Helpful hints for hydrometer use
Accuracy of hydrometer readings depend upon:
1. CLEANLINESS OF THE HYDROMETER, HYDROMETER JAR AND THE LIQUID IN WHICH THE READINGS ARE TAKEN
For uniform and reproducible readings, the surface of the hydrometer and especially of the stem must be clean so that the liquid will rise uniformly and merge into an almost imperceptible film on the stem. The readiness with which this condition is fulfilled depends somewhat on the character of the liquid. Such liquids as mineral oils and strong alcoholic mixtures readily adhere to the stem. Weak aqueous solutions of sugar, salts, acids, and alcohol require scrupulous cleaning of the hydrometer stem. Before a test is made the hydrometer should be thoroughly washed, rinsed and dried by wiping with a clean, lint free cloth. The hydrometer jar should be thoroughly washed and rinsed before the clean test liquid is added.
The hydrometer and liquid should be at the temperature of the surrounding atmosphere to prevent changes in density during the testing. To ensure uniformity of density and temperature, the liquid should be completely stirred shortly before the observation is made. Stirring is accomplished with a perforated disk or spiral on the end of a rod long enough to reach the bottom of the container. Stirring from top to bottom disperses liquid layers of different densities and aids in attaining temperature uniformity. Readings should not be made until both liquid and hydrometer are free of air bubbles and are at rest. Temperature of the liquid should be determined by a precision thermometer and recorded along with the hydrometer reading. Thermometer ASTM 12F (-5/215 °F in 0.5 degree divisions) or ASTM 12C (-20/102 °C in 0.2° divisions) are thermometers designed specifically for this application and are recommended by most ASTM test methods).
3. PROPER IMMERSION
Use a hydrometer jar with an inside diameter of approximately 1″ (25mm) greater than the outside diameter of the hydrometer. This will minimize any undue effect surface tension will have on the readings of the hydrometer. In general, a hydrometer jar of 50mm (inside diameter) by 375mm (inside height) is suitable for the majority of hydrometers.
Immerse the hydrometer slowly to a point slightly beyond that at which it floats naturally (not more than 2 scale divisions), then allow it to float freely. Take care that the hydrometer is not in contact with the sides of the hydrometer jar when reading.
Related documents which are informative on the use of hydrometers:
ASTM E-100 Standard Specification for ASTM Hydrometers
ASTM E-126 Standard Test Method for Inspection and Verification of Hydrometers
ASTM D-1298 Standard Method for Density, Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method
ASTM D-891 Standard Test Method for Specific Gravity, Apparent, of Liquid Industrial Chemicals
Our thanks to ASTM (ASTM E-100 and E-126), NIST (NBS Circular 19), Chase Instrument Company, Kessler Instruments and others for information gleaned from their literature and incorporated into the above suggestions.