SCIENTIFIC SECTOR CATALOGUE - Flipbook - Page 22
Hydrometers
Hydrometers
The traditional hydrometry scale of speci昀椀c
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 section use this
scale.
For the speci昀椀cation 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 speci昀椀c 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 speci昀椀cally 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.
All hydrometers can be supplied with UKAS
certi昀椀cation*, certi昀椀cate traceable
to recognised national standard or
statement of conformity, if required.
* UKAS calibration carried out via a third party laboratory.
Measurement for life.
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