The carbon residue test method gives a measure of the tendency of a fuel to produce carbon deposits.  The results are dependent on the specific test conditions and are therefore only an approximation of possible engine fuel deposits.  High carbon residue results can be indicative of excessive amounts of glycerol in the biodiesel sample.

Significance

The referee test method for carbon residue detection is ASTM D4530, known as the Micro Conradson method for carbon residue.  In this method a weighed amount of sample is place in a small glass vial and heated to 500°C in an inert, nitrogen atmosphere for a specified time.  During the test the sample undergoes coking reactions and any volatiles are removed by the nitrogen.  Any remaining carbonaceous material is reported as a percentage of the original sample as carbon residue (micro).  D4530 is equivalent to ASTM D189, the Conradson method using a similar setup on a larger scale.  The micro method offers the advantages of better control of test conditions, smaller samples and less operator attention.  Another acceptable method is ASTM D524, known as the Ramsbottom Carbon Residue method.   The Ramsbottom method involves a weighted sample placed into a special glass bulb which is then placed in a metal furnace at 550°C.  All volatile matter is evaporated out of the bulb, while the heavier residue undergoes cracking and coking reactions.  This method is generally limited to samples that are mobile below 90°C, and are not numerically the same as results using methods D4530 or D189 (although some approximate correlations have been derived).   ASTM D6751 allows for a maximum of 0.05% (by mass) carbon residue in a biodiesel sample using the Conradson micro method.