CALIBRATION: The set of operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument ...., and the corresponding known values of a measurand.
The hydrometer is inspected visually for defects in numbering, graduating, condition, or any obvious physical defects or damage, for any loose ballast material, and the presence and correct placement of a 'scale slippage indicator'.
Calibration: Per Astm E-126
The instrument is then tested in carefully prepared solutions of appropriate media, against NIST CALIBRATED reference hydrometers at the required test points (as specified by ASTM or the customer). At least three readings of the clients instrument (and at least six readings of the NIST reference standard) are taken at each test point. Readings are resolved under magnification to approximately one-tenth of the smallest scale division and are carefully noted. We report the mean of a minimum of three closely correlated readings.