Refractometers are excellent devices to measure salinity and specific gravity in salt water and reef aquariums as long as they are calibrated correctly. I typically calibrate my refractometer every couple of uses. We are going to go through the steps of how to calibrate a refractometer. Yours may be very similar, but it is best to check your manufacture to make sure each step is correct for your refractometer.
We begin by using Accubrate by Aquatic Experts which is calibrated at 77 degrees Celsius, salinity of 35 parts per 1000, and specific gravity of 1.0264. Now I mentioned the temperature is 77 degrees and you don’t have to worry about your tank being 77 degrees or your room being 77 degrees what you want to make sure of is that your refractometer and the solution you are testing are the same temperature. In this case they are both room temperature. You’ll notice this refractometer has ATC, which stands for Automatic Temperature Compensation, this means it will automatically adjust the temperature and give you correct readings every single time you use it.
First, rinse off the refractometer to remove any salt crystals, dust, etc. It is preferable to use distilled water for this rinse. Alternately you may use R/O DI water. If that is unavailable, use R/O water. It is good to use the purest water you can get. I’m am going to open the lens up and to rinse off the prism. You do this by dropping the distilled water onto the lens. You just want to make sure it and the cover slide are rinsed well to make sure no salt deposits remain. Once comfortable that it is clean, we use a clean, very soft cloth to dry it off. When the refractometer is dry we are ready to calibrate.
Next, shake the bottle of calibration solution. Accubrate has a built-in dropper, place 3 to 4 drops onto the refractometer and then close the refractometer lid. Make certain that the entire surface of the prism is covered. If there are any air bubbles, gaps or spaces, open it and add another drop just to make sure there are no spaces when we read it.
Looking through the eye piece, adjust the lens until you can see clearly. You will notice a scale on the left-hand side of the scale we will see specific gravity and in this case, we are shooting for the calibration of 1.026 - 1.0264. On the right side of the scale we will see our salinity which we are shooting for 35 parts per 1000. Notice the blue section on the top and clear section at the bottom. On the very bottom of the blue edge is what we will read. Right now, I am reading about a little less than 35 parts per 1000 of my salinity. I remove the rubber cap found on the top side of the refractometer, use a screw driver and slightly turn it and then look again to see if it says 35 parts per 1000. If it does not say 35 parts per 1000 continue to adjust it until it does.
Cover the calibration screw with the rubber cap when finished calibrating. Use distilled water to clean the prism. Remove a bit of salt water from your aquarium and leave it sitting for a few minutes so that both the water and the refractometer are the same temperature. Drop a few drops of saltwater onto the prism, make sure surface is completely covered. Hold the refractometer to the light. Adjust the end of the refractometer so the scales can be seen. Our example reads 1.023 specific gravity. This is a fish only fish tank so I want to keep the salinity low. (If it was a reef tank and it should be 1.026 to 1.027 - In this case we would create a slightly higher salinity in the return water in order to increase the salinity of the aquarium.)
For our purposes, I want this tank to be at 1.023 soI am happy. Some aquarists like to keep their fish tanks lower closer to 1.020 because they think parasites are less prevalent in lower salinity; in this case simply remove some water from the aquarium and add a little fresh R/O Di water to lower the salinity. Rinse off the refractometer one last time with the distilled water to make sure that it is clean and ready for next use. Recalibrate after a couple times of use. Put this refractometer back in the case and you are done.