Hello,
I have got an issue. Im making a device which has to simulate injection of air in the pipeline.
I need a valve (probably solenoid valve) which will open on specific value of pressure (done by centrifugal compressor). So say, Im waiting in the specific place in the pipeline, where the valve will be placed for for instance 5 bars. When the value is reached the valve is opened and after a second closed again.
I would like to make everything myself at the best case.
Thanks for your advices 
I have got an issue. Im making a device which has to simulate injection of air in the pipeline.
I'm not sure what you mean by "simulate".
To really control a valve based on pressure, of course you'll need a driver circuit to trigger the valve, and you'll need a pressure sensor. I'd start with that... Start with reading pressure and/or controlling a valve with the Arduino.
Once you've got the input output circuitry and basic software working it's simply a matter of an [u]if-statement[/u] to open the valve if the target pressure is reached, then a [u]delay()[/u] before closing the valve.
for instance 5 bars.
With any "analog" reading, it's usually best to look for a value greater-than, less-than, or a range of values rather than looking for some exact value. if you are reading integer Bars you might be OK, but the real-world pressure might be 4.999 Bars or 5.1 Bars, etc. so you have to be careful with the measurements and logic.