Mechanical Measurements

Hydrometers

John D. Wright
Tel: 301/975-5937
E-mail: john.wright@nist.gov

Do not ship instruments or standards to the mailing address listed below. Contact the technical staff for the shipping address. 

Mailing Address:
National Institute of Standards and Technology
100 Bureau Drive, Stop 8361
Gaithersburg, MD 20899-8361

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Reference Standard Hydrometers (16010C-16020S)

NIST provides measurement services for reference hydrometers to measure liquid density. These reference standard hydrometers are generally used as laboratory standards to calibrate other hydrometers.

Reference hydrometers accepted for calibration include specific gravity, proof spirit for alcohol solutions, API degrees for petroleum measurements, degrees Baume heavy and degrees Baume light, and other arbitrary scales, all subject to discussion with the technical contacts. Specific gravity hydrometers cover the specific gravity range of 0.65 to 2. The hydrometer scale should be divided into 0.001, 0.0005, 0.0002, or 0.0001 units of specific gravity. Proof spirit hydrometers cover the range of 0 to 200 proof. The alcohol hydrometer scale should be divided in whole, half, fifth or tenth percents. Reference hydrometers should be made of smooth, transparent glass, free of bubbles or other imperfections. The hydrometer should bear an inscription that indicates the purpose of the instrument. This inscription should denote the reference temperature at which it is to be used. The maker's name or trademark and an identification number should be inscribed on the hydrometer scale. Hydrometers accepted for calibration must conform to specified requirements. The technical contacts listed above can give guidance on these requirements.

Visual inspection should be made by the customer before shipping to NIST for calibration. The hydrometer should be packed in a safe reusable shipping container.

NIST provides a three-point calibration on these hydrometers (16010C). Additional points can be provided at an additional charge if requested (16020S). The calibration at NIST consists of comparing NIST master hydrometers directly with the customer's hydrometers in a liquid of low surface tension.

The expanded uncertainty for NIST master hydrometers depends on stem scale. Components of the expanded uncertainty include the Type A Standard Uncertainty, composed of the imprecisions of such process components as scale readings, thermal gradient of the calibration fluid, etc., and Type B Standard Uncertainty, based on direct comparisons to solid density standards (plummets), density of water, and precision mass standards.

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References-Hydrometer Measurements

ASTM Specification E100-03, Standard Specification for ASTM Hydrometers, Book of Standards 14.03 (July 2004).

An Improved High-Precision Calibration Procedure for Reference Standard Hydrometers, H. A. Bowman and W. H. Gallagher, J. Res. Natl. Bur. Stand. (U.S.) 73C (June 1969).

Testing of Hydrometers , J. C. Hughes, Natl. Bur. Stand. (U.S.), NBS Circular 555 (Oct. 1954).

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