BS 718:1991

Five series of hydrometers for liquids with densities between 600 and 2000 kg/m³ at 20°C and three sub-series for petroleum products for densities between 600 and 1100 kg/m³ at 15°C. Materials, permitted errors, corrections for surface tension, temperature and meniscus, guide to purchasers, method of use, density of water from 0°C to 40°C, vessels for hydrometer observation.

This British Standard specifies requirements for five basic series of glass hydrometers of constant mass which are graduated to indicate density (kg/m³ or g/ml) when used at 20 °C.

Each series comprises hydrometers which between them cover the interval 600 kg/m³ to 2 000 kg/m³ (0.6 g/ml to 2.0 g/ml). The hydrometers are graduated appropriately for use in liquids of low, medium or high surface tension.

The standard also specifies three sub-series of hydrometers which are graduated either to indicate density when used at 15 °C or to indicate density when used at 20 °C. They have smaller tolerances on scale error, are limited to the range 600 kg/m³ to 1 100 kg/m³ (0.6 g/ml to 1.1 g/ml) and are for use in liquids of low surface tension.

A single hydrometer principally suitable for the measurement of the density of petroleum fuels is included.

This British Standard does not cover hydrometers with a built-in thermometer (thermohydrometers).