By Sujan Dahal

Hydrometer is a device for measuring some characteristics of a liquid, such as its density (weight per unit volume) or specific gravity (weight per unit volume compared with water).In simple words, hydrometer is a scientific instrument used to measure relative density of liquids.

Hypatia of Alexandria, is given credit for inventing the hydrometer (or hydroscope) in the late 4th century or early 5th century. The two Baumé hydrometers (invented by Antoine Baumé), one for specific-gravity determinations in liquids denser than water and the other for liquids less dense than water, are calibrated with the special Baumé scale (also constructed by him) in 1768.

Hydrometer is based on Archimedes’ principle ( that a solid body displaces its own weight within a liquid in which it floats). It consists of two parts : stem and bulb.The slender tube above the larger tube is the stem.  It is hollow glass with a rounded end.  The straight, cylindrical stem is calibrated by which the density or specific gravity of the test liquid is measured.  The stem ends as the glass cylinder enlarges into the bulb. Again, the bulb consists of 2 parts:  the ballast and the glass bead.  As the hollow bulb enlarges from the stem, its diameter increases, stabilizes, and rounds at its end in the shape of a hemisphere.  This rounded end contains the ballast (small steel spheres) inside, at the end of the bulb.  The small spheres are held in place by epoxy.  The ballast weights the hydrometer in the liquid.  Attached to the outside of the hydrometer, at the end of the bulb, is a small, glass bead.  This glass bead serves to buffer the hydrometer, preventing the bulb from coming into direct contact with the bottom of the liquid’s container.

To use hydrometer, first retrieve sample(of which density are you measuring) & insert hydrometer.Then, obtain the original gravity reading. Make sure you are reading the bottom of the meniscus to get most accurate data. Now calculate with temperature using Standard Hydrometer Temperature Correction Chart and repeat the process.Finally, you get your final reading.Different scales are used on hydrometers depending on the types of liquids being measured. Some of its major scale are Brix or Balling Scale, Baume Scale, Alcohol Scale, and many more.






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