The 3-27 Conversion Test

By Rick Da Tech


Homebrewers use the 3-27 conversion test to determine if our biodiesel has completed the chemical reaction from vegetable oil to biodiesel. It is not an ASTM test and does not tell us if we meet ASTM standards. ASTM 5684 is a set of directions used by commercial biodiesel producers to measure the amount of glycerin in biodiesel.

ASTM 5684 specifically tells us four things:

  • The quantity of Free Glycerin in our biodiesel.
  • The quantity of glycerin in our biodiesel that is attached to monoglycerides.
  • The quantity of glycerin in our biodiesel that is attached to diglycerides.
  • The quantity of glycerin in our biodiesel that is attached to triglycerides.

The 3-27 test indicates how soluble our biodiesel is in methanol. Pure biodiesel dissolves completely in methanol without a trace. Pure vegetable oil does not dissolve in methanol at all and forms two distinct phases of methanol and vegetable oil. Monoglycerides and diglycerides partially dissolve in methanol. Unlike pure biodiesel, methanol becomes saturated with monoglycerides and diglycerides at some point and refuse to dissolve more. Methanol becomes saturated with diglycerides more quickly than monoglycerides. So, the 3-27 test is a go/nogo test with a visual indication showing us if our biodiesel has more vegetable oil, diglycerides, and monoglycerides than methanol can absorb. The ASTM test measures the concentration of free glycerin, but our 3-27 test is blind to it since it fully dissolves in methanol.

Monitoring your process: The test can be used in mid process to determine how long to run your reactor, or monitor your conversion at different stages in the process. Not Quantitative: While there must be some relationship between the amount of fallout and the degree of conversion, so far the test is not quantitative. We can’t say that a 1mm bead of oil fallout means “x percent” conversion and if you see a 2mm bead it's twice as impure.


Performing the Test

Add 3ml of biodiesel in a glass container with 27ml of methanol. The methanol must be 20°C (68°F), and the biodiesel must be near that temperature. The methanol must be pure and not recycled or recovered from glycerin.

Mix and let settle for 10 to 15 minutes. Any liquid that settles to the bottom by the end of 15 minutes is a fail. It passes when the biodiesel completely dissolves in the methanol. If it is a little cloudy with no precipitation, that is a pass.

You can perform the test on washed and dried biodiesel, or raw, unwashed biodiesel after you let it settle for a few minutes.


The test is one part biodiesel to nine parts methanol. Increasing the sample size increases the sensitivity of the test. There are references to tests with 10:90, 25:225, 1:9, and others.

The 3/27 test is a qualitative test, meaning a go/nogo test. Some people use it quantitatively as a way to compare one batch to another. However, you should resist the temptation to come up with a formula that converts fallout volume to percent conversion. Those formulas do not work every time for everyone.

There have been tests that compared the results of the 3/27 test to the results of ASTM 5684. If you have a clear and bright sample with no fallout, you are likely to meet or come very close to ASTM specifications for conversion. The two tests do not always agree. However, the 3/27 test is considered by most to be the standard for home-brewed biodiesel.

Related Links:

3/27 and beyond infopop thread

Biopowered Wiki Page on 3/27