Density
A number of techniques as well as standards exist for the measurement of density of materials. Such techniques include the use of a hydrometer (a buoyancy method for liquids), Hydrostatic balance (a buoyancy method for liquids and solids), immersed body method (a buoyancy method for liquids), pycnometer (liquids and solids), air comparison pycnometer (solids), oscillating densitometer (liquids), as well as pour and tap (solids). However, each individual method or technique measures different types of density (e.g. bulk density, skeletal density, etc.), and therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question.
Beer measurement
The most common method measuring the density of a liquid is with a hydrometer; hydrometers can be calibrated with a number of scales. A common scale is that of specific gravity (SG); that is to say the density of a liquid relative to the density of pure water (at a standard temperature). Specific gravity can also be measured by a pycnometer or oscillating U-tube electronic meter. Water has a SG of 1.000, absolute alcohol has a SG of 0.789. Other density scales are discussed below.