Specific Gravity (Specific Gravity)

Specific Gravity (SG), also called relative density, is the dimensionless ratio of a substance's density to the density of a reference material at standard conditions. For liquids and solids, the reference is water at 4°C (density 1,000 kg/m³). For gases, the reference is dry air at standard temperature and pressure (density 1.225 kg/m³ at 15°C and 1 atm). A liquid with SG of 1.05 is 5% denser than water; a gas with SG of 0.55 is 45% lighter than air.

In CBG, biogas, and waste-processing contexts, SG governs several safety and operational outcomes. Methane has SG of 0.55 relative to air — significantly lighter than air, so leaks rise to the ceiling and accumulate in high points of buildings. CBG safety design therefore places gas detectors at the highest point of any enclosed space, and ventilation must be sized to clear leaks upward. Hydrogen sulfide has SG of 1.19 — heavier than air, so leaks accumulate in pits, sumps, and floor-level spaces. H₂S detection requires sensors at low points in confined-space entry. Carbon dioxide has SG of 1.53 — also heavier than air, accumulating in low spots and posing asphyxiation risk in confined spaces around biogas upgrading systems. Liquid digestate has SG of 1.02–1.08, slightly heavier than water, with implications for pump and tank sizing.

SG also drives commercial measurement in CBG sales. CBG dispensed at retail is sold by mass (kg), but underlying flow measurement in pipelines is volumetric (Nm³), and the conversion uses biogas-specific density that depends on composition. Higher methane purity (lighter gas, lower SG) means more energy per kg but fewer kg per Nm³. The Bureau of Indian Standards IS 16087 specification therefore mandates SG measurement as part of CBG quality testing, alongside methane content and calorific value. Custody-transfer flow meters on CBG cascades use ultrasonic or Coriolis principles to handle gas composition variations, and meter calibration must be checked any time upgrading system performance shifts. Trade-off in measurement: ultrasonic meters are cheaper and have no moving parts but require composition correction; Coriolis meters directly measure mass flow but cost 3–5× more.