Refactometer????

So I posted a question a few weeks ago about eavapotion in my tank. I was worried that my hydrometer was not accurate because I haven't had to add any freshwater. So I bought a refactometer yesterday and confirmed my salinity was good and the hydrometer confirmed my reading. Today i tested my salinity with the refactometer and I was high like .030 sg.. I started to weird out how that could happen so fast. I decided to recheck my calibration again like I did the first day. I used the same rodi water to check the zero on the calibration. To my surprise the zero drifted so I zeroed it once again. I rechecked the salinity and it was fine like the day before. My question is how often to you have to zero a refactometer, I would think only once as long as you don't bang it around... its a cheaper meter I will post a pic. Bought on amazon.. fyi I have been an instrument tech for 15 years so I understand limiting variables during cals. I did clean the meter glass each time , used differant droppers for fresh and saltwater. One variable I haven't thought about is light I just checked it outside in the sun both days dunno how much lighting even effects a refactometer .. ideas???

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