Earlier, the control signal of industrial control instruments was 0~10mA, that is, “0” corresponds to the “fully closed” state of the instrument, and “10” corresponds to the “fully open” state of the instrument.
​With the development of computers, modern instruments are increasingly controlled by computers and DCS systems, which also brings problems; there are two states that cannot be distinguished (both signals are “0” states)
1) Industrial control instruments The control signal is “0”
2) When the computer fails, the break signal will generate a “0” signal. In order to distinguish the two states and prevent misoperation, the 0~10mA signal is gradually phased out. Now the control signal of industrial control instruments is changed to: 4~20mA. That is: 4mA corresponds to “fully closed”, and 20mA corresponds to “fully open”. rise.