Refractometer

[eagle1]

Finally received my refractometer and after calibration, used it on my main and hospital tanks. I couldn't believe how far off my swing arm device was.
After setting QT to 1.009 using the refractometer, I tested it using the swing arm and it read around 1.005. I checked the main tank with the refractometer and it read 1.024. The swing arm read around 1.021.

 

[bang guy]

You should be using the Salinity scale, the S.G. scale is not accurate on an ATC refractometer. 14ppt is the number to shoot for.

 

[eagle1]

It does look like if I shot for SG of 1.009, I'm really at 12 in PPT scale.
Why the discrepancy? And how did you find out which side is the correct one? Just curious.

 

[bang guy]

Refractometers measure salinity, not specific gravity. Saltwater hobbiests should have no interest in S.G. if they have a refractometer.
The descrepancy is because S.G. changes with temperature and salinity does not (not significantly anyway).
Any scale that equates salinity with specific gravity is only accurate at one specific temperature.

 

[donmgicwon]

Originally posted by Bang Guy
Any scale that equates salinity with specific gravity is only accurate at one specific temperature.

Does that mean a refract. with a scale like the one above is no good or that you just need to make sure your measurement is taken at the same temp. that the refract. was calibrated at? I am confused at your statement, which is similar to one made in another post. Please help. I want to make sure I am geting an accurate measurement, after spending $50.00 I'm not convinced a bought a good tool and I may as well just kept my swing arm.

 

[donmgicwon]

Originally posted by Bang Guy
You should be using the Salinity scale, the S.G. scale is not accurate on an ATC refractometer. 14ppt is the number to shoot for.

ATC = (auto. temp. compensating)?:notsure:

 

[bang guy]

Originally posted by donmgicwon
Does that mean a refract. with a scale like the one above is no good
I don't understand why you're confused.
Sea water is about 35ppt. So use the PPT scale and ignore the S.G. scale. I do not understand why they even put the S.G. scale on refractometers. You perchased an instrument that precocely measures salinity. It's an outstanding investment.
So bottom line:
35ppt for saltwater aquariums
14ppt for hyposalinity to kill parasites.