salinity calibration confusion

[KStatefan]

Randy has a whole article on this

http://reefkeeping.com/issues/2006-12/rhf/index.php

 

[Randy Holmes-Farley Verified Community Expert]

So then my next question pertaining to using 0 as the calibration point not being the way to go. Is that because the response isn't linear?

I gave the exact reasons above in post # 17.

The response is approximately linear, but not all devices have the proper linearity even if made perfectly. A brine refractometer has a different slope of refractive index vs salinity than does a seawater refractometer (see the article posted just above for details in what that means). Even a true seawater refractometer may have the slope wrong if it is not made perfectly.

How many $50 analytical instruments are made perfectly?

 

[Atrumblood]

I gave the exact reasons above in post # 17.

The response is approximately linear, but not all devices have the proper linearity even if made perfectly. A brine refractometer has a different slope of refractive index vs salinity than does a seawater refractometer (see the article posted just above for details in what that means). Even a true seawater refractometer may have the slope wrong if it is not made perfectly.

How many $50 analytical instruments are made perfectly?

Ok fair enough. Thanks again

 

[GrizFyrFyter]

The quality of the equipment varies. I ran this test with the 4 refractometers we had at the LFS I worked at, just to show the manager why calibrating at 0 doesn't work.

One of the four was actually accurate with a 0 calibration. The other three were off by 0.002 to 0.007 sg.

You can use a commercial calibration solution but I have found several bottles that were not what they claimed to be and had a customer crash his tank after using a bad solution and making adjustments to his tank based off that calibration. His sg was 1.017 by them time he asked for help.

I prefer to either mix a calibration solution myself with a set amount of salt in a set amount of water (math doesn't lie) or use a floating hydrometer (not the swing style, the weighted glass meter that floats in the water) to check my refractometers monthly.

 

[Atrumblood]

Randy has a whole article on this

http://reefkeeping.com/issues/2006-12/rhf/index.php

The article was great. The graphs definitely helped me understand it better. Very nice write up @Randy Holmes-Farley

 

[Randy Holmes-Farley Verified Community Expert]

Thanks.

Happy reefing. :)