Calibrating refractometer

[Irsug1]

Hi all,
So from Randy's article, RO should be 1ppt?
I am wondering if my 35ppt solution is old and no longer accurate. I have left the bottle uncapped overnight a few times over the last 3 plus years.
When I tested RO, my refractometer read far below the zero line. Thoughts?
Thanks in advance.

 

[john.m.cole3]

I would buy or make some new calibration fluid. Even one night w/ the cap left off will be enough to throw off your readings. think about how much water evaporates from your tank each day. About 1% right? Say that happens 5 times... now your calibration fluid is at 36.75 ppt. cross checking w/ RO water will not confirm your refractometer is calibrated correctly.