Salinity doubts

[vincephuket]

Hi everyone, I have a few doubts about salinity/density.

My refractometer is calibrated at 20 C I believe (looking through it at the bottom of the two scales, density and salinity, it s written 20 C).

I used deionized water at 27 C to set the zero with the screw driver. Then tested my tank water which is at 27 C and the instrument reads salinity 37 and density 1028. I know it s too high, but my question is: are these readings correct or should I compensate in someway as the temp is different?

 

[fcmatt]

get a calibration fluid made for refractometers for 8 bucks. Better yet buy 2 diff brands. Sleep well knowing it is more accurate.

 

[Randy Holmes-Farley]

Hi everyone, I have a few doubts about salinity/density.

My refractometer is calibrated at 20 C I believe (looking through it at the bottom of the two scales, density and salinity, it s written 20 C).

I used deionized water at 27 C to set the zero with the screw driver. Then tested my tank water which is at 27 C and the instrument reads salinity 37 and density 1028. I know it s too high, but my question is: are these readings correct or should I compensate in someway as the temp is different?

Complicated question without more info.

What refractometer exactly?

Does it claim to be ATC (automatic temperature compensation)?
Does it claim to be a true seawater refractometer?

Buying or making a 35 ppt seawater standard would go a long way to understanding its accuracy.

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

http://reefkeeping.com/issues/2004-06/rhf/index.htm

 

[vincephuket]

Complicated question without more info.

What refractometer exactly?

Does it claim to be ATC (automatic temperature compensation)?
Does it claim to be a true seawater refractometer?

Buying or making a 35 ppt seawater standard would go a long way to understanding its accuracy.

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

http://reefkeeping.com/issues/2004-06/rhf/index.htm

No brand, but yes with ATC, in the instructions it says to calibrate it with distiller water (set zero) at 25 C (68 F) which is what I did.

What should I do? I live in Thailand, standard solutions are not easy to find...

 

[Randy Holmes-Farley]

No brand, but yes with ATC, in the instructions it says to calibrate it with distiller water (set zero) at 25 C (68 F) which is what I did.

What should I do? I live in Thailand, standard solutions are not easy to find...

The claim that the temperature needs to be at 25 deg C (or any specific temperature) for calibration of a refractometer with ATC does not make any sense, but is very widespread. I think it is a false carry over from refractometers that do not have ATC. Any temp is fine, as long as you are within the ATC range and the ATC works. Calibrating at tank temp would eliminate any concern about the effectiveness of the ATC though.

Instructions about distilled water are common, and commonly incorrect since that is not appropriate unless it is a true seawater refractomer. Much more widely used are brine (sodium chloride solution) refractometers which will be a bit off when calibrated with distilled water (even if functioning perfectly).

For the moment, I'd assume it is correct within 2 ppt, and lower the salinity slowly while checking the refractometer with a DIY solution made from table salt as in the above link. It's best if you have a scale, better than nothing if you do not.

 

[vincephuket]

The claim that the temperature needs to be at 25 deg C (or any specific temperature) for calibration of a refractometer with ATC does not make any sense, but is very widespread. I think it is a false carry over from refractometers that do not have ATC. Any temp is fine, as long as you are within the ATC range and the ATC works. Calibrating at tank temp would eliminate any concern about the effectiveness of the ATC though.

Instructions about distilled water are common, and commonly incorrect since that is not appropriate unless it is a true seawater refractomer. Much more widely used are brine (sodium chloride solution) refractometers which will be a bit off when calibrated with distilled water (even if functioning perfectly).

For the moment, I'd assume it is correct within 2 ppt, and lower the salinity slowly while checking the refractometer with a DIY solution made from table salt as in the above link. It's best if you have a scale, better than nothing if you do not.

Thanks Randy, will prepare that solution and re calibrate!