If you make biodiesel using vegetable oil that is contaminated with water, you can have problems. And the more water in the oil, the more problems you’ll have. Glop problems, washing/emulsion problems, reduced yield and so on. So here’s a way to actually test your oil to find out if it is contaminated with water or not, and if so, how much water. Please note that you can’t just look at the oil and know if it’s wet or not. It’s easy, so jump right on in. The short version is: weigh, heat to dry, weigh again, do math.

You need an accurate scale that can precisely measure weight with a resolution of 0.1 grams (a tenth of a gram) or better. Ebay has lots of vendors that can supply a manual triple beam balance that works beautifully. There are digital versions as well. The scale should have an upper capacity of about 500g or more to do this test well. You could fake it with a top capacity of 100g, but you have to use a much smaller sample and you lose some precision.

1. Find a container to put an oil sample in. Gladware works nicely (semi-disposable “Tupperware” and available at any supermarket or Wal-Mart). It’s about the right size, cheap, lightweight, re-usable, see-through and heat resistant at the temperatures we’re working with.

2. Weigh the empty container to the nearest tenth of a gram and record that somewhere.

3. Take a 300 to 400 mL sample of oil. The sample should be representative of all the oil you are going to react. So typically, I dump all my oil into my processor tank and stir it for ten or fifteen minutes. Then I take the sample.

4. Weigh the sample. Record this number. Subtracting the weight of the container gives us the actual weight of the oil. Record oil weight. I designate this as “wet weight”.

5. Heat the sample in a microwave until the oil reaches a temperature of 250° F(~120° C). Test the temperature of the oil periodically OUT OF THE MICROWAVE. Keep it at this temperature until all boiling stops. You should heat for a minute or two and then test the temperature. Then heat again, and check temp.

The thermometer above has actually lapsed itself and is reading 260° F

Harbor Freight sells a nice probe thermometer with a 6” metal probe with a round thermometer head at the end. A candy thermometer would work as well. Check your thermometer for accuracy in boiling water and confirm a reading around 212° F (100° C). Some are a mile off. During the heating phase, you will get a qualitative indication for the presence of water. If your sample starts to spit and boil around 212° F (100° C) you know there is some water. We’ll get quantitative data in a few minutes.

6. Allow the sample to cool off for ten minutes or so. This allows any residual emulsified water to evaporate, plus it’s safer.

7. Weigh the sample again, record that number. Subtract container weight. This is your “dry weight” of the oil. If you’re a purist from chem. lab, you would do the whole heat/weigh thing again and again until the weight stops going down to be sure the water is all gone. I make the assumption that if the sample is 250° F(~120° C) and there’s no boiling, the water is gone.

8. You can see where this is going. Take wet weight and subtract dry weight. The difference is the weight of the water that was in your sample. Now divide water weight by the original “wet oil weight” and this will give the decimal fraction water content of the sample. Multiply by 100 to get percent.

Here’s an example:
Container wt: 26.2g
Total wet wt: 335.6g
Wet oil wt: 335.6g - 26.2g = 309.4g
Total dry wt: 321.6g
Dry oil wt: 321.6g – 26.2g = 297.4g
Water wt: 309.4g – 297.4 = 12.0g water
Water content: 12.0g / 309.4g = 0.03878
% water by wt: 0.03878 x 100 ~ 3.9% water by weight

If I’m careful, and without heroics, I can measure the presence of 0.2 grams of water in a 500 gram oil sample.

That would be:
0.2g/500g = 0.0004 water, or 0.04% water.

So even if you’re sloppy, and only measure gram amounts, you can still learn a lot about water content with a scale and a microwave. As an example, my microwave is 1,100 watts and I do the test on high for 3-6 minutes, depending on how cold and how wet the oil is. I typically do one or two minutes per session and then check temperature. It has crossed my mind you could use the defrost function for really wet oil to minimize popping issues if needed. If it pops a lot, you could be losing oil and it will make your results look wetter than it reallly is. It would take longer of course, but it's basically an unsupervised process. Nuke for ten minutes on defrost and check temp, etc.

Summary:
Weigh
Heat
Weigh again
Perform the math