How to us a Hydrometer t o measure salinity

1. 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 Fromthe LaMOTTECOM PANY, www.lamotte.com to determine the salinity of the sample. Ocean Watch, 2013