How to test for biodeisel viscosity

Written by colin rowe
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How to test for biodeisel viscosity
A plastic bottle is perfect to test biodiesel viscosity (mineral water image by Olga Shelego from

A major criticism of biodiesel is it is only really useful in warmer climates because it is more viscous than regular diesel. In cold climates, even the engine heat cannot reduce viscosity enough for easy use. To address this problem, biodiesel is used with regular diesel in mixtures that vary according to ambient operating temperatures. Measuring the viscosity of homemade biodiesel is vital to working out how much regular diesel to put in your car. You can measure the viscosity of biodiesel with a few household items and a stopwatch.

Skill level:

Things you need

  • 1.5-litre plastic bottle
  • 2-millimeter copper pipe
  • Drill and 2-millimeter drill bit
  • Biodiesel
  • Stop-watch
  • Calculator
  • Thin marker pen
  • Tape measure
  • Scissors
  • Thermometer
  • Bucket

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  1. 1

    Cut the bottom off the plastic bottle using the scissors so the biodiesel can be poured in easily.

  2. 2

    Remove the cap of the bottle and drill a hole in it.

  3. 3

    Insert the copper tube into the hole in the lid so about one centimetre of it is in the bottle.

  4. 4

    Replace the cap on the bottle, put you finger on the end of the copper tube and fill the bottle with water. This is to check for leaks around the cap which could affect the results. Make sure you dry the bottle completely after emptying the water.

  5. 5

    Measure out and mark a vertical 10-centimeter section on the side of the bottle. The line nearest the cut-off end of the bottle is the start line and the one nearest the cap is the finish line.

  6. 6

    Place the bucket underneath the bottle so you do not lose the biodiesel by testing the viscosity.

  7. 7

    Heat up the biodiesel to 40 degrees Celsius. This needs to be very accurate so the readings are meaningful.

  8. 8

    Put your finger over the end of the copper pipe and fill the bottle with biodiesel to the start line.

  9. 9

    Remove your finger from the end of the copper pipe and simultaneously start the stopwatch. Make sure the bottle is held perpendicular to the bucket.

  10. 10

    Measure how long the biodiesel takes to get to the finish line and repeat at least two more times so you have an accurate result. If this test is done properly, the results should be within a few tenths of a second of each other.

  11. 11

    Divide the results in seconds by ten to find out how far the biodiesel travelled in one second. This will give you a value in centistokes, which is the standard measure of viscosity in fuel oil.

Tips and warnings

  • Make sure you clean and dry the bottle after every test to get the best results.
  • Use a smooth-sided plastic bottle to get the best results. Any curves or moulded designs in the bottle will affect the results.
  • This is only ever going to be a rough guide, but it should be good enough to work out what kind of fuel mixture you will need.

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