The threat of water in the fuel is real, but it shouldn't be a problem. You
must have a business process in place to manage the problem. The most likely
cause is being delivered fuel with already absorbed water. The second threat
is fuel is hygroscopic and absorbs moisture from the atmosphere. This is a
low threat in cold temperatures because the air will not contain much water.
However, this is a big problem in warm humid climes. The consequences are
tank corrosion, bacterial growth and fuel system failure. The decision to
fill your tanks are really fuel cost and saving opportunities based. Your
protection against the threat are corrosion proof tanks, biocide additives
and a centrifuge water/fuel separator (fuel polishing system). The risk
cannot be avoided, only mitigated.
Steve

"Marc Auslander" wrote in message
...
Common knowledge is that we should top up our fuel tank for the winter
sleep. Argument is that otherwise the temp changes will continuously
condense water into the tank, as moist air is pulled in, condensed,
and then expelled.

I have no doubt that the effect is real, but wonder how big it
actually is. How much water per gallon (or liter) of air would
condense in one cycle of some plausible temperature range and some
plausible outside dew point?

(I think I know how to calculate this, but wonder if its already been
done?)