Marine Aquariums

Refractometer calibration


Hello. I purchased the H2Ocean refractometer along with a Sybon  calibration fluid. If I calibrate it with reverse osmosis water it does not  properly match up the the sybon fluid calibration. all samples are room  temperature. The calibration fluid is set at 35ppt. It is showing a difference of 4 or 5 ppt. My thought is to trust the calibration fluid over  the RO water sample as that is what the tank water should be at. Do you have  any suggestions?


I never really trust these so called calibration solutions.

Looking on the internet the solution appears to be designed to read 35ppt at 25c whereas refractometers are calibrated and measure at 20c

Most solutions are correct when used for conductivity calibrations but I am not convinced that the refractive index is correct and this is what a refractometer measures.

To correctly calibrate your refractometer you must have the instrument at 20c (not the solution) during calibration. After that you can use at other temperatures as it is ATC.

Add some RO water and set to zero
You should then be able to read the correct salinity.

Note that the sybon refractometer is for measuring brine and not seawater and will give a reading 1.5 higher than the true seawater salinity.

This will hopefully help


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