How to Use a Refractometer to Measure Salinity (Step by Step)

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Here is an in-depth instructional step-by-step guide of how to calibrate and use a refractometer to measure the salinity in your saltwater aquarium.

Maintaining stable salinity levels in your saltwater aquarium is one of the essential parts of the hobby.

Incorrect measurements can make or break your tank, and if you are not careful, it can be detrimental for all the organisms that inhabit the ecosystem you’ve created for them.

Luckily for us, most saltwater organisms are relatively forgiving and can tolerate specific ranges of salinity fluctuations. However, it’s our responsibility to maintain stable parameters and don’t let the salinity in our tanks fluctuate too much. That’s why reef aquarists use different types of tools to measure the salinity in their tanks and make sure everything it’s on point.

The most popular tools for measuring the salinity levels in a reef tank are the hydrometer and the refractometer. I use a refractometer, which is the most accurate way to measure salinity in your reef tank.

In this article, I’ll show you step by step how to use your refractometer, how to calibrate it, and how to take care of it.

Let’s get started!

How to Use a Refractometer


What is a Refractometer?

A refractometer is a simple device most commonly used for measuring concentration in liquids. It’s used in food, agricultural, chemical industries and often in the saltwater aquarium hobby as a tool for measuring salinity levels.

How to Calibrate a Refractometer?

To use the refractometer appropriately, you first need to calibrate it. Without calibration, you’ll never be sure if the readings are accurate. Don’t sweat about it; it’s nothing complex. You just need to follow a few steps. 

How to Calibrate a Refractometer

Step 1: Place a few drops of RO water on the main prism

Open the daylight plate, and place a few drops of RO water on the main prism. You can use distilled water as well. 

Recently, I noticed people use sodium chloride solutions to calibrate their refractometers, which in theory is actually better than using RO water, but I prefer to stick with my old method; it’s working just fine. 

Close the daylight plate, so the water spreads across the whole surface of the prism. You don’t want to see bubbles or dry spots on the prism. It should be without air and covered with water. 

Step 2: Hold the refractometer in the direction of a light source 

After you’ve done the first step, you need to allow the sample to temperature adjusts for approximately 45 seconds before proceeding with step two. 

Hold the refractometer in the direction of a light source and look through the eyepiece. You should see a circular field with graduations down the center. If you do not see it clearly, adjust the focus until the picture is cleared up. 

When you are testing the salinity in your saltwater aquarium, the upper part of what you see should be blue, while the lower part should be white. But in this case, we are not testing the salinity; we are calibrating the refractometer.

If you calibrate your refractometer with RO water, the field should be all blue. It means that the salinity is 0, which is because we are not using saltwater. If it’s not all blue, we need to adjust the refractometer using a calibration screw. Proceed to step three to finish the calibration process. 

Step 3: Use the calibration screw to calibrate the refractometer accurately 

Use the calibration screw that came with your refractometer to calibrate it accurately. 

Look through the eyepiece, and screw until the boundary between the upper blue field and the lower white field meet precisely on the zero scale. 

Congratulations, your refractometer is now calibrated, and you can start using it. 

How to Use a Refractometer?

Using a refractometer to measure salinity is not rocket science, and it’s pretty easy when you learn it for the first time. Here’s a step-by-step process on how to properly use the refractometer.

Step 1: Place a few drops of saltwater on the main prism

Use the pipette provided in the package and collect water from your tank. Open the daylight plate and place a few drops of saltwater on the prism.

Wait for approximately 45 seconds for the sample to adjust to the ambient temperature. Most modern refractometers are equipped with ATC ( automatic temperature compensation system), which means shifts in the temperature within the acceptable range should not affect the accuracy.

Step 2: Look into the eyepiece in the direction of a light source

After step one is completed, look through the eyepiece in the direction of a light source to see the readings.

The boundary between the lower portion of the blue area and the upper portion of the white area is the reading of the salinity levels in your reef tank.

Step 3: Clean the main prism after you’ve done the measurement

After you’ve done the measurements, clean the prism with a few drops of RO or distilled water. Then, use a soft eyeglass cloth to wipe the remaining water residue.

How Accurate are Refractometers for Salinity?

In the past, refractometers were quite expensive, and most hobbyists chose to use a hydrometer as a tool to measure the salinity levels in their tanks.

Today, refractometers are affordable for most people, and there isn’t any excuse not to use them as a primary tool for measuring salinity.

Refractometers are pretty accurate, and unlike hydrometers which lose accuracy after a certain period of usage, they can be used for an extended period without compromising the accuracy.

Final Thoughts

A refractometer is an excellent tool for measuring the salinity in your reef tank. If you use it carefully, it will serve you for years.

To make sure you have accurate measurements, calibrate the refractometer every month. Do not expose it to bad working conditions, and always rinse it with RO water after you are done with the measuring.

I hope you enjoyed reading this article and you’ve found it informative. If you have any other questions, feel free to ask in the comment section below.

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