How to us a Hydrometer
to measure salinity
What is a Hydrometer?
A hydrometer is a tool scientists use to measure the specific gravity of liquids.
Specific gravity is the ratio of the mass of a liquid to the mass of an equal
volume of pure water. The measurement made with the hydrometer can be used
to calculate the salinity of a water sample.
Because the density of a liquid changes with temperature, hydrometers are
calibrated for different reference and sample temperatures. Classically, specific
gravity was measured using the density of water at 4ºC (the temperature of
maximum density for pure water). Today, most hydrometers used in water
quality testing are calibrated with a reference temperature of 60ºF. These
hydrometers are inscribed with "60ºF/60ºF" on their necks denoting that the
reference density was measured at 60ºF and the sample should be at 60ºF.
Because specific gravity measures the mass of a liquid over the mass of
distilled water, specific gravity is unitless. Hydrometers come with different
scales depending on the characteristics of the liquid the user is measurin..
How to use the Hydrometer
1) Fill the glass cylinder with sample water.
2) Put the hydrometer with the wide end of the bulb down. It will bob up and
down in the sample. Note that the sample may overflow from the cylinder.
3) Make sure the hydrometer is not in contact with the sides of the cylinder and
take the reading.
Reading the Hydrometer
Care should be taken when reading the hydrometer; it is very easy to
misinterpret the scale. Once the hydrometer has stopped bouncing up
and down and the hydrometer is not touching the walls of the cylinder, a
reading can be made. Note that a meniscus forms on the neck of the
hydrometer. See the example to the right. The correct reading
of this hydrometer is about 0.982
Use the accompanying “Specific Gravity Conversation” table
From the LaMOTTE COM PANY, www.lamotte.com
to determine the salinity of the sample.
Ocean Watch, 2013