Refractometers should be calibrated with a seawater calibration fluid.
Swing Arm Type Hydrometers are only good for hockey pucks
Pinpoint Meters... no comment but I gave up using them a long time ago.
Floating hydrometers I don't use either.
Make sure the GHL Cond probe is cleaned and swish it through the calibration solution to get rid of any air bubbles.
Cheers,
Tim
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