Working principle, solution and high assignment of some secondary instruments.

As for the question you asked, let me talk about my own opinion first.

Generally, the current secondary instruments include intelligent hand-held instruments, digital display instruments, paperless recorders, etc. They can be used as secondary instruments (including secondary display instruments, secondary control instruments, and also integrating display and control), and the old ones are commonly known as rod instruments (if controlled, they usually have handwheels).

Nowadays, secondary intelligent instruments, such as digital display meters, can input a variety of signals (resistance, mv, ma), and there are modules that can be switched, such as Changhui swp series used in our factory. You input different signals, as long as you set different codes, it will automatically convert. The output is generally 4-20ma or 1-5v.

Connecting wires: field primary instrument-safety barrier (isolation and explosion-proof)-secondary instrument -PLC (DCS).

If it is controlled, go back to the original road and then control the opening of the valve. The whole system is a closed-loop pid control loop.

As for the internal structure, there are detection unit, transmission unit and output unit. The specific circuit structure should be similar to that of a multimeter (personal feeling)

250 ohms is the standard resistance, which realizes the conversion between milliampere and volt. ..

In addition, the old secondary instrument should be equipped with a temperature transmitter if it is a thermal resistance input, because the safety barrier can convert 4-20ma into 1-5v, but if the measured resistance can only be converted by the temperature transmitter, it can be changed to 1-5v (that is, the resistance can be converted to voltage), so the new intelligent secondary instrument will not have this problem.

I'm not an expert either. I hope I can help you!

Wang Yu, Measurement Department, Automation Department, Tianjin Iron and Steel Group