Red Sea Hydrometer with Liquid Crystal Thermometer

Versatile swing-arm hydrometer accurately measures both specific gravity and water temperature. Clever design allows periodic or continuous reading of the most important water quality parameter in saltwater aquariums. Measure from 1.010 to 1.030 specific gravity and 64-97°F.

Please click on "More Information" for directions, specific gravity conversion charts, and salinity information.

Periodic Measurement

1. Ensure that the Black Plug is inserted into the flow outlet at the bottom of the Hydrometer
2. Submerge the Hydrometer into the water until the water inlets are below the surface of the water. To prevent incorrect readings due to air bubbles forming on the pointer, move the Hydrometer within the water to be tested for a few moments or until no bubbles are visible.
   1. The Specific Gravity (S.G.) can read accurately with the Hydrometer remaining in the water providing it is held such that the surface of the water is exactly at the level of the Water Inlets.
   2. Alternatively carefully raise the Hydrometer vertically out of the water and place it on a flat surface. Do not tilt the Hydrometer, as water will spill from the Water Inlets.
   3. After use, flush the Hydrometer with warm, fresh tap water to avoid salt deposits, which can form on the swing-arm and distort future readings.

Continuous Real-time Measurement

3. Ensure that the Black Plug is removed from the flow outlet at the bottom of the Hydrometer and stored in the holding area under the scale.
4. Submerge the Hydrometer approximately 15 cm/6" below the surface of the water. To prevent incorrect readings due to air bubbles forming on the pointer, move the Hydrometer within the water to be tested for a few moments or until no bubbles are visible.
5. Using the suction cup provided, attach the Hydrometer to an inner wall of the aquarium or sump that is easily visible from the outside.
6. Wait a few moments for the reading to stabilize.
7. To avoid salt deposits forming on the swing-arm, clean the Hydrometer once a month by soaking it in a cleaning solution for at least 30 minutes.
8. Rinse thoroughly before returning to the aquarium.

Hydrometers, Salinity, Specific Gravity, & Temperature

The most basic water quality parameter in a saltwater aquarium is the Salinity, which is a theoretical measure of the amount of salt contained within a volume of water, normally expressed as grams per liter (g/l) or parts per thousand (ppt). (Note: Salinity should not be confused with manufacturer's instructions for the preparation of artificial seawater expressed in g/l or lb/gal.)

Unfortunately, it is not possible to measure Salinity directly. However, it has a temperature-dependent relationship with Specific Gravity, which is easily measured with a hydrometer. For a particular Salinity, specific gravity increases with decreasing water temperature and ,therefore, a S.G. measurement is only meaningful if the temperature is also known.

Conversion of S.G. to Salinity at a constant temperature of 25°C/77°F

Effect of temperature on S.G. for a constant Salinity of 30.6 ppt

Salinity in the Saltwater Aquarium

It is essential to maintain correct salinity in a saltwater aquarium because of the fine balance between the concentrations of salts in a fish's body tissue, compared with that of the surrounding water. The ability fish have to control small fluctuations of these concentrations in nature is called osmoregulation. However, if the salinity of the water in an aquarium is outside the range of naturally occurring salinities, then fish and invertebrates will experience osmotic stress, which can lead to their death.

The most common occurrence of salinity variation is the gradual increase in Salinity due to evaporation. Constant and accurate measurements of S.G. are, therefore, essential to ensure correct conditions. A salinity of 30.6 ppt, i.e. a specific gravity of 1.021 at 25°C/77°F is recommended for most saltwater aquariums.