Specific Gravity

Specific gravity strips the units off density by dividing by water's 1000 kg/m³. What is left is a pure ratio, and that is the whole reason the quantity has survived: the number is the same in every unit system. A fluid of SG 1.19 is 1.19 times denser than water whether you work in kilograms per cubic metre, pounds per cubic foot, or anything else. It also reads as a float-or-sink test at a glance — below 1 floats on water, above 1 sinks. Gasoline sits near 0.74, sea water at 1.025, concrete around 2.4, steel at 7.85.

The practical use is converting a label into a weight. A drum of concentrated hydrochloric acid is marked SG 1.19, so its density is 1190 kg/m³, and a 205 L drum holds $205 \times 1.19 = 244$ kg of liquid — worth knowing before it goes on a hand truck. Same arithmetic in a mechanical room: a 1000 L loop charged with 40% propylene glycol at SG 1.045 holds 1045 kg, and the extra mass shows up in the pump's power draw.

The instrument that reads it is Archimedes' principle made into a tool. A hydrometer is a weighted float that sinks until it displaces its own weight, so the depth it settles to is a direct readout of the surrounding fluid's density — no calculation, just a scale on a stem. Brewers watch wort fall from about 1.050 to 1.010 as sugar becomes alcohol, and the drop estimates the strength without opening the vessel. Battery technicians read the same instrument against a different scale: a healthy lead-acid cell shows about 1.265 charged and 1.120 discharged, because the sulfuric acid is genuinely consumed as the cell delivers current. Petroleum has its own derived scales, Baumé and API gravity, which are specific gravity rearranged so the numbers run the other way.

The error the name invites is treating specific gravity as a density. It is not one, and it has no units. Converting to kg/m³ by multiplying by 1000 works only because water happens to be 1000 kg/m³; in pounds per cubic foot the multiplier is 62.4. Feed an SG into an equation that wants ρ — hydraulic power, buoyancy, pressure head — and the answer is off by a factor of a thousand. The two quantities are also often quoted in the same breath on a product sheet, which does nothing to help.

Then the fine print that separates a careful figure from a rough one. A specific gravity is meaningless without two temperatures, because both the sample and the water reference expand. Standards state it as a basis: 60/60 °F means sample and reference both at 60 °F, while a 20/4 basis compares a 20 °C sample against water at its 4 °C density maximum, and the two differ by a few tenths of a percent — trivial for a drum count, not trivial in custody transfer. A hydrometer reading therefore needs a temperature correction, and a warm sample always reads low. Gases are a separate convention entirely: the specific gravity of natural gas, about 0.6, is referenced to air, not water, and reading it against water is an 800-fold error. Finally, using SG to infer a concentration requires the right table for that specific solute, and for a few solutions the relationship is not even single-valued — sulfuric acid reaches peak density near 98% and gets lighter above it, so one reading can correspond to two very different strengths.