How does moisture enter a tank?

That may not be an inherent or common issue, moisture in the tank is most likely due to compressor not getting all water separated from air or condensation during rapid pressure loss. If the tank goes empty during use, there is nothing preventing it from scavenging water from the regulator or other sources when valve is open especially if under water. Various reasons to keep pressure in the tank, it may reduce fatigue due to the cylinder not shrinking to its minimum diameter. If the tank is fully evacuated the seal may allow water in if there ends up being a vacuum pressure differential and valve is not tight. The valve seat with Teflon face may seal better under pressure. Pressure gauges do not read low pressures in order to ensure tank is not fully evacuated.

Moisture can enter a tank in several ways depending on the type of tank and the environment it is exposed to. Here are some common ways moisture can enter a tank:

1. Condensation: When the temperature inside the tank is cooler than the temperature outside, moisture in the air can condense and collect inside the tank. This can happen in any type of tank, but it is more common in metal tanks.
2. Leakage: If the tank has any cracks, holes or damaged seals, moisture can enter the tank through these openings.
3. Humidity: If the air around the tank is humid, moisture can enter the tank through vents or openings.
4. Direct exposure to water: If the tank is not sealed properly or is exposed to rain or other sources of water, moisture can enter the tank through the openings.
5. Inlet air: If the tank is filled with air that is not dry or conditioned, moisture can enter the tank through the inlet air.

It is important to prevent moisture from entering the tank as it can cause corrosion, contamination, and other issues. Proper maintenance and regular inspections can help prevent moisture from entering the tank.