Specific gravity at 60/60°F

where specific gravity at 60/60°F (16/16°C) means that both oil and water are at 60°F (16°C).

Given this relationship, heavier liquid fuels are denoted by lower API gravity values. The specific gravity is lowest for Grade No. 1 and increases with increasing grade number. Conversely, the Degree API gravity is highest for Grade No. 1 and decreases with increasing grade number. The gross heating value in Btu/lbm (kJ/kg) is generally highest for Grade No. 1 and decreases with increasing grade number. However, since the density, in lbm/gallon (kg/liter), increases significantly with increasing grade number, so does the heat value per gallon (liter).

There are several ASTM standards for determining the heating values of liquid fuels, depending on the precision of results desired. When the heating value is determined by calorimetry, the procedure most frequently used is described in the ASTM D 240, Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter.

For fuel oils, the gross heat of combustion is reported in preference to the net, which is the quantity required in practical engineering applications. Since high accuracy requirements for energy density determination are rarely a high priority for these fuels, the procedures described in ASTM D 4868, Standard Test Method for Estimation of Net and Gross Heat of Combustion of Burner and Diesel Fuels, suffice in most instances. Calculations are based on density, sulfur content, water content, and ash content (the method is not applicable to pure hydrocarbons).

Table 5-5 includes the HHV and LHV, in English system units, of some commonly used liquid fuels. Table 5-6 lists approximate heat content of various petroleum products in million Btu per barrel, as adopted by the U.S. Energy Information Administration (EIA) per the Bureau of Mines. Table 5-7 lists the chemical elements and compounds found in commonly used fuels for combustion.

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