INTRODUCTION

The density of a substance is the ratio of its mass to its volume. It is always necessary to state the units of density because it may be expressed in a number of different mass and volume units.

Specific gravity is the ratio of the mass of a volume of a substance to the mass of an equal volume of some substance taken as a standard. For convenience, the following standards are generally used: for water, 39.2 F (4 C) and 60 F (15 C), and for gases, the standard is dried air at the same temperature and pressure as the gas for which the specific gravity is sought.

API hydrometers were developed for crude oils and are defined at 60 F. The specific gravity (SG) also was defined at 60 F as the density of a fluid at 60 F referred to the density of water at 60 F. Thus, direct conversion between the API gravity and specific gravity is possible.

A complete set of charts has been prepared for the conversion of hydrometer readings at temperatures from 0 F to 150 F ( ?18 C to 66 C) for the range of API gravities from 0 to 99; these charts are published in the Petroleum Production Handbook (McGraw-Hill). For example, using the API charts, assume that an API hydrometer reading is 12 at an observed temperature of 85 F (29 C). Enter the 10-19 API Chart at a temperature of 85 F, move to the column under the observed value of 12, and find that the API gravity at 60 F...