The volume in the fuel tank is fairly small, most of our boats it would be less than 0.1 cubic meter. At around 90 degrees Fahrenheit that would be about 1/10 of an ounce of water in the air in an empty tank.
http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm If the temperature starts around 60F and drops to 20F about 0.03 ounces of water would condense in a 0.1 cubic meter tank. Air changes volume about 3% for a 40F temperature swing, so per month another 0.03 ounces of water might get in. Note that the numbers assume the maximum amount of water in the air, or 100% relative humidity. If the winter relative humidity is 50% then cut the numbers above in half. Michael Brown Windburn C&C 30-1 Date: Fri, 26 Oct 2018 10:37:21 +0000 From: "Della Barba, Joe" <joe.della.ba...@ssa.gov> Subject: Re: Stus-List [EXTERNAL] Re: Filling diesel tank Urban myths? In Maryland we have high humidity and large temperature changes during the winter. I had to rebuild the wood supports for my fuel tank because so much water would condense on it the wood rotted! The aluminum tank is frequently covered with drops of water on the outside during the winter. Joe Coquina Sent: Thursday, October 25, 2018 5:34 PM Subject: [EXTERNAL] Re: Stus-List Filling diesel tank Urban myths? I?m not in total disagreement, but the urban myth may be more of a Great White North myth. In that, when it gets cold out it stays cold until spring. But otherwise, when it get cold at night and warms up in the day, it best to keep the tank either full or empty. The reason is that a metal tank with some liquid will dehumidify the air inside (and outside) the tank during the daytime when ambient air starts to warm relative to the night time lows. Think about a tank half full after a cold night. The cold fuel is a big heat sink and will keep the tank fuel and metal container much colder than the warming daytime air. Warming air + cold metal surface = condensation, i.e. cold sweat. A full tank has less condensing surface and less air inside. An empty Aluminum tank will warm and cool with ambient air so there will be little if any condensation. I guess one could close up the tank (i.e. close of breather line) to try to eliminate the reoccurring condensation, but that?s likely not as easy as it sounds (and end up being a bigger PITA than filling the tank). With any luck, Global Warming will rid the Canadians of some urban myths?. - Paul E. 1981 C&C Landfall 38 S/V Johanna Rose Fort Walton Beach, FL
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