Relative Density (Specific Gravity) is the ratio of the mass of a sample to that of an equal volume of a standard substance (typically water), where 20/20 refers to the temperature of the sample and standard substance respectively.
Relative Density is commonly used within the food, beverage, chemical & pharmaceutical industries.
Pharmacopoeia (the quality standard for all companies concerned with research, development, and manufacturing or testing of medicinal substances, including cosmetics and veterinary products) contains specifications for measurement techniques as well a product specific methods for analysis, known as monographs. There are over 120 monographs in Pharmacopeia that cite density measurement.
Digital density meters including the DSG Series from Bellingham + Stanley, a Xylem brand, utilise the effect of density on the natural resonant frequency of an oscillating U-tube filled with the liquid under test. A simple equation relates the resonant frequency to the mass, volume and density of the test liquid.
Digital density meters are highly precise and often include integral thermal control and dehumidifiers to ensure reading reliability.