Check the purity of your biodiesel batch with this hydrometer. Usable biodiesel has a specific gravity between 0.85 and 0.90.
Hydrometers are little floats with calibrated stems used to measure the specific gravity of a liquid. Specific gravity is the ratio of the density of a liquid to the density of an equal volume of pure water. The most familiar applications of a hydrometer are checking a battery charge, checking anti-freeze protection, calculating the salinity of a water sample, or checking the purity of biodiesel fuel. See below for optional cylinders to hold liquid samples in which to float the hydrometer.