Providing the correct and stability salinity is very important, and to do so you need to have the ability to measured its value accurately. Until recently I used the swing arm type hydrometer (and probably will for a quick check), however, they need to be calibrated. Out of the box the swing arm hydrometers give a reading that can very significantly between units. Finally I spent the $120 for one and here is some details on it and how to use it.

The model is the 10ATC, supplied by AquaDirect in Australia. The ATC stands for automatic temperature correction, which means that it doesn't matter what the temperature of the refractomter is (10-30^oC) it will provide an accurate reading for the salinity.

The Aquadirect salinity refractometer, 10ATC.

With it comes the refractometer, a dropper to put water onto the prism, a calibration screw driver plus some instructions.

Parts that come with the refractometer.

Use of the refractometer is as simple as placing a drop of water onto the prism with the dropper, allowing a second or two (literally since the amount of water is very small and will rapidly reach the temperature of the refractometer) for the water to reach the temperature of the refractometer then look through the eye piece and read off the salinity. The water needs to cover the prism completely and without any water bubbles between the prism and the cover.

Correct water coverage between the cover and prism on the left, incorrect on the right.

The view through the eye piece when measuring saltwater should look something like this. The upper section is coloured blue, the lower colourless and the intersection of the boundary with the scale indicates the systems salinity. In the image below it shows a salinity of 34.5 ppt or 1.026 specific gravity.

Before it can be used to measure the salinity of the saltwater, it needs to be calibrated using pure water. Calibration is performed by viewing through the eye piece and adjust the calibration screw unit a reading of 0 ppt is visible.

Calibration screw to adjust the zero salinity reading to zero.

The view through the eye piece should look like this, with the upper section coloured blue, the lower clear and the boundary between the two crossing right over the top of the zero reading on the salinity scale.