What is specific gravity?
Broadly speaking, it is the ratio of an object’s density relative to the density of another substance, usually water.
Is specific gravity the same as density?
No. Density is the ratio between a substance’s mass and volume; specific gravity is a dimensionless unit derived from two measurements of density.
How do I find specific gravity?
For most purposes, such as float switch applications, it’s the ratio of the substance’s density to that of water, which in such cases would have an SG
of 1.0. Substances with a lesser density than water would have an SG of less than 1, and a denser substance more.
Since water has a density of 1 gram per cubic centimeter, the equation for establishing a liquid’s specific gravity would be: Specific Gravity = (density of liquid) / (1 gm/cm³).
Do liquids and solids have specific gravity?
Yes. Jet fuel, for instance, has a specific gravity of about 0.62; pure gold of almost 20.
Can temperature affect specific gravity?
Yes, it can. When using water as a reference for establishing specific gravity, it is almost always assumed that the water is at 4°C, when it’s densest. In cases where the temperature or pressures of the two substances differ, the discrepancy must be corrected. Water itself becomes less dense as it becomes warmer until it evaporates and becomes steam vapor at boiling point.
Can altitude or elevation from sea level affect specific gravity?
Since atmospheric pressure decreases at higher altitudes, elevation does affect specific gravity of air and can have an impact on the operation of liquid pumps which rely on atmospheric pressures to operate.
What’s the significance of specific gravity when choosing a float switch?
When selecting a float or level switch for any application, it’s important to choose a switch made out of a material with a specific gravity that is compatible with the liquid medium and desired function. Since substances with higher specific gravities sink in liquids with lower ones, something required to float in a certain liquid must have a lower specific gravity.
For instance, a polypropylene level switch might have a specific gravity of about 0.80, which would make it ideal to float in water, but not gasoline, which only has a specific gravity of about 0.70. For more information and the specific gravities of many common liquid media, see our application note on the subject. All our level sensors and float switches also have their specific gravities on their product pages.