I also must respectfully disagree with Brokenot. A prime example is condensed dew on a vehicle or the front lawn. That volume of moisture appears over the space of a few hours... And unless it is below freezing, it happens just about every night... Granted, the vehicle skin has more surface area and access to more moist air, and yes it takes more time inside a fuel tank, but the moisture is available...
Disagree away, that's (part of) what forums are for. I'm very familiar with condensed dew on the grass and such, but do an experiment if you'd like. Take a gas can, (or any other comparable container you have available), and set it outside exposed to the same conditions you mentioned you observed when all of the moisture condensed on the lawn. Leave it 1/4, 1/2, 3/4 full....however much you like. Let it sit for several days or weeks, and see how much moisture it "gains" from the atmosphere. Of course be "fair" during your test. The container should be capped but still vented...just like a fuel tank is. Your test should also be conducted with a fluid that you know isn't
already moisture-laden, and it should be fluid that water will separate from, (and be visible apart from), your "test fluid" for the duration of your test. (Something like regular engine oil or mineral spirits fresh from a sealed container maybe...?)
It's important to note that I never said there was, (or could be), no moisture in a fuel tank....I see it every day at work. (I've been turning wrenches full-time since 1983.) What I said was, that moisture in fuel tanks due to condensation is overly exaggerated. The vast majority of the moisture you'll see in the fuel bowl on a carbureted small engine, or in the bottom of a fuel tank after it has sat in storage didn't condense and end up there, it was in the fuel all along and it separated while the equipment sat.
We have well over a hundred pieces of rental equipment that are "seasonal" in nature and spend entire months sitting. It gets cold here at night, and warms up significantly during the day. Like most places, we have dry periods and periods with high humidity. If there were significant fuel tank moisture gains due to condensation, we'd see it plain as day. Back when I lived on the farm we didn't see significant "condensation gains" either. Not in the bulk fuel tanks, or the seasonal equipment that spent months sitting idle.
The water ends up at the bottom of the tank while the equipment sits. If it's a gravity-fed small engine fuel bowl, then water ends up there. If it's a conventional fuel tank with a pick-up tube oriented near the bottom of the tank, well...there's a good possibility that some water will be sucked up and cause issues when the machinery is fired up for the season.
We see water, and the first thing we think of is condensation. And most of what we see isn't a product of condensation that occurred
after we put the fuel in the tank and capped it.
Next time you see one of the lab coat-wearing sales pitch guys or gals pitching a product that battles condensation in your fuel tank, tell them to calculate how many times the fuel tank's air space volume needs to be not only
changed out, but also needs to have
every available drop of moisture wrung out of it as it's being changed out...in order to accumulate the amount of water they're displaying in their beaker. Even at some ridiculously high humidity level that would make their situation semi-plausible, they still won't be able to explain why/how the tank exchanged it's available air space volume the huuuuge number of times it would take to accumulate the amount of moisture they're claiming.
Their numbers will not add up.
