That scale will be accurate as long as you use the refractometer at the temperature it was calibrated at.
However, don't compare the refractometer specific gravity with the number you get from a hydrometer. They were probably calibrated at different temperatures.
The hydrometer may be calibrated for measuring the specific gravity at 25C water temperature, while the refractometer is probably calibrated for use of the instrument at room temperature, about 20C.
That's why I try to use the ppt (part per thousand ) measurement, but a hydrometer cannot measure that.
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Mitch
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