How to Measure Salinity Using a Refractometer

Maintaining the correct salinity in man-made marine or reef ecosystems is important to the health of its inhabitants. When saltwater evaporates, the salt is left behind, and the salinity of a tank or pool can quickly rise to a dangerous level. The best approach is to check salinity frequently and add small amounts R.O (reverse osmosis) or distilled water to maintain the desired salinity. Depending on the refractometer, you can measure salinity through "specific gravity" or percentage of salt. Natural ocean water generally maintains a specific gravity between 1.024 and 1.027. Salinity should be approximately 32 to 37 ppt (parts per thousand).

Things You'll Need

R.O. or distilled water
Eyedropper or syringe
Soft cloth or tissue

Instructions

- 1
  Calibrate your refractometer by placing two to three drops of R.O. or distilled water on the meter. Look through the eyepiece and turn the calibration screw until the scale reads 0. Some refractometers come with "calibration solution" which is said to tune the refractometer more accurately. Follow the manufacturer's instructions to calibrate with a solution. Dry the refractometer thoroughly with a soft cloth or tissue after the calibration is complete.
- 2
  Take a small sample of the water you wish to measure using an eyedropper or 1/2 mm syringe without a needle.
- 3
  Open the lid to the meter. Holding the refractometer level, place three to four drops of water onto the meter. Close the lid.
- 4
  Look through the eyepiece. Turn the focus knob until the numbers look sharp. The upper field of the meter will appear blue and the lower field will appear white.
- 5
  Look for the number that corresponds with the line where the blue turns white. This is your specific gravity or salinity. Most refractometers have the specific gravity on the left, the salinity on the right.
- 6
  Rinse the refractometer's meter and lid with fresh water (tap is fine) and dry completely with a soft cloth or tissue.