How to use a Refractometer to Measure Salinity

Different angles of a refractometer device with case
Reefractometer from AquaMaxx

In the early days of the hobby, we’d use a floating glass hydrometer to measure the specific gravity in our tanks and when mixing up saltwater. The strength of the seawater determines how high the hydrometer floats in the water. Glass hydrometers worked best by taking a water sample with a tall cylinder and placing the hydrometer into it. If you put the hydrometer into your aquarium, it would bounce up and down making readings impossible. Imagine how long it took to mix a batch of saltwater! The latest generation of aquarium hydrometers use a pivoting pointer inside a water collection chamber to indicate specific gravity and salinity levels. Hydrometers are calibrated to be most accurate when the water is 77°F (25°C). While hydrometers are still used today, many reef aquarists have made the switch to a refractometer. So, if you’re considering the switch, here’s what you need to know about saltwater refractometers and how to use them.

What’s a Refractometer?

Monitoring the salinity of your reef aquarium is an important step in maintaining a stable environment for your fish and corals. There are two ways to define the strength of saltwater; one being Salinity, which uses parts per thousand (ppt) as the unit of measure, and the other is specific gravity which is the density of a substance (seawater) in ratio to a standard (pure water) – both are measured by a refractometer. Seawater has salinity that measures about 35 ppt, or in specific gravity around 1.026. Pure water has a specific gravity of 1.00, but when salts are added to it, the specific gravity increases.

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Refractometers are optical instruments used for measuring concentrations of liquids. They’re used to measure sugar solutions, brine, engine coolant and other liquids. The main component of a refractometer is the prism. A few drops of liquid are placed on the prism. The clear lid is closed, creating a thin layer of water across the prism. When light enters the liquid layer, it changes direction. This is called a refraction. The refractometer measures the amount of light shift, called the angle of refraction. This refraction is converted to a useful unit of measure. With saltwater refractometers the units are specific gravity and salinity. By looking through the eyepiece, you’ll see the measurement scale indicating salinity and specific gravity.

Why switch to a refractometer?

A refractometer is considered by many to be a more accurate way to measure salinity. It has no floating parts like hydrometers. The big feature is that a refractometer can be calibrated to ensure accuracy. Unlike hydrometers, refractometers have automatic temperature compensation (ATC) so you don’t have to worry about your tank’s water temperature when taking a measurement. Many aquarists have made the switch to a refractometer to make mixing up batches of saltwater easier too.

How to use a Refractometer

It’s easy to use a refractometer, just follow these steps:

  1. Open the prism cover.
  2. Place a few drops of water on the prism.
  3. Close the cover.
  4. Look through the eyepiece.
  5. Adjust focus.
Instructions on how to use a refractometer infographic

As you look through the eyepiece, you’ll see the scale for salinity and specific gravity with a distinct shift in color between the upper and lower section of the scale. That lateral line of color separation is the salinity level in specific gravity and PPT measurements (as seen in photo below).

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Example of the measurement scale when looking through the refractometer eyepiece

Tips: Try to rid any of the sample water bubble pockets under the cover to get accurate readings. If the measurement line on the scale is blurry, repeat the steps with another sample from tested water. Before using your refractometer, it is recommended to perform a calibration to make sure the reading is accurate. Here’s why calibration is important.

Calibrating your Refractometer

Most of the refractometers sold for aquarium use are designed for measuring brine (sodium chloride) solutions. The refractive qualities of brine is a little different than true seawater. This can be corrected by calibrating the refractometer with a salt standard that compensates for this difference.  A product like Brightwell’s Refractometer Calibration Standard will allow you to bring your unit back into proper calibration.

  1. Open the cover and wipe the prism with a soft cloth to remove finger prints. There should be no salt residue on the prism or cover. Rinse with RO water if necessary and dry with a soft cloth.
  2. Add a few drops of calibration solution to the prism and close the cover.
  3. Wait about 45 seconds for the ATC to correct for temperature.
  4.  If the reading does not match the calibration solution, make small adjustments with the calibration screw.

Your refractometer’s manual may say to calibrate to 0-salinity using distilled or RO water. You’ll get much more accurate salinity readings by calibrating with a saltwater calibration solution. That’s because the refractometer will be dialed-in at the salt level you’re working with.

Tips for Accurate Readings

  1. Keep the prism free of finger prints. Use a soft eyeglass cloth to keep it clean. Never use a paper towel or regular cloth. It will scratch the prism.
  2. Rinse off saltwater with RO water. Any residual salt will cause inaccurate readings the next time you use the refractometer.
  3. Dropping or rough treatment can damage the internal ATC system or misalign the optics. Handle with care.
  4. Calibration once a month will ensure accurate readings.
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Digital Salt Meters

A digital refractometer from the manufacturer, Milwaukee Instruments

Electronic salinity meters are also available. The Hanna Salinity Tester measures the conductivity of saltwater and converts it to salinity and specific gravity. The Milwaukee Seawater Digital Refractometer is a true optical refractometer that displays the salinity and specific gravity on the LCD display. Both devices are made specifically to measure seawater. Like refractometers, they have ATC and require periodic calibration. However, digital testers are more expensive, and will require the batteries to be replaced.

Conclusions

Refractometers were at one time too expensive for marine aquarists. Today the price has dropped to an affordable level, making it easy for aquarists to have the accuracy and convenience that only a refractometer can provide. The calibration ability, automatic temperature compensation and precision of salinity measurement places the refractometer in the “must have” category for serious reef aquarists.

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