The principle is simple.
In fact, the spring scale.
Don't know is to ask mechanical or electronic.
From the beginning of the machine first.
Pressure-relief devices will have a first probe, one end of the probe with a spring to resist, probe into one end of the pipe and at the other end of the pressure-relief device is sealed, when air pressure is greater than the jacking force of spring, connected to the exhaust system on the raft back to be opened.
Began to stress, when the pressure less than the jacking force of spring, recovery.
Electronic principle is the same, different is to rely on touch point contact to open the valve gate.
Also can adjust the size of the air pressure.
Principle is in different position of the spring contraction device contact switch.
When the touch point contact, through to the valve door magnetic coil, on and off of the valve gate, open the valve door, and reduced pressure.
When spring after for open contact, magnetic coil power, closure, the valve door closed.