Hydrometers for Surface Tension Analysis

Hydrometers are simple instruments based on Archimedes’ principle. Consisting of a glass tube equipped with a graduated scale on one end and a weighted bulb which floats freely when submerged in liquid, hydrometers measure its density according to when it reaches certain scale markings on their graduated scales. They’re useful tools in many fields including petroleum hydrometry, alcohol/battery hydrometry, saccharometers lactometers and soil hydrometry.

Most National Metrology Institutes (NMIs) use tridecane or distilled water as the standard reference liquid for hydrometer calibration, due to their low surface tension and wetting ability. As with other hazardous and flammable liquids, their handling requires extra precaution.

Temperature variations should be avoided as much as possible to ensure optimal readings from hydrometers, while their accuracy can fluctuate depending on surface tension differences in liquid being measured; correction tables are available to account for these effects.

Hydrometer calibration requires two steps; measuring the density of the reference liquid and weighing the hydrometer. Cuckow’s method offers an easy, straightforward solution with combined uncertainty better than 0.3 division (k = 1) per scale mark used.

Care should be taken not to disturb the fiducial mark on the base of a hydrometer when measuring. A fiducial mark consists of a thin horizontal line printed on its scale which coincides with another thin horizontal line etched onto its stem; should they fail to align perfectly, the hydrometer must be returned back to its supplier for repair or replacement.