How does Water get into the Tank?
With the onset of winter the question arises if the tank in our motorgliders should be filled to the brim before the sailplane is stored for the winter, or should it be completely empty? In an empty tank there is the possibility to consider that condensation water is formed during the winter and settled in the bottom of the tank.
But this theory is wrong!
Think about it:
An empty tank in a DG-808C, for instance, contains about 20 liters of air. The aircraft is either hangared or trailered. The tank is sealed. So where should the exchange of air come from? It can only come from the overflow pipe because of external changes in air pressure.
If we assume that in the course of a maximum winter hiatus of 7 months the air pressure fluctuates every day by about 10 hPa because of weather and temperature changes (4 hPa is probably more realistic). That would be a daily change of 10 / 1013 of the atmospheric pressure or about 1 % . In 210 days that would be an exchange of 210 % of 20 liters or 42 liters of air.
If we assume at an average temperature of 10 C air can absorb a maximum of 9.41 g of water per cubic meter. If we further assume that new air flows in at 100% relative humidity and flows out at 50% relative humidity we can expect 5 g per cubic meter of condensation or 0.2 g for our 42 liters.
And there is still the question why the water should condense out of the air. This takes place mainly on the cold outer surface of a tank, which we will not have here, since the tank is made out of fiberglass and isolated from the fuselage surface.
In any case, the volume of condensation is so small as to be unnoticeable.
The theory that a tank should be brim full during the winter to avoid condensation, is false. But this should not prevent you from pressing the drainer in the spring. Can you have water in the tank?
Definitely, as we know from personal experience:
Our sales manger went to the airport with a customer. He filled the tank, gave the customer a thorough briefing, and he taxied to the start point. Full throttle, the airplane gained speed and the engine stopped!
What a way to conduct a demonstration flight! 
Start attempts down to an empty battery were without success. With the help of a car battery he could get the starter to a high temperature, but the engine did not start. Finally he removed one of the spark plus and saw the water drops on it.
Finally - and much too late - the drain cup was used and showed half full of water!
Cause: This was at the time that leaded super gas was phased out, and we got the last of it out of the gas station into our canister. But we did not know that. The canister was contaminated with water, and that is how the water got into the tank. Always consider the source where you get your gas, that is usually where the water comes from.
Because you can never be sure that something like this could happen to you - and the engine fail shortly after lift-off - pay attention to our safety instructions and use the drainer regularly, if you are flying a self launcher.
Another cause of water in the tank can be parking the airplane outside in the rain. But this pertains more to motor airplanes which might have leaky tank caps. Our composite tank caps are absolutely tight, and you are unlikely to park your sailplane outside without a cover (Jaxida cover or similar). If you have an automatic fueling installation like the DG-808C you can even tape the tank cap .
How much should be in the tank during the winter?
It would be best if the tank would be empty. You don't have to pump it dry, but never fill it to the brim before final disassembly. For the tank material it is of no matter if the tank contains gas or air. The reason for this recommendation has to do with the nature of the two cycle mixture:
It is probable that after a long time parked the gas oil mixture demixes itself to some degree and then you have a completely undefined fuel when you start the engine.
Should you leave your tank half full and trailer the airplane for a while before the first start you are o.k. again. If you fill the tank the mixture will become correct again. Only if the tank is brim full can the mixture not reconstitute itself, and that could cause problems on your first start.