Dear all,
i have some question regarding control system.
I'm working in a gas plant, and in this plant there is a u-manometer with 2 kind of photocell,
the first one is photocell for safety level, and the second is photocell for reaction control.
I'm sorry i can not share the P&ID because i do not have it.
But could anyone tell me how this configuration work?
Here i attach the simple P&ID (it's all i have)

[link Point to another website Only the registered members can access]