Thermal ballast and moisture ingestion, adsorb vs. absorb
I'll second this, it's more urban legend than scientific fact. I don't believe condensation is as great a concern as many believe.
I carried out condensation testing of fuel tanks over the course of a winter in Virginia several years ago. I removed the access hatches to two tanks, one steel another aluminum, once a week for the entire winter, and looked inside. I never saw any signs of condensation, and the environment here is truly ideal for it, temperature swings of 40F are not uncommon in 24 hrs as warm fronts move through. If my wife leaves the garage door open during one of these events, water condenses on everything, tool chests, engine blocks, vices etc. ...
Ok...so the skeptics seem stuck on
"how come I've never seen any water". Here's why they've never seen it. For Steve D., here is (specifically) why your tests produced a 'false negative'.
First, let's summarize some scientific facts I think everyone agrees on:
1) Diesel fuel
absorbs water, up to it's saturation point, which is temperature dependent. Diesel fuel can absorb water directly from the air, or it can absorb it from condensation that
adsorbs on the inside of the tank, or it can absorb it from a leaky-fuel fill. It doesn't matter where the water comes from. This dispels the myth that "there's no condensation in my tank because I haven't found any water". Y
ou won't see any of the water, no matter where it came from until (a) the fuel becomes saturated and then (b) the water precipitates out.
2) Water that is dissolved in saturated diesel fuel will precipitate out into liquid water at the bottom of the tank when the temperature of the fuel-water mix drops. Again, this is because the saturation point is temperature dependent. (
Precipitation is different than
condensation, which seems to be creating confusion here).
3) Where a cold, solid surface contacts moist air ('cold' meaning below the dew point), water will condense (adsorb) on that surface. Always. It just does. Water will continue to condense on the cold surface until the temperature of that surface rises above the dew-point.
4) Where a cold
liquid surface contacts moist air, water will be absorbed directly into the liquid at a rate that is dependent on the relative saturation points of the liquid vs. the air, and on their relative temperatures differential.
Now, inside a tank
half filled with diesel, in an environment where the temperature of the
fuel is below the dew point of the moisture-laden air,
the fuel is cooling the aluminum around the perimeter of the tank. Inside the tank, this is where the condensation will form, right near the fill level of the diesel fuel in the tank, and
nowhere else. You haven't seen it (a) because you're not looking in the right place, and (b) your tank is in a floating vessel, and any slightest motion is continually rinsing any condensate off the sides of the tank, right along the tank fill level line.
This may become immediately and intuitively obvious the next time you look at a half-empty can of still-cold beer outside on a humid day. The condensation on the cold aluminum only goes up to (slightly above) the liquid level in the can. The upper sides and top of the can will be bone dry, because there is nothing to keep the temp of the aluminum below the dew point.
Next time you hear someone say
"I looked inside the tank and didn't see any droplets on inside surfaces of the tank", now you know why they didn't see any. It's because of insufficient
thermal ballast.
re:
"If my wife leaves the garage door open during one of these events, water condenses on everything, tool chests, engine blocks, vices etc. ..."
No, not everything. Look harder next time. An empty aluminum can sitting on your vice will not have condensation on it. A thin sheet of aluminum or steel standing next to the engine block will not have any condensation on it. However, (as you observed) engine blocks, vices and the like will have lots of condensation because they are
good sources of thermal ballast. They stay cold for a long time even as the air warms up. Compare this to a thin sheet of aluminum which will warm up rapidly with the surrounding air.
So...re-try your garage test like this:
Take a
half-full aluminum tank of diesel. Put it next to that engine block. The next time you see condensation on the engine block, look, you will see it on the fuel tank,
but only up to the liquid level in the tank.
This is because the liquid fuel in the tank provides the thermal ballast necessary to keep the aluminum below the dew-point long enough for the condensate to form.
Now think to yourself...what sort of magic can it be that is preventing condensation from forming on the
inside of the tank (along the tank-fill line) while you are looking at the condensation on the outside of the same tank?
One possibility is that the fuel in the tank is very dry, very far from it's saturation point. In this case, the fuel itself can be acting as a dessicant, absorbing water quickly from the air, reducing the water content in the air and thereby limiting the amount of water adsorbing (condensing) in the tank. This is only temporary. Over time, the fuel will get closer and closer to saturation point and as it does, condensate formation will increase.
A prevous poster in this thread referenced another test performed by CMS, much as yours was, but with a lot more excruciating detail. He probably invested a hundred hours over the course of three years...but again using an
empty aluminum tank. Of course he never found any condensate, just like he's never found any condensate on (or inside) an empty beer can! Boaters can thoroughly understand the concept of ballast in terms of the difference between the density of lead or cast iron and water, grasping the concept of thermal ballast should be a short step from there.
https://www.physicsforums.com/threads/thermal-ballast-or-whatever-its-called.951156/
Another issue has to do with the fuel tanks being located in a boat bilge versus in a garage, or strapped to the underside of a car. This is a critical element, and another reason you got a 'false negative' in your tests.
During the seasons and in the areas where 'moisture ingestion' of fuel is the biggest problem, the water temperature is significantly colder than the peak ambient air temp. This not only ensures your bilge will be much cooler than the ambient air, it keeps the fuel (aka thermal ballast) in your tanks that much cooler also. As one might expect, if you are in an area (or a season) where the water temperature is much closer to the air temp (like in the Carribean), you would see much less moisture ingestion. It doesn't matter if you are in 90% humidity, if you don't have a sufficient temperature differential, you're not going to have condensation.
To summarize...the three factors that drive moisture ingestion of diesel fuel in vented tanks are temperature differentials, dewpoint and thermal ballast. Any testing that does not accurately represent all three factors in a marine environtment
simultaneously is bound to fail.
In particular, the recurring errors I have seen in all these testing efforts is the
exclusion of thermal ballast, and a
failure to consider bilge ambient temperatures.
Lastly...another repeated misconception in this thread takes the form of
"if there was (liquid) water in my tanks, I would be seeing it in my filter bowl".
This is also wrong...I'll cover that separately.
