Refractometer Errors

[Reefer Rob]

Low measurements of Mg, kH and Ca in my top off water, and high salinity readings from the water I've been getting my frags in from other aquariums and aquarium stores has made me doubt the accuracy of my refractometer. So I started looking around, and it turns out a lot of the cheaper refractometers sold for aquarium use are for measuring salinity, not sea water.

Salinity refractometers are designed to measure sodium chloride. Sea water has other elements in it that give a false salinity reading on these types of refractometers. This article by Randy Holmes Farley summarizes the information I was able to find.

I know I'm being picky, and stability is more important than the actual salinity, but I still think it's nice to start with an accurate instrument.