3g/L。Automatic, quality-grading equipment was designed for grain purchasing, which could detect and grade grain impurities and moisture using the capacitance measurement method, and bulk density using the vacuum negative pressure method. Experimental results showed that after the separation of impurities, the grain purity was 99.5%，the repeatability error of impurity separation was less than 0.3%, moisture error was within ±0.5%, the repeatability error of moisture was less than 0.2%, and the repeatability error of bulk density was less than 3g/L.