Editing API gravity (section)

==Measurement of API gravity from its specific gravity==
To derive the API gravity, the specific gravity (i.e., density relative to water) is first measured using either the [[hydrometer]], detailed in [[ASTM]] D1298 or with the [[oscillating U-tube]] method detailed in ASTM D4052.

Density adjustments at different temperatures, corrections for soda-lime glass expansion and contraction and [[Meniscus (liquid)|meniscus]] corrections for opaque oils are detailed in the [[Petroleum Measurement Tables]], details of usage specified in ASTM D1250. The specific gravity is defined by the formula below.

:<math>\mbox{SG oil} = \frac{\rho_\text{crudeoil}}{\rho_{\text{H}_2\text{O}}}</math>

With the formula presented in the previous section, the API gravity can be readily calculated. When converting oil density to specific gravity using the above definition, it is important to use the correct density of water, according to the standard conditions used when the measurement was made. The official density of water at 60&nbsp;°F according to the 2008 edition of ASTM D1250 is 999.016&nbsp;kg/m<sup>3</sup>.<ref>Report on the Development, Construction, Calculation and Preparation of the ASTM – IP Petroleum Measurement Tables (ASTM D1250; IP 200), 1960, published by the Institute of Petroleum, 61 New Cavendish Street, London, W.1 and American Society for Testing Materials, 1916 Race Street, Philadelphia 3, Pa.</ref> The 1980 value is 999.012&nbsp;kg/m<sup>3</sup>.<ref>API Manual of Petroleum Measurement Standards, Chapter 11.1 – 1980, Volume XI/XII, Adjunct to: ASTM D1250-80 and IP 200/80</ref> In some cases the standard conditions may be 15&nbsp;°C (59&nbsp;°F) and not 60&nbsp;°F (15.56&nbsp;°C), in which case a different value for the water density would be appropriate (''see'' [[standard conditions for temperature and pressure]]).