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Keeping a tank full in order to somehow prevent 'condensation is foolhardy at best and at worst (especialy if the fuel isnt being used quickly) can lead to fuel degradation, water ad/absortion, enhanced biological growth (fouling).
Water 'uptakes' into fuel by 'chemical equilibrium' because the 'new' fuel is essentially dry and the atmosphere is moist. This 'mass transfer' will continue until the oil is fully saturated with water ... and then the water will begin to 'settle down' due to gravity. It makes NO difference if the tank is full, half empty or nearly empty ... as a long as there is unsaturated fuel in the tank and the tank is in contact with the moist atmosphere the water will 'enter' the oil. The AMOUNT of oil in the tank governs the AMOUNT of water the oil 'uptakes' .... simple chemistry (- law of partial pressures).
Once a tank with oil saturates and the water begins to 'drop out' at the bottom to form a distinct 'layer' of free water with the saturated mixture on top .... you now have a 'chemical pump', allowing the water to drop out and if the 'ambient conditions' are 'right', more water will 'equilibrate' into the oil and only to later 'drop out' at the bottom and cause more 'equilibrium imbalance' to allow more 'water uptake' ... ad nauseum.
The amount of 'saturation' is dependent on the AMOUNT of oil in the tank. Empty tanks do not magically 'fill up with water, do they?'. If they did we wouldnt need to drill water wells as all we'd have to do is put out empty tanks to get all the water we need ... and we know this isnt true.
'Condensation' ONLY means that the oil is already fully or nearly already saturated with water. The LEAST amount of fuel in the tank will 'uptake' the LEAST amount of water.
So, keep only enough fuel in the tank for your relative immediate needs plus some reserve; buy your fuel only from 'high turnover/fresh' (dry oil) sources, if possible; drain/empty the tank when long term storing; avoid buying your oil from low-turnover sources - marinas where the fuel sits for long time (water saturated) before pumping.
Why 'store' an unnecessary amount of oil if 'chemistry' tells you that it will soon 'saturate' with WATER ??????
Better to keep a minimum quantity that can only 'uptake' a minimum amount of water; buy 'fresh' (dry) when and as needed rather than 'storing' it only to have it saturate all by itself.
Last edited by RichH; 04-28-2011 at 02:12 PM.