Specific Gravity Hydrometers
The specific gravity Hydrometer 60/60°F scale compares the relative density of a given liquid with that of water, at 60°F (15.6°C). The instruments listed are similar to the hydrometer included in ISO 650 and in the (now superseded) 1960 issue of BS 718.
General Purpose and Specification Hydrometers
The traditional hydrometry scale of Specific Gravity (density, relative to that of water at 60°F) used in the 1960 edition of the British Standard 718 is still much in demand and the General Purpose hydrometers at the beginning of this catalogue use this scale. For the Specification Density Hydrometers to BS 718 : 1991, the internationally accepted basis of scale is grams per millilitre (g/ml) with a reference temperature of 20°C (or, exceptionally 15°C). For use in tropical countries, ISO 387 proposes that hydrometers be adjusted at a higher reference temperature, 27°C (as opposed to the previously used high adjustment of 84°F). Instruments adjusted at this higher temperature can be supplied to special order.
In addition to being adjusted for use at a specific reference temperature, hydrometers are adjusted with regard to the Surface Tension of the materials whose density is to be determined. Surface tension may be expressed specifically for example 55 mN/m (millinewtons per metre) or described as a band. Low surface tension is less than 35 mN/m and medium surface tension is between 35 and 55 mN/m.