Tropic Marin Hydrometer Set

Salinity - what is it?

Salinity provides an indication of the total amount of salts dissolved in the aquarium water. It is usually expressed in parts per thousand (‰). On average, one liter of natural sea water contains 35 grams of salt, which is why its salinity is 35 ‰.

Another way to determine salinity is to determine the density. The unit of density is g/cm ³ or, more simply, g/l. A density measurement measures how much a single liter of a liquid weighs. One liter of pure water weighs approximately 1000 g, so its density is 1.000 g/l. In addition to the water particles, sea water also contains various salts, which is why it is somewhat heavier than pure water. So 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. You should always take this connection into account when determining the salt content in your pool.

In order for the physiological processes of the animals in your aquarium to run optimally, the salinity value must always be within a certain range. At a water temperature of 25°C, the salinity in your tank should be around 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 Archimedes principle, i.e. the higher the density of the water, the more weight the spindle loses and the further it protrudes.

To take the measurement, it is best to place the spindle in a corner of your technical tank with a calm flow or in a separate container (the measuring cylinder is ideal for this). It is important that the hydrometer can float freely and does not sit on the bottom of the container.

Once the spindle has settled in the water and is no longer moving 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 have to read the scale value at the level of the water.

Hydrometers are usually very precise and larger spindles in particular have a high resolution. However, you should remember that the density depends on the temperature, which is why you have to correct the value you have determined using a correction table when measuring above 25°C 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, a certain degree of sensitivity is required when handling a spindle, as it is made entirely of glass and can easily break if force is applied.