How to calibration a refractometer

Calibrating a refractometer is an important process to ensure accurate readings. Here is a general procedure for calibrating a refractometer for marine aquariums:

Clean the prism with distilled water and dry it with a soft cloth.
Place a few drops of calibration solution on the prism.
Close the daylight plate and press it lightly so that the solution spreads across the entire surface of the prism without any air bubbles or dry spots.
Allow the sample to remain on the prism for approximately 30 seconds.
Look through the eyepiece and adjust the calibration screw until the reading matches the value of the calibration solution.
You can find more detailed instructions on how to calibrate your refractometer in its user manual or by searching online.

I hope this information helps! Let me know if you have any other questions.