What is Hydrometer? How to use the hydrometer?

Hydrometer Definition

hydrometer can be defined as an instrument used for measuring the relative density of fluids works on the principle of buoyancy.


It consists of a glass barrel and bulb syringe for sucking up a sample of the electrolyte to float an enclosed glass hydrometer calibrated to read in terms of specific gravity. The depth to which the float sinks in the liquid indicates the relative weight of the liquid compared to water and gives us a measure of the specific gravity of the liquid. The hydrometer floats low in the fluid if the specific gravity is low and it floats high in the fluid if the specific gravity is high. The float is made of glass and is equipped with a paper scale built inside the hydrometer with marks on it which must be read on a level even with the liquid surface and this reading indicates the specific gravity of the liquid.

The correct method for reading the hydrometer is shown in Figure The eye should be on a level with the surface of the liquid in the hydrometer barrel. Disregard the curvature of the fluid where the surface rises against the float, barrel and the stem due to surface tension. Keep the float vertical.


Hydrometer readings should not be taken immediately after water has been added. The water should be fully mixed with the underlying electrolyte by charging before hydrometer values are reliable. During reading a hydrometer, the barrel must be kept vertically and just the required amount of acid be drawn up into the barrel with bulb fully expanded to lift the float freely so that it touches neither the side nor the top or bottom stoppers of the barrel.

The hydrometer barrel and float must be kept clean with soap and water so that the float will not stick to the sides. The float must be inspected in small periods of time for cracks which would cause acid to enter the airtight float and make any wrong reading. If the paper scale inside the float is wet, it is an indication that the float leaks: and should not be used.

Hydrometer with different measurement scales.
Hydrometer with different measurement scales.

Ranges Or Uses

The hydrometer sinks deeper in low-density fluids such as gasolinekerosene, and alcohol, and less deep in high-density fluids such as brine, milk, and acids. It is usual for hydrometers to be used with dense fluids to have the mark 1.000 (for water) near to the top of the stem, and those for use with lighter fluids to have 1.000 near the bottom. In so many industries, a set of hydrometers are used (0.95-0.9, 1.0-0.95 etc.) to measure a different range of specific gravities that may be found.

Watch the video below to know how to use the hydrometer

Types Of hydrometer

  1. Lactometer.
  2. Saccharometer.
  3. Thermohydrometer.
  4. Battery hydrometer.
  5. AntiFreeze Tester.
  6. Acidometer.
  7. Barometer.
  8. Salinometer.
  9. Urinometer.


The electrolyte in a fully charged battery is usually 1.23 times as heavy as an equal volume of pure water when both liquids are at the same temperature. The battery electrolyte would, therefore, be described as having a specific gravity of 1.230 meaning that its weight is 1.230 times the weight of pure water. When the battery discharges, the sulphuric acid in the electrolyte combines chemically with the plates and the remaining electrolyte becomes lighter in weight. By finding the relative weight of the electrolyte we can know how much acid has been combined with the plates and, also estimate how much electricity is still left in the battery. But actual weighing of the electrolyte would be inconvenient, so an instrument called the Hydrometer is used.

Watch another video below to know how to use the hydrometer.

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