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. 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 for with a reference temperature of 60ºF. These hydrometers will be 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 measuring. The hydrometers used for the Bayouside Classroom program range from 0.940 to 1.010 and are calibrated at 60ºF/60ºF. The measurement made with the hydrometer can be used to calculate the salinity of a water sample.
Using a hydrometer is quite simple. The user must be careful, though, to not break the the hydrometer or the cylinder as they are made from glass. Follow these easy-to-follow directions:
- Fill the glass cylinder with sample water.
- Put the hydrometer with the bulb end down. It will bob up and down in the sample. Note that the sample may overflow from the cylinder.
- Assure that the hydrometer is not in contact with the sides of the cylinder and take the reading.
READING THE HYDROMETER
Extreme 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. Just as reading the meniscus in a graduated cylinder, the user must take the reading where the plane of water is and not where the water clings up the neck of the hydrometer. See the image to the right. The correct reading of this hydrometer is about 0.982.
Once the reading has been made, clean the cylinder and hydrometer. Carefully place them into their protective packaging so they will not break.
Click here for a convenient specific gravity to salinity calculator (data used to create this calculator is from LaMotte Company, Chestertown, MD, USA).