Density and API gravity are used in custody transfer quantity calculations and to satisfy transportation, storage, and regulatory requirements. Accurate determination of density or API gravity of crude petroleum and liquid petroleum products is necessary for the conversion of measured volumes to volumes at the standard temperatures of 15°C or 60°F.

Density and API gravity are also factors that indicate the quality of crude petroleum. Crude petroleum prices are frequently posted against values in kg/m³ or in degrees API. However, this property of petroleum is an uncertain indication of its quality unless correlated with other properties.

Field of Application-Because the thermohydrometer incorporates both the hydrometer and thermometer in one device, it is more applicable in field operations for determining density or API gravity of crude petroleum and other liquid petroleum products. The procedure is convenient for gathering main trunk pipelines and other field applications where limited laboratory facilities are available. The thermohydrometer method may have limitations in some petroleum density determinations. When this is the case, other methods such as Test Method D 1298 (API MPMS Chapter 9.1) may be used.

This test method is suitable for determining the density, relative density, or API gravity of low viscosity transparent or opaque liquids, or both. This test method, when used for opaque liquids, requires the use of a meniscus correction (see 7.2).