Red Sea refractometer VS Amazon ATC refractometer

[MmmmBalf]

There's a reason you shouldn't use 35ppt calibration fluid with the Red Sea. Calibration fluid is salt (Sodium Chloride) and water, mixed to the salinity of 35ppt. That's what most other refractometer's measure. Red Sea have made their instrument to measure the refractive index of sea water, with all of it's different salts and other elements, which is different to plain salty water (brine). You can't calibrate it using brine, it's not designed to measure the refractive index of brine. It will apply it's sea water algorithm and come up with a different reading.

Unfortunately I don't think there's a sea water calibration fluid available. So RO water it is.

 

[Bozoman]

I agree that it is always best to calibrate any instrument using a solution as close to the measurement you are going for, but that also depends on the calibration solution staying true. Keep in mind that 35ppt calibration fluid can evaporate over time and become more concentrated.

 

[Randy Holmes-Farley Verified Community Expert]

There's a reason you shouldn't use 35ppt calibration fluid with the Red Sea. Calibration fluid is salt (Sodium Chloride) and water, mixed to the salinity of 35ppt. That's what most other refractometer's measure. Red Sea have made their instrument to measure the refractive index of sea water, with all of it's different salts and other elements, which is different to plain salty water (brine). You can't calibrate it using brine, it's not designed to measure the refractive index of brine. It will apply it's sea water algorithm and come up with a different reading.

Unfortunately I don't think there's a sea water calibration fluid available. So RO water it is.

That is not correct.

The 35 ppt calibration fluids available, including my DIY, are NOT 35 ppt sodium chloride. They are solutions, either seawater mimics (such as the Pinpoint) or sodium chloride (my DIY) solutions, made to the same refractive index as 35 ppt seawater, not 35 ppt sodium chloride, even if made with sodium chloride.

These solutions should read 35 ppt in any refractometer designed to measure seawater.

I discuss this in great detail in these articles:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm

Refractometers And Salinity Measurement
http://www.reefedition.com/refractometers-salinity-measurement/

 

[Randy Holmes-Farley Verified Community Expert]

I agree that it is always best to calibrate any instrument using a solution as close to the measurement you are going for, but that also depends on the calibration solution staying true. Keep in mind that 35ppt calibration fluid can evaporate over time and become more concentrated.

That is certainly true.