I think I read somewhere that the Hanna measure salinity by conductivity. Salt makes water conduct electricity. Pure water does not conduct electricity, and therefore RODI water at 000 does not conduct electricity. Our TDS are also conductivity meters calibrated to read PPM. So using RODI to do the calibration makes since in that context. The thing that makes me do a think is that other impurities cause water to conduct electricity also. We add trace elements to our water. So they must effect the accuracy, IF I am correct. The refractometer measures salt content by refraction, and the swing arm measures specific gravity or the gravitational weight. However the swing arms are prone to all kinds of problems, such as deposits, friction, temperature, wear, bearing slop, and salt grit & trace elements binding in between the bearing surfaces.
It is my understanding that the refractometer method is the most accurate for a reasonable price, and the digital conductivity meter method is the next most accurate but more simple to operate. I do not have a Hanna yet but plan to get one at some point and intend to calibrate my refractometer with the standard fluid provided then test a sample of tank water with it and then with the Hanna and to then calibrate the Hanna per the refractometer reading AND RODI water to see if they agree. If they do not I will then use the reading off the refractometer to calibrate the Hanna BECAUSE it is what I have been using and running my tank by. This does not mean I think the Hanna is wrong it just means I will use it for being handy and to keep my water the way it is currently.
That is a lot of mumbo jumbo.
In fact, I just logically talked myself out of buying a digital salinity meter and will stick with my refractometer.
