Specific Gravity and Biodiesel
By Rick Da Tech
API Gravity of Diesel
One of the field tests for Diesel Fuel is to measure its specific gravity. ASTM D287, Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) is the official ASTM test procedure used by the petroleum industry. This test measures density using a hydrometer calibrated to the API Gravity scale. API Gravity is inversely proportional to specific gravity. As API gravity goes up, specific gravity goes down. The formula that relates API to SG is:
API = (141.5 / SG) - 131.5 (1)
The petroleum industry uses API Gravity as an indication of the energy content of the fuel. As API Gravity increases energy content decreases. Since specific gravity is the inverse of API Gravity, a higher specific gravity means a higher energy content fuel. As Specific Gravity increases, power output and mileage both increase. The API limits for D2 Diesel are 30 to 45. That translates to specific gravities of 0.876 to 0.802.
Measuring specific gravity is an accepted field test used in the distribution network to determine the quality of diesel fuel. It can alert us to the presence of contaminants in diesel like gasoline, water, and biodiesel. Since it is so useful as a field test for diesel, there is a strong desire to apply this test to biodiesel.
The Specific Gravity of Biodiesel
The specific gravity of Biodiesel varies with its fatty acid composition, and its glycerin content, both free and bound. The same basic principles apply. A denser biodiesel has higher energy content and results in better mileage and increased power. Since the fatty acid content dictates the specific gravity, a denser vegetable oil processes into a denser biodiesel.
Vegetable oils typically have a specific gravity from 0.903 to about 0.921, depending on its fatty acid composition and temperature. The densities recorded in literature are at different temperatures. I have found three different temperatures for measuring density, 15.5C, 20C, and 25C. As the temperature increases the density decreases, for example, olive oil at 15.5 C ranges from 0.915 to 0.918. At 25C olive oil ranges from 0.910 to 0.920.
Biodiesel has a range of about 0.86 to 0.90. So, if you want to find out if your oil is biodiesel or vegetable oil, you can measure its specific gravity with a hydrometer, and anything above 0.90 is probably vegetable oil, and anything below 0.90 is probably biodiesel.
An old-timer in biodiesel suggested using ice to determine if its oil or biodiesel. Ice has a specific gravity of 0.917, causing it to float in oil (usually) and sink in biodiesel. However, some oils, like canola with its specific gravity of 0.913 to 0.916, is less dense than ice so that ice would sink into it. I guess you could try it with a lot of ice to cool down the oil, so it would float even in less dense oils.
Some contaminants can alter the specific gravity of biodiesel. One of those contaminants is methanol. Biodiesel containing methanol has a specific gravity about 0.005 lower than biodiesel that has had the methanol removed. Soap, monoglycerides, and diglycerides also affect specific gravity.
Unfortunately, there are too many variables to use specific gravity as a reliable quality test, like we do with diesel fuel. However, we can still use it to grade the energy content of biodiesel from batch to batch and estimate the performance we can expect from it.
Densities of Vegetable Oils and Fatty Acids University of Nebraska, Lincoln, 1992 pdf
Density of Cooking Oil The Physics Factbook
The Specific Gravity Of Biodiesel Fuels And Their Blends With Diesel Fuel University of Illinois 2004
API Gravity - The Engineers Toolbox