Regarding zone-based control of instruments

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Dear sea friends: Hello everyone! I encountered a problem: when providing nitrogen protection for a tank at atmospheric pressure, the foreign party used control valves on both the nitrogen supply line and the exhaust line, with the tank’s top pressure gauge used for control. The control principle is as follows – when the pressure at the top of the tank is less than 3 KPa, the inlet valve opens while the outlet valve closes ； When the pressure is higher than 5 KPa, the opposite is true ； What kind of control method is this? Some people say it’s hierarchical control; I’m not sure if that’s correct Another issue is this: it is said that a pressure gauge can control two control valves. In normal conditions, both valves are open; only when the pressure rises does the intake valve close slightly while the exhaust valve opens more, and vice versa ； What is this control method called again? Thank you all!