Looking for some salinity testing advice

LeafsReefer · Jun 10, 2023

I'm looking for some advice on conflicting readings that I'm getting. This is my first time doing salt water so while I'm cycling I took a water sample to a local store and it measured low, 1.021. I recalibrated at home and got things set up properly. I wasn't sure if I was doing things correctly or if something was wrong with my refractometer because I wasn't always seeing the blue area so I took it to the store and they helped me out and calibrated for me with RODI water like I do. I think I wasn't putting enough water in so I wasn't always seeing the line right. I'm measure at 1.025, but just to make sure I got it tested again. This time I went to a shop that is closer to me thinking it wouldn't be a big deal, but they measured my sample at around 1.022 and they calibrate with calibration solution. I took the rest of my sample and I retested at home and got 1.025 again. I cleaned it with RODI water and tested the RODI water and it was calibrated to 1.00.

Does anyone have any advice on how to resolve this?

AstroCoral · Jun 10, 2023

Do not calibrate with RODI water, you need 35ppt calibration solution for a refractometer. The margin of error becomes quite large if using RODI water since it is far from the specific gravity you are trying to achieve. 35ppt is a lot closer to your reef tank than 0ppt, the margin of error will multiply as you move away from where you calibrated it thus calibration solution is essential.

Additionally, some refractometers are better than others. I have one refractometer that will lose calibration in a matter of hours while I have another one (exact same brand) that will stay calibrated for months.

LeafsReefer · Jun 13, 2023

AstroCoral said:
Do not calibrate with RODI water, you need 35ppt calibration solution for a refractometer. The margin of error becomes quite large if using RODI water since it is far from the specific gravity you are trying to achieve. 35ppt is a lot closer to your reef tank than 0ppt, the margin of error will multiply as you move away from where you calibrated it thus calibration solution is essential.

Additionally, some refractometers are better than others. I have one refractometer that will lose calibration in a matter of hours while I have another one (exact same brand) that will stay calibrated for months.

Thanks for the tip. This seems to have helped.