The National Institute of Standards and Technology (NIST) provides calibration services for hydrometers used to measure liquid density. Hydrometers with scales for density, specific gravity, proof spirit for alcohol solutions, API degrees for petroleum measurements, degrees Baume heavy and degrees Baume light, and others are accepted for calibration. NIST calibrations are done by weighing the hydrometer while it is immersed to specified scale markings in a liquid of known density (tridecane). Normally a calibration is done at three points, at approximately 10 %, 50 %, and 90 % of the scale. The calibration report includes a simple equation for each calibration point that gives the user a correction to the hydrometer reading that accounts for the surface tension of the liquid in which the hydrometer is used. The calibration results have an expanded uncertainty (with coverage factor of 2) of 100 parts in 106 or less of the liquid density. The apparatus was validated by comparisons using liquids of known density: water, toluene, tridecane, and trichloroethylene. Results from these comparisons agreed within 40 parts in 106 or less (see Table 3). The new calibration method is consistent with earlier, manual calibrations performed by NIST using reference hydrometers (see Figure 5).
and Bean, V.
NIST Calibration Services for Hydrometers., Special Publication (NIST SP), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=832342
(Accessed December 2, 2023)