1, the voltage signal is unstable, easy to be interfered in the long-distance transmission, the current signal is relatively stable. The current signal is used in the transmission process, and the voltage signal (range of voltage values) is used in the specific control of the control instrument and the DCS system.
2, the range of 0 ~ 20mA signal and fault signal can not be distinguished. Less than 4mA indicates line failure, or suspicion of zero drift. The birth of a two-wire instrument prompted the change of standards, and the new standard of current signal is 4~20mA.
The 3,4-20mA standard is set for 2-wire sensors, where the 0-4mA current is needed to supply the sensor's own minimum operating current. If 0-20mA is used, the sensor will not be able to obtain sufficient signal when the signal is small. Working energy.
4. In fact, for 4~20MA signals, the DCS system generally counts more than 20.2mA and less than 3.8mA as faults.