Specific gravity is the ratio of a material's density to the density of an identified reference substance. The ratio has no density units because like units cancel. Its meaning depends on the reference and conditions; a number labeled SG is incomplete if those details are missing.
Specific gravity is the ratio of a material's density to the density of an identified reference substance. The ratio has no density units because like units cancel. Its meaning depends on the reference and conditions; a number labeled SG is incomplete if those details are missing.
Water and air are different references
The Plastics Pipe Institute's handbook glossary defines material specific gravity using water as the reference, usually at an identified temperature. That is a material-property context, rather than a universal rule that every SG value compares with water.
Swagelok's valve-sizing bulletin explicitly distinguishes liquid specific gravity, using water as one, from gas specific gravity, using air as one. A gas value and a liquid or solid value therefore cannot be compared as though their reference densities were identical.
Retain the document's definition, reference temperature and other stated conditions. Density itself is mass per volume and carries units. Specific gravity compares densities; copying a unit-bearing density into an SG field silently changes what the number describes.
The surrounding specification supplies the purpose
Our ABS and PVC comparison includes specific gravity in a manufacturer's material-property table. That ratio does not, by itself, establish impact resistance, pipe stiffness, sound performance or approval for an application. Those questions need their own evidence.
A flow specification can use a fluid's specific gravity for a different purpose. The valve flow coefficient entry explains why a coefficient needs the actual fluid and operating conditions. A shared connection size and SG value do not promise a particular installed flow.
The pool-heater gas-line guide also identifies different specific-gravity assumptions in named tables. Understanding the label does not authorize substituting one table's assumptions into another or choosing gas pipe without the required qualified sizing work.
What to retain when comparing data sheets
Ask which material or fluid was evaluated, what reference was used and which test or calculation conditions accompany the number. Keep the original units with any density measurements and retain the source document rather than only transcribing an SG value.
Where pressure enters the same calculation, gauge and absolute pressure supplies another reference distinction. The two labels answer different questions.
Do not infer drinking-water safety, contamination, fuel compatibility or a component's pressure rating from specific gravity. For a replacement or design decision, have the responsible provider identify the complete material, service conditions and applicable approvals.