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RC Fuels Nitromethane, Castor Oil, Synthetic, heli fuel, 4 stroke, etc...Fuel Q&A is here!

water in fuel

Old 09-15-2005, 07:18 PM
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WS
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Default water in fuel

Has anyone ever tried putting the fuel in the freezer to remove water? Would this work? Methanol freezes at something like -97C and nitromethane at -29C. Would such a process cause any water to freeze, making it easy to remove?
Old 09-15-2005, 09:09 PM
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Default RE: water in fuel

I've tried it and it doesn't work. What it will do though is give you some idea of how much water is in the fuel because below a certain critical temperature all the oil will suddenly come out of solution and fall to the bottom. The temperature that this happens at depends entirely on what % of water is in the fuel. A few years ago I did some experiments adding water to fuel and put it up on a web page at http://www.holdfastmac.com.au/Waterfuel.html

The critical temperatures I found may vary slightly with different blends of fuels but probably not by much.
Old 09-16-2005, 04:09 PM
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Jim Thomerson
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Default RE: water in fuel

A mix of two miscible substances like water and methanol freezes at a lower freezing point than either of the pure components. The mix with the lowest freezing point is the eutectic mix. So a water/methanol mix is likely to freeze at a much colder temperature than you can get in a home freezer. Many years ago we used to mix antifreeze and water to get a mixture with a low enough freezing point to protect the cooling system. Makes you wonder why we don't have to do that with modern antifreeze.

Downunder's web page is a good read. It is interesting that the oil comes out of solution as it does. Molecules which have a charged area will fit into the structure of water. These are called hydrophylic molecules. Methanol has charged areas on the OH and thus fits in quite well. Oils generally do not have charged areas and are called hydrophobic molecules. Oils that mix with methanol, or water, would need, I think, a charged area somewhere on the molecule, but most of the molecule is uncharged. So I guess a certain amount of Brownian movement, molecular vibration, heat, is necessary to keep them 'shooken up' in the methanol. Correction welcome, as this is more chemistry than I actually know.
Old 09-16-2005, 08:30 PM
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Default RE: water in fuel

Ahhhh....now I know why fish are watertight. Fishoil is hydrophobic

I must admit I was quite surprised to see how the oil all came out of suspension at once when it reached that critical point, I expected to see it gradually settle out as it cooled. I like the theory of Brownian movement but I applied quite a bit of Brianian movement (shaking vigorously) to add to the Brownian movement but this had no affect on the critical temperature. Just kidding you know more chemistry than I do!
Old 09-16-2005, 09:19 PM
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Jim Thomerson
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Default RE: water in fuel

Actually, fish are not watertight, fish oil or not Freshwater fish are saltier than the water they live in and pee a lot. Saltwater fish are not as salty as the water they live in, so they tend to dry out. They drink saltwater and excrete the salt through salt glands. Some bony fish can do both and move back and forth between fresh and salt water. Sharks, skates, and rays store urea in their blood to partially balance their lack of salt, which is why you soak shark meat before you cook it, to get the urea out. Interestingly enough, the South American freshwater rays have the lowest amount of blood urea of any known vertebrate. Did I mention I am an ichthyologist?
Old 09-16-2005, 11:46 PM
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Default RE: water in fuel

In frigid Minnesota, the windshield washer solution is a mixture of methanol and water. Purposely to keep it from freezing. And it still is liquid at 30 to 40 below.
Old 09-17-2005, 03:35 PM
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Jim Thomerson
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Default RE: water in fuel

I think when fuel is exposed the methanol evaporates at a much faster rate than the fuel absorbs water. That's why fuel feels cold when you get it on you. Has anyone done the experiment of putting an open container of fuel on a scale and tracking the weight change? If water absorption is the big thing, it should gain weight: If it is methanol evaporation, it should lose weight. Either gaining water or losing methanol is not good, so keep your fuel can capped.

My apologies for the fish post. At least I didn't post the full notes for the hour lecture.
Old 09-17-2005, 09:31 PM
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Default RE: water in fuel

No need to apologise Jim, after all where else but on a model aircraft forum could you find out that fresh water fish pee a lot? I knew your area of expertise which is why I mentioned the water tight fish

As for how fast fuel absorbs water, that's a bit hard to say. A few days ago I was doing some messing around with fuel and water and I forgot to put the cap back on the container and it was open for maybe 24 hours. I filled a 10cc syringe with the fuel and put it in my freezer and no oil seperated out so any water content had to have been less than 0.25%. This isn't all that surprising because there was about 3 litres of methanol in the container so even 0.25% equates to 7.5cc of water in 3 litres of methanol. But our temp here is only around 60F with a fairly low humidity so there's not all that much moisture in the air. Plus it was sitting in still air inside the house so the methanol may have simply dried out the small amount of still air inside the container. The worst case then would be a very hot, humid and windy day. And raining
Old 09-18-2005, 10:20 AM
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Jim Thomerson
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Default RE: water in fuel

I haven't done the weighing experiment because I don't have a good enough scale. I know guys who have taken their airplane into the post office to weigh it on the postal scale in the lobby. However, I don't think that is a viable option for the fuel-water experiment.[:-]

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