Refractometer Errors

Reefer Rob · #1 04-23-2007, 03:55 AM

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.

Der_Iron_Chef · #2 04-23-2007, 01:26 PM

Interesting read. Can your "problem" be solved by correctly calibrating your instrument, or do you need to buy another refractometer altogether? I guess it depends on whether or not the refractometer in question was actually designed for what it's being used.

From everything I've read on various forums, refractometers are the easy, flawless way to determine salinity/specific gravity. Now I'm learning that that's not necessarily the case! I think I'll prepare one of his DIY calibration testing samples to check mine out.

Thanks for the heads up.

andsoitgoes · #3 04-23-2007, 03:29 PM

This came up when I was having issues with my digital salinity meter. It's very frustrating and very difficult to find something that will actually match up.

With the long-term success people have had using refractometers, hydrometers (both floating and swing arm) - in the end, I think keeping it consistent is more important than anything.

So I think making a standard, and keeping that as accurate as possible to base your test against is the easiest solution, but looking at the 5-page document - it just makes me go back to thinking if keeping things consistent is more important than a minor difference which might still be in existence unless you're doing this regularly?