If your making the piston system why not just have a very small piston pushing on a strong spring and a micro switch operated by how far the spring is compressed. If you put a screw on the opposite end of the spring to the piston you could adjust the operating pressure.
When we talked about an analogue system we meant a comparitor with one input connected to the pressure sensor output and the other input to a reference voltage. The reference voltage would normally be set using a potentiometer but as you do not want to use a potentiometer you could calculate the required resistor values. The comparitor chip would have to be followed by a transistor to give enough current to drive a suitable relay. As you planed to use an arduino that would also require a transistor an relay on the output. So the comparitor chip (An LM393 at about £0.5) and a few resistors would replace the arduino.
Some hysteresis could be designed into the compatitor circuit to avoid rapid on / off cycling. Many of us could draw you thhe circuit to use a comparitor.

Les.

 

Thank you Les but it's important for me that the pressure remain between a specific range. This is the pressure switch I will use https://climate.emerson.com/en-sg/s...1002/emerson-ps1-pressure-switch-single-en-sg
I will be able to chose separately max and min pressure on the system.

The Arduino solution will help me because with arduino I can control the pressure only by attaching 2 socket, pressure sensor and pump.
I'm sorry and I can't understand why nobody can help me with the second line of the code. Anyway this forum remain the best for me.

 

You have stated that your project is for a Commercial application ........
And yet You have no idea of what is required to achieve your end goal.
The people attempting to help You here still have only a vague,
general idea of what You are trying to accomplish.

You have also stated ..........
"" The Arduino solution will help me because with arduino I can control
the pressure only by attaching 2 socket, pressure sensor and pump. ""
after having multiple, highly qualified Engineers guessing at various different solutions
because they have very little information to work with.

Everyone in this Forum wants to guide You to the best solution for your problem,
but when we see that there may be serious problems, or dangerous conditions created,
because of your limited understanding of the device being modified,
there may be a reluctance to provide any further assistance.

~50-Bar can be extremely dangerous.

You have also stated that the type of controller that You want is
already available from established manufacturers.
I would suggest that You take advantage of their research and experience
in building safe, and reliable, controllers,
by purchasing the appropriately designed controller for your needs
from one of these established manufacturers.
.
.
.

 

After many test i found the right number to put on the code.

Pressure = map(analogRead(ReadPressure), 102, 921, 0, 80);
Setting = 5 * (map(analogRead(ReadSetting), 0, 1023, 27, 0));
Threshold = Setting - 10;

The number "27" is important because without changin the value of the potentiometer the pump will stop soon or later.
If I put "50" the pump will stop at 86.6Bar
If I put "10" the pump will stop at 20Bar

At the momento I'm unable to understand how it work but Im happy to have discovered it.

thanks to everyone for the precious advices