Salinity - what is that?
Salinity provides an indication of the total amount of salts dissolved in seawater. It is usually expressed in parts per thousand (‰). On average, a liter of natural seawater contains 35 grams of salt, so its salinity is 35 ‰.

Another way to determine salinity is to determine density. The unit of density is g/cm3 or somewhat simpler g/l. A density measurement involves measuring how much a single liter of a liquid weighs. A liter of pure water weighs about 1000 g, so its density is 1,000 g/l. However, seawater contains various salts in addition to water particles, which is why it is somewhat heavier than pure water. Accordingly, its density is approximately 1.025 g/l.

Since liquids expand or contract depending on the temperature and thus change their volume, the temperature has a decisive influence on the salinity or density. When determining the salinity in your tank, you should always take this relationship into account.

In order to ensure that the physiological processes of the animals in your tank can run optimally, the salinity value must always lie within a certain range. At a water temperature of 25°C, the salinity in your tank should be about 33 to 36 ‰. This corresponds to a density of 1.022 to 1.024 g/l.

Measuring salinity with the hydrometer

The measuring method itself is based on the Archimedean principle, i.e. the higher the density of the water, the more weight the spindle loses and the further it protrudes.

For the measurement you place the spindle best in a flow-calmed corner of your sump or in a separate container (the measuring cylinder is ideal for this). It is important that the hydrometer can float freely and does not touch the bottom of the container.

After the spindle has settled in the water and no longer moves up and down, you can read the density on it. There is a scale on the neck of the spindle for this purpose, and you must read the scale value at the height of the water level.

Hydrometers are usually very accurate and especially larger spindles have a high resolution. However, you should keep in mind that the density depends on the temperature, which is why you have to correct the value determined by you with the help of a correction table in order to obtain the exact density.

When using a hydrometer you should make sure that it is always free of impurities and salt crusts. In addition, some sensitivity is required when handling a spindle, as it is made entirely of glass and can easily break if force is applied.