hydrometer is a simple (typically glass) instrument used to measure the density of liquids.

Usable biodiesel has a density of 0.86-0.90 g/ml (grams per millilitre – pure water is roughly 1g per ml). Therefore a hydrometer which covers that full range is required to check the quality of biodiesel produced. A brewer’s hydrometer is unsuitable as the general density range for brewers measuring liquid as sugar turns to alcohol is 0.990-1.070 g/ml.

The hydrometer used in the commercial production of biodiesel is a BS718 M50SP with a range of 0.850-0.900 g/ml. It is 270mm long with 0.001 g/ml divisions, and +/- 0.0006 g/ml at 15 degrees Celcius.

For domestic (non-professional) production of biodiesel a cheaper option would be a BS718 M100 with a range of 0.800-0.900 g/ml, 0.002 g/ml divisions, and 0.002 g/ml accuracy at 20 degrees Celcius. This hydrometer is 250mm long.

Stevenson Reeves Ltd has the following very useful guide: How to Use a Hydrometer.

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