I did a water change yesterday and as always used my refractometer to check the salinity of the water i was making.
My refractometer instructions said to wait 30 seconds before reading to make sure the temperature balance was correct.
Just by chance yesterday I'd left the water on the prism for a few hours and before cleaning it I looked through it again. The salinity had been 1.006 at first look but was 1.011 a few hours later.
Is this normal? Which reading is accurate and how long should I leave (or not leave) before taking a measurement? Could it be that I haven't been allowing a long enough time for the temperature to settle for an accurate reading?
Taking a wild guess, I would assume that in those few hours, some of the water evaporated, raising the salinity. Salt doesn't evaporate, it stays behind.
