Instrument maintenance of differential-pressure method, gravimetric method and humidity sensor method are simple; and only need regular maintaining operation. However, in equal-pressure method, infrared sensor method and electrolytic method, users must pay careful attention to aspects such as the maintenance of sensor, tail gas treatment and the removing of impurities inside test systems. At the same time, users should operate carefully to avoid sensor depletion. Allowing for the higher cost of sensor, it is essential that users carry out special maintenance.