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Title: Methods Of Deterkining Specific Gravity
Author: E. J. Schumacher
Source: 1969 Gulf Coast Measurement Short Course (Now called ASGMT)
Year Published: 1969
Abstract: The determination of specific gravity is an important function in the work of Gas Measurement. In measuring gas by orifice meters, the specific gravity of the gas being measured is certainly one of the major variable factors in determining actual volumes measured, if not the greatest. Unless the coefficient is corrected to an accurately determined specific gravity of the gas being measured, the measurement will not be correct. Before going any farther into discussion, we should define Specific Gravity. One can find many definitions for specific gravity. In fact one can pick almost any gas handbook or gas journal and find a different definition in each one. But technically speaking in the gas business, specific gravity can best be defined as the ratio of the molecular weight of a gas to that of the molecular weight of air, where the molecular weight of air is assumed to be 28.9644. 1 Since molecular weight measurements are not easy to obtain, especially in the field and to be practical, instrumentation is used to obtain specific gravity by determining the relative density of a gaseous substance to that of dry air, assuming air as unity. Relative density being the ratio of the weight of a volume of gas to that of an equal volume of dry air, where the weight of both gas and air are taken under equal pressure and temperature conditions. To accurately obtain specific gravity, it is of necessity to adjust relative density readings for pressure, temperature and deviation from Boyles Law, found between air and gas, except where instruments are calibrated to a gas of known specific gravity.

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