If you run auto fuel in your aircraft engine it's best to avoid the kind of auto fuel that contains ethanol. Sometimes you cannot be sure if the fuel you're using has ethanol or not. This test is an admittedly unscientific version of the ethanol test on the EAA website.
I use a pint size Mason jar for the test with about a inch of water. Photo 1 shows the jar with water and a black strip of tape marking the water level. I colored the water with a little grape juice for the photographs.
Next add 2 or 3 inches of the fuel to the jar. Put the lid on the jar and shake it for a minute or so to mix the water and the fuel. Photo 2. The water will dissolve any ethanol in the fuel sample.
Finally, let the jar sit for several minutes. The water/ethanol will settle to the bottom of the jar and the gas will rise to the top. Note in Photo 3 that the "water" level is now nearly to the top of the tape. The increased volume of water is due to the dissolved ethanol. If there is no ethanol in the fuel sample, the water level will not rise.
Most folks only care if ethanol is present or not, but (Photo 4) you can calculate the approximate percent of ethanol in the fuel as follows: (1) measure from the tape mark to the new water level, in this example the water level went up 7/16 inch (.437") and (2) measure from the tape mark to the top of the gas, in our example it's about 2 1/4 inch (2.250") then (3) divide the ethanol amount (.437) by the total fuel added (2.250) and we get 0.19 or 19% ethanol.
How accurate is this test? Well, the pump where I got my fuel sample was labeled 15% ethanol (and I measured 19%). I've also tested several samples labeled "pure gas, no ethanol" and only one of them showed a slight indication of ethanol (the water level on that one sample rose about 1/8 inch, suggesting about 5% ethanol). So I'm thinking this test is plus or minus 5% accurate...or maybe the fuel compounders have a 5% tolerance!
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