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Why testing salinity?
In sea water aquaria it is important to hold the salinity constant, because most of the animals are adapted to a very narrow range of salinity. Therfore you must proof the content of salt every month and correct it.
There are some methods to measure the concentration of all salts. The most common in sea water aquaristics are: electrical conductivity, specific density (araeometer), and the optical refraction with a refractometer.
The electrical conductivity can be used for several purposes: measuring the salinity, for proving reverse osmosis units and ion-exchange resin units, and for testing "Kalkwasser". Because of the very complicated technique (high precision in a wide scale from 1 to 55,000 µS/cm) the aquaristic conductivity meters are not the best: wrong measuring with accuracy over 2,000 µS/cm in sea water are possible. The calibration is not done regularly or is done with old calibration fluids.
Araeometers are very fragile and you have to handle with care. Short araeometers (shorter than 20 cm) are not precise and not suitable for sea water aquaristics. Long units are precise but you must use them always at the same temperature or you have to calculate with tables to the right density. You can make very easily faults: taking not the right level, too small test zylinder or araeometers are covered with algae.
Refractometer is an optical unit to measure the frefraction index. With temperature compensated refractometer you can measur fast and safe the salinity. For testing you only need some drops - not more.
Principle of a refractometer
If light is running from an optical thinner to an optical thicker medium, the light beam is refracted. The grade of refraction depends mainly on the kind of medium and its concentration, futher on temperature and wavelength. In the refractometer this circumstance is used. It is important that for every substance an own scale is mounted. With a scale e.g. for sugar it is not possible to measure sea salt. It is important, too, that an non-temperature compensated model is only used at that temperature the scale is made for. Otherwise you must calculated with the help of tables to the right concentration.
Working with and maintenance
To measure with a refractometer is very easy. Put only 2-3 drops onto the main prism and shut the light plate. If you see through the refractormeter into a light source (day light, lamp) you can see the measuring value at once. In the AquaCare refractometer you have two scales: one for the specific gravity in kg/kg and one for the salt concentration / salinity in ppt or ‰. After reading the value you can clean the refractometer with a soft cloth. You need any batteries for this unit.
To measure a long time with high precision you must take care of the refractometer. It is not permited to measure organic substances like alkohol, water with sediments or chemical aggressive solutions like acids or bases. The coating of the prism will be destroyed. This optical measuring device is not made for submersed duty. Please store the refractometer in its transport box.
Measuring range: 0 to 100 ppt (Promill)
Resolution: +/- 1 ppt (Promill)
Measuring range: 1.000 bis 1.070 kg/kg
Resolution: +/- 0.001 kg/kg
Calibration temperature: 20 +/- 1°C
Measuring temperature: 10 to 30°C
Storing temperature: 4 to 40°C