Veegee STX-3 Salinity Refactometer calibration issues

[Soilworker]

So I have the Veegee STX-3 Salinity Refractometer and from what I have read its supposed to be pretty accurate. The manual says to calibrate with RO\DI water which I have done and my tank's salinity comes out to 1.025, right where I want it.

I bought BRS's Refracto Juice which is said to have a salinity of 1.0264 sg or 35ppt. If I use my RO\DI calibrated refractometer it reads 2ppt higher, so 37ppt. If I instead calibrate my refractometer with the Refracto Juice and measure my tank's salinity then the tank salinity reads 2ppt higher, about 1.028 (or 37ppt) or so.

The question is what do I do? The refracto juice appears to be off by 2ppt when compared to my RO\DI calibrated refractometer but I don't know what to trust or what to do at this point. This isn't a cheap refractometer so I can't imagine the device is faulty

 

[Soilworker]

An additional data point, my Apex conductivity based salinity probe reads 35.5 ppt currently at 78F.

 

[Randy Holmes-Farley]

I do not know which, if either is correct, but it is not obvious either is out of spec.

The STX-3 claims accuracy of +/- 1 ppt.

If the calibration fluid had similar specs, then there's no inherent discrepancy.

Calibration fluid 35+/- 1 ppt might be 36 ppt.

STX-3 reading a real 36 ppt might give 35-37 ppt.