How It Works
Specific gravity (SG) is the dimensionless ratio of a substance's density (or specific weight) to that of a reference — usually water at 4 °C (1,000 kg/m³). An SG less than 1 means the substance floats in water; greater than 1 means it sinks. You can calculate SG from either density ratios or specific weight ratios.
Example Problem
Olive oil has a density of 920 kg/m³. What is its specific gravity relative to water?
- SG = 920 / 1,000 = 0.92
Since SG < 1, olive oil floats on water.
When to Use Each Variable
- Solve for Specific Gravity (Density) — when you know the substance and reference densities, e.g., checking if a material will float or sink in water.
- Solve for Substance Density — when you know the specific gravity and reference density, e.g., converting an SG measurement from a hydrometer to a density value.
- Solve for Reference Density — when you know the SG and substance density, e.g., determining what reference fluid was used in a reported SG measurement.
- Solve for Specific Gravity (Specific Weight) — when working with specific weight ratios instead of density ratios, e.g., using weight-based measurements in field conditions.
- Solve for Substance Specific Weight — when you know SG and reference specific weight, e.g., calculating the weight per unit volume of a petroleum product.
Key Concepts
Specific gravity is the dimensionless ratio of a substance's density to a reference density, usually water at 4 degrees C (1,000 kg/m3). Because it is dimensionless, SG has the same numeric value regardless of the unit system. An SG less than 1 means the substance floats in the reference fluid; greater than 1 means it sinks. SG can also be expressed using specific weight ratios, which is equivalent when both measurements are at the same gravitational acceleration.
Applications
- Petroleum industry: API gravity is derived from specific gravity to classify crude oil grades
- Brewing and winemaking: using hydrometers to measure sugar content via SG before and after fermentation
- Geology and mineralogy: identifying minerals by comparing their SG to known reference values
- Quality control: verifying purity of chemicals, acids, and solutions by checking SG against specifications
Common Mistakes
- Confusing specific gravity with density — SG is dimensionless, while density has units (kg/m3 or g/cm3)
- Ignoring temperature — both the substance and reference densities change with temperature, so SG varies unless temperature is specified
- Assuming water density is always 1,000 kg/m3 — at temperatures other than 4 degrees C, water density deviates slightly
- Using SG to predict buoyancy in fluids other than water — a substance with SG = 0.9 floats in water but may sink in a lighter fluid
Frequently Asked Questions
What is specific gravity used for?
Specific gravity is widely used in geology (mineral identification), brewing (measuring sugar content), petroleum (API gravity), and quality control to verify material purity.
Is specific gravity the same as density?
No. Density has units (kg/m³), while specific gravity is a dimensionless ratio. Numerically, specific gravity relative to water at 4 °C equals the density in g/cm³.
Can specific gravity change with temperature?
Yes. Both the substance and reference densities change with temperature, so specific gravity measurements should state the temperature conditions (e.g., SG at 20 °C/4 °C).
Reference: Lindeburg, Michael R. 1992. Engineer In Training Reference Manual. Professional Publication, Inc. 8th Edition.
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- Density Converter — convert density units between kg/m3, lb/ft3, and g/cm3.
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