ppm是什麽单位？pphm和ppm的换算关系？

PPM [简明英汉词典]

=Parts Per Million 百万分之几, 百万分率

ppm [简明英汉词典]

part per million 百万分之...

pphm [简明英汉词典]

=parts per hundred million 亿分率(0.000001%)