A hydrometer is a laboratory instrument used to measure the specific gravity of liquids. The gravity is defined as the relative density which is arrived at a density of that liquid divided by the density of water.
There is a very interesting history behind the invention of hydrometers. It is believed that it was developed by the Greek scholar Hypatia of Alexandria after a bishop named Synesius of Cyrene requested him to do in a letter. So, its origin can be traced to as early as a 4th or 5th century! The history books show its usage in the 11th, 12th and 18th centuries as well.
Made from a glass material, a hydrometer comprises a long cylindrical stem. There is a bulb weighted at the bottom either with lead or mercury to allow it to float upright and lend stability in the water. There is also a scale printed on its side to measure specific gravity. In order to ensure better accuracy, the reading has to be adjusted according to the temperature of the liquid at the time of measurement.
This scale also varies as per the industry application. For instance, the petroleum industry uses API gravity, chemistry and pharmacology industry use Baume scale, brewing and wine-making industry use Plato and Brix for the sugar-based liquid industry.
A hydrometer is specifically used for measuring low-density liquids such as gasoline, alcohol and kerosene as well as high-density ones such as acids, brine and milk. The only difference is that it will go deeper in high-density whereas it will not be less deep in low-density. During the use, it is placed in the container holding that liquid. The value at the surface of the liquid actually provides the specific gravity of the liquid.
There are different types of specialized hydrometers named after their uses:
- Lactometer for milk
- Alcoholometer for wines and alcohols
- Saccharometer for sugar
- A barometer for tanning liquids in leather-making.
- Urinometer for urine analysis
- Acidometer for acids
- Batter hydrometer for lead-acid battery
- Salinometer for liquids with salt content
- Thermohydrometer for fuel oils like petroleum
Hydrometers are even used for soil analysis – a process which grades fine-grained soils, clays and silts.
The uses of hydrometers are as varied as it can get as far as there is any type of liquid concerned. Choose a hydrometer based on the industry lab you are working in.