Refractometers: What they Do and How to Use Them In Your Aquarium
Veterinary & Aquatic Services Department, Drs. Foster & Smith

What does a refractometer do?

RefractometerSerious hobbyists know that the best way to measure the salinity and specific gravity of aquarium water is to use a refractometer. These precision optical instruments are incredibly accurate even at very low salt levels, and are equally easy to use. Recently, these instruments have become more affordable, so now every aquarium owner can have one to help monitor water conditions.

How do I use it?

Place 1 to 2 drops of aquarium water onto the prism glass and close the plate cover. Once a fluid sample is placed on the measuring surface, the light passing through it slows and bends. The refractometer focuses this bent light on a tiny internal scale. The scale is magnified by the eyepiece lenses so it is easily visible.

Look through the eyepiece and view the scale. The top portion of the scale appears blue and the bottom part appears white. Take your reading at the intersection of these two colors. One side of the scale measures salinity and the other side measures specific gravity.

To effectively monitor your marine aquarium water conditions, consider investing in a refractometer. The accuracy and affordability make it worthwhile.

Salinity & Specific Gravity are easily viewed when looking through the Refractometer
Illustration of how a refractometer works

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