How to Calibrate a Refractometer for Salinity

Refractometers measure the salinity of sea water, which is an extremely important parameter in marine aquariums. Salinity controls the salt balance between marine creatures and their environment, as well as the levels of many other ions that hobbyists typically do not measure independently. Hobbyists must test salinity to make sure it remains within the correct range for their fish and invertebrate animals, such as corals and sea anemones. Aquarists can calibrate the refractometer by turning the calibration screw to raise or lower the scale, in relation to the path of light entering the instrument.

Things You'll Need

  • Refractometer
  • Distilled water
  • Pipette
  • Clean tissue
  • Calibrating solution
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Instructions

    • 1

      Ensure the refractometer is designed to measure seawater. Different types of refractometers are available to measure various types of liquids.

    • 2

      Lift the protective shield on the refractometer.

    • 3

      Use a pipette to draw up a drop of distilled water, and place it on the glass prism of the refractometer. Wipe the prism with a clean, soft tissue and leave the prism to air dry for a few seconds.

    • 4

      Draw up one or two drops of distilled water with a pipette, and drip these on the glass prism of the refractometer. Close the protective shield, and place the refractometer to your eye.

    • 5

      Turn to face the light, and look through the viewfinder. Light will cover part of the scale, while the rest will be dark. Read the value on the scale at the line between light and dark. Distilled water has a refractive index of "0". The refractometer will reflect this figure, if it is correctly calibrated.

    • 6

      Turn the calibration screw on the underside of the refractometer to make adjustments, as necessary. Turn this screw very slowly, until the correct refractive index displays on the scale. The calibration screw will raise or lower the scale relative to the path of the light. Continue to turn the screw until the refractive index is "0" -- indicating the refractometer is calibrated.

    • 7

      Lift the protective shield and wipe the prism dry, using a clean, soft piece of tissue paper.