Calibrating refractometers

[Ori]

My refractometer had been giving me consistent readings of 0.024 but today it’s been giving 0.025. I topped off with freshwater but still the same reading. When I put RO water in it it gives a reading below zero, but loosening the screw makes it go lower and it can’t be tightened anymore. What should I do?

 

[NY_Caveman]

That seems strange it will not tighten. I will note I had to calibrate mine every time I took it out, but it worked well. I also rinse it with RO/DI after every use.

 

[Super Fly]

do u calibrate the refractometer often? It should be calibrated weekly using calibration fluid. Also, temp of the refractometer should be same as water. I keep mine in 1F of house where temp is around 78 while the tank is in basement (lower temp) and bring it downstairs whenever I need to use it. If the refractometer was left in basement (where temp is lower than 78 degrees) then it will give different reading.

 

[SurfLife]

..It should be calibrated weekly using calibration fluid.

I always feel the need to warn people of my unfortunate intro to saltwater when I see this mentioned. When I started out I purchased a new refractometer that included calibration fluid. I calibrated it before using and a few times after that. Everything was dying except my 2 clowns and a mushroom coral(all still living). Inverts were dying instantly.
I was testing everything and my salinity was always 1.026.
I eventually purchased the Milwaukee digital. Calibrated with the new fluid and tested tank. 1.041.
Retested with old refractometer. 1.026.
Used new calibration fluid with old refractometer. 1.041.
Something as simple as a little packet of bad calibration fluid and I was ready to quit.
Lesson learned.
I also learned clown fish are some tough animals.

 

[skim]

What you should do is what the instructions say to use. If it says to use RO/DI or Distilled and zero the unit do it as that is the way it was designed. Take the Milwaukee unit, it uses distilled water for calibration. The thing that will influence the readings the most will be temp. The closer the unit is to the water temp being tested the more accurate it will be, even if they have ATC as this is only for about 2 degree difference +/-, so if your tank is 80 and room temp is 75 then you will want to bring the units temp. up.