Refractometer wonky??

[Myka]

Vertex refractometers like most in the hobby are "saltwater" refractometers, as opposed to "seawater" refractometers. "Saltwater" refractometers measure the amount of Sodium chloride content, as opposed to "seawater" refractometers that measure entire mineral content of seawater. The scale is different for each refractometer.

Why does this matter? If a "saltwater" refractometer is calibrated using distilled or RO/DI water it will read 36.5-38 ppt when you measure 35 ppt seawater. A "saltwater" refractometer needs to be calibrated using calibration fluid at 35 ppt.

When using RO/DI water you are assuming your TDS meter is also calibrated properly. If your TDS meter is off then your refractometer will also be off.I suppose if you're in a pinch you could just set your refractometer 1.5 ppt higher than 0 when measuring distilled or RO/DI water, but calibration fluid costs about $5 and removes one more point of possible error.

Also, the temperature of the refractometer when calibrating and measuring is important. The temperature of the one or two drops of water you add to the crystal doesn't matter so much as such a small amount of water will quickly become the temperature of the refactometer. The refractometer should be close to 20C when measuring or calibrating for highest accuracy.

I think your RO/DI water is off because you adjusted your refractometer by 0.013 and it just so happens your reading was also off by roughly 0.013. Make sure temperature is correct, recalibrate properly, and re-test.

EDIT: Here's a good article. http://www.theaquariumsolution.com/s...ts-measurement