non dive shops.
Good luck hope you have a honest dive shop that treats you fair!
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Speaking of moisture getting into the tanks, I once watched a tank monkey put all of the tanks in the water, aluminum tanks tend to float bottom up, then attach the fill whips and open the valve. This blows the water that was between the k-valve and the whip into the tank. That might explain why I only got a few years out of my first set of 72’s.
There was (RIP) also a NJ LDS that wasn’t doing PM on their compressor and had a reputaion for pumping water into tanks
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I’m not sure how far away your LDS is, but if it is nearby then yes, I’d be running over there and spending the $7 for an air fill personally. I’ve never stored tanks completely empty for the reasons stated above.
If you can’t, then as Greg stated I’d recommend a cool, dry, climate controlled space inside your house somewhere until you can get them filled.
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Do you mean that you had your tanks Hydro tested ? They still have to be VIP’d with sticker before a fill anyways. So it doesn’t matter if its full or empty. The tank will have to be empty for the VIP inspection anyways. If its a steel tank and even if a small amount of water was left inside from a hydro test. It will cause rust very quickly. Get them to the LDS for a VIP/fill. Its not a good idea to store your tanks empty anytime.
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A vip from a place that does not do scuba fills will not allways save you money some scuba shops will require any empty tank to get a new vip before they will fill your tank even with a current vip. Allso some will not accept a vip from others if it is not a vip from a recognized scuba fill shop.
If it is empty and they see any problem they will charge for a Vip + cleaning, possibly tumbling, and a fill.
I took a bunch of my tanks directly to a certifed hydro facility and they did a hydro tumbled tanks as necessary, cleaned, did a vip required by DOT and included a vip sticker which not all hydro facilities include the sticker. My shop accepted the sticker they provided for air fills only my nitrox tanks required O2 cleaning and a new viz for oxygen clean with the oxygen vip sticker.
There was shop that insisted they sell me the big nitrox bumper sticker tank wrap which is not required by law. But I guess shops can make up there own requirements over and above what DOT and the law requires. I will not do business with those shops. Just seems to be a red flag they may not be totally honest!
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Good luck I hope your dive shop is honest and only charges you for what is really necessary? It is realy hard to know for shure as for you do not get a video of the inside of your tanks or the work they perfomed. All of the things discussed is important for your safety and the safety of the fill operator’s
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Empty tanks aren’t necessarily a concern. I have tanks that had been stored for years without valves in them that pass both Hydro and VIP no problem. Worth noting, is that steel tanks are more susceptible to internal corrosion and sensitivity to moisture than aluminum ones, but all metal can and will corrode if not cared for. As far as 200-500PSI: Not necessary. 30-50 PSI is all you really need if you’re concerned about keeping contaminants out. Gas can’t naturally move from an area of lower pressure to an area of higher pressure, and water doesn’t diffuse across your closed valve and tank wall, so as long as there’s ANY positive pressure inside the tank, it won’t take on contaminants. I don’t recommend storing your tanks full for long periods of time becuase it’s hard on the valve seats. Accordingly, if there is water vapor condensation already present in the tank with the tank valve closed, this vapor cannot evaporate and escape. It’s really the latter that causes damage to tanks, rather than the amount of pressure stored in them. Higher pressures will also accelerate corrosion in the presence of catalysts / contaminants, due to the higher partial pressures of your primary oxidizing agent: Oxygen. If in doubt, remove the valve and let the tank air out until no condensation can be seen or felt inside, then replace it in a low humidity environment and have the tank filled from a reputable source.
Use your tanks often, get fills from a properly maintained compressor, and you shouldn’t have to worry much about this so long as you purge the valve before attaching it to a whip or regulator.
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