If you need calibrate the Sugar brix meter, proceed as below:
1.Power on
2.Drop about 0.3ml purified water, close the lid
3.Press "ZERO" button two times
4. 
twinkling
5.Press "ZERO"
6.0.0% brix, calibration succeed
GVDA Sugar brix link: https://www.gvda-instrument.com/measuring-instruments/digital-regractometer/digital-sugar-brix-meter.html
Take a look our Sugar brix meter models show as below:
