If nothing more, I hope this article alerts aquarists to some of the issues behind the use of hydrometers and specific gravity for measuring salinity. For those interested in additional discussion of hydrometers and how they relate to specific gravity and salinity, there is a nice discussion in Stephen Spotte's "Captive Seawater Fishes", along with a chart for correcting 60 °F/60 °F specific gravity measurements made at other temperatures.
Swing arm hydrometers normally show a salinity scale and an S.G
My husband got my refractometer at , where “low-cost” salinity hydrometers range from $105 to $258. One model measures salt content in parts per thousand; others measure the percentage of salt by weight of the solution. You can translate percentage of salt to either weight or volume by using the tables on pages 38 and 39 of my book.
Measuring Salinity with a Hydrometer
Note also that you can accurately measure brine strength only before you’ve added sugar (as is generally done for meat curing, though not for fermenting vegetables). Once you’ve added sugar, the hydrometer will measure the density of the solution, not the salinity of the brine.
Determination of salinity by density using a hydrometer.
Salinity is one of the most important chemical parameters to monitor in reef aquaria. While there are a variety of ways to measure salinity, including using refractometers and , hydrometers can be an accurate and inexpensive method. Unfortunately, not all hydrometers are suitably accurate, and gave a recipe for a standard solution with which to calibrate, or at least check the operation of, typical hydrometers. Monitoring the salinity level is an important issue itself, wholly apart from deciding what salinity level to target in an aquarium. Fortunately, a number of different methods for monitoring salinity are available to aquarists, including specific gravity (via hydrometers), refractive index (via refractometers), and conductivity (via electronic meters). In order to get the most out of any one of these methods, however, aquarists must have confidence that it provides reliable information. Each method can provide perfectly adequate information for aquarists, assuming that the device used is properly manufactured, calibrated, and used. Alternatively, each device can provide misleading information if any of these factors is not optimal.