西门子双极性换算关系 4-12-20mA，假设电流电压比800/5,6300/100， 采样原值X,实际工程值Y，注意：12MA工程值理论是为0，可适当减零漂(SIEMENS bipolar conversion between 4-12-20mA, assuming the current voltage ratio 800/56300/100, sampling value X, value engineering Y, attention: 12MA value engineering t