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Introduction to the technical terms related to petroleum product density meters: Density: The mass of a unit volume of liquid at 15°C and 101.325 kPa, expressed in kg/m3. Densities at other standard temperatures, such as those at 20°C for certain products and in certain locations, are also used, and a few units such as kg/l and g/ml are still in use. Relative density (specific gravity): At a specific temperature, it is the ratio of the mass of a given liquid to the mass of an equal volume of pure water at the same temperature or at different temperatures. The two standard temperatures will be clearly specified below. Common standard temperatures include 60/60°F, 20/20°C–20/4°C; the term specific gravity was used in the past and can still be seen today. API gravity: A conventional scale used to indicate the relative density of oils at 60/60°F, with the relationship expressed as follows: API = 141.5/A (relative density at 60/60°F) – 131.5. Unless otherwise specified, the standard temperature defined as the reference temperature is 60°F. Apparent density refers to the density gauge reading observed at a certain temperature, rather than the density value at a specified standard temperature. These values are merely densitometer readings and not the density at other temperatures, relative density, or API gravity. Cloud point: The temperature at which a cooled liquid becomes cloudy as wax crystals begin to form under specified conditions. Set point: The lowest temperature at which a cooled sample of oil and petroleum products is still capable of flowing under specified conditions. Wax Appearance Temperature (WAT): The temperature at which wax solids form in petroleum and petroleum products when cooled under specified conditions.