Divers like being underwater- it's what they do. When one considers the amount of prep time it takes to go diving, it's understandable that divers want to stay underwater as long as possible. Divers want their tanks full.

All too frequently, divers get short air fills. Why is this so common? What's the problem? Aside from differences between a diver's pressure gauge and the filling station's gauge, the problem is one of physics. Charles' Law tells us that the pressure of gas inside a fixed volume (like a tank) increases, the temperature will increase accordingly. Translation: tanks warm up as they are filled. Physics also tell us that in a fixed volume, pressure will decrease as temperature decreases. Translation: tanks that were just filled will cool off and be less full.

Now, what can divers do to make sure they get good air fills? They need to know what makes a good fill, then have patience.

A few rules for a good air fill:

1. The tank should be filled to (or near) it's maximum working pressure.
2. No cylinder should ever be pressurized past its maximum working pressure.
3. The maximum recommended fill rate for tanks is 300psi per minute.
4. Water should never be allowed to enter the tank.

How to get a good air fill
Following the rules above, tanks should be filled slowly and out of water, then allowed to cool. After the tank has cooled, it should be filled again to reach the maximum working pressure.

How not to get a good air fill

1. Do not overfill tanks so that when they cool, the maximum working pressure will be achieved. This breaks rule #1 above, risking damage to the cylinder.
2. Do not put a tank in cool water so that it cools faster. This contributes to breaking rule #4 above. The NOAA Diving Manual recommends that "Cylinders should not be placed in a water bath for filling. The risk of water entering a cylinder while immersed in water and the resulting corrosion is potentially more hazardous than the risk of over-heating during filling."

Generally, keep in mind that filling a scuba tank to its full capacity is not as simple as it may seem. In fact, managing to fill a scuba tank in a retail store environment can be something of an art. Divers should have patience with the staff and give them ample time to fill tanks completely and properly. Divers should consider leaving their tanks at the store for the day.

If you or the person filling your tank is in a hurry, just agree to settle on a short fill and maybe offer to pay a little less for it. It's better for your tank, safer, legal, and the few hundred psi you'll be short will only reduce diving time by a few minutes anyway.

Finally, remember that only individuals with proper training should be allowed to fill scuba cylinders.