The Specific Gravity based on water is not applicable to flow services that are 100% gas. Specific gravity for liquid is weight of liquid with respect to water and specific gravity of gas is weight of gas with respect to air. These 2 fluids are totally different – one is non-compressible & another is compressible. Their properties are drastically different (in case of gas, its volume & hence density will change with change in pressure but not in case of water).
We could consider adding a toggle to switch between water and air. Since the weight of gases is very small, most users simply ignore it. So I would ask if inclusion makes enough difference to make this enhancement worthwhile.
For most gases the specific gravity should be entered as near zero unless it is under very high pressure. The specific gravity of gas is not useful without pressure, temperature and compressibilty factor which change with operating case.
Regardless, a new enhancement has been logged:
Enhancement 108544:Tracker#:4904 - Allow option to reference specific gravity of dry air for gases in pipe content