Q: GHS does not give the expected result when converting API60 to specific gravity. Why?

A: What GHS calls "specific gravity" is normally "relative density". At 39.2 deg F they're the same thing. True specific gravity cannot be used to find the weight of a volume without knowing the temper ature. For the most part in naval architecture work we do not factor in the temperature; it is customary to speak of the specific gravity of liquids without reference to temperature.

Relative density is the density of a given substance divided by the maximum density of water, which occurs at 39.2 deg F. The metric system is based in this, so that at SpGr=1.0 you have 1000kg.m^3.

The formula commonly used to convert API degrees to specific gravity at 60 degrees F is referring to real specific gravity. At 60 deg F water has a density that is about 0.999 of its maximum density. S o when GHS converts API degrees to specific gravity it multiplies the result by 0.999 to get back to relative density.

However, GHS gives you the option of using 60 deg F as the reference temperature for specific gravity. If you declare this (see the SPGR command) then the API -> spgr conversion comes out as expecte

Either way, the actual weight/volume for a given API remains the same.

Copyright (C) 2011 Creative Systems, Inc.