The refractometer is a very useful tool in the aquarists arsenal.
It is a very accurate device which is used to measure the amount of salt in the water i.e. the specific gravity.
A tool which is quite often used is the hydrometer however one of the downfalls of the hydrometers it that it must be regularly cleaned. Failure to do so means that false readings may be obtained. What quite often happens is that a reading is taken and then the water is simply poured back into the aquarium and the hydrometer (swing needle type in this example) is then placed back on the shelf ready for its next use.
What happens in this example is that the water dries however a small amount of salt is left behind. This salt is not visible to the naked eye however it is enough to make the needle heavy and therefore give you a false reading. Over time the readings get worse as the needle gets heavier and heavier and unknown to you the specific gravity in the aquarium is totally wrong.
With the refractometer readings are much more accurate. There still is maintenance to be performed which is basically where the refractometer is calibrated however this does not need to be done to the same type of frequency as of the hydrometer.
So how easy are they to use?
Very easy actually. Basically a small sample of water is taken, normally one to two drops of water. This is then placed onto the prism glass where it can be viewed through the eyepiece.
What happens when the water is placed onto the prism glass is that light is passed through the water. This light then bends and is focused on a scale which can be read through the eyepiece.
The scale is normally very easy to read but this does depend upon the manufacturer chosen.
In my opinion a refractometer is a fantastic purchase for the serious aquarist. They are very easy to use and allow for stability to be maintained which is one thing we aim for.