Welcome to the magic world of Node-Red.
Getting Arduinos talking to Node-Red is fun.
Just on what you said:
The PSI data should be sent out - as it seem reading it: by the serial port? - to the RPI.
That's ok, I guess.
What I would suggest you do is try this first to establish a base line that the Arduino is doing the right thing.
Load the IDE and open the serial port to the Arduino.
You should see the data being sent out line by line.
The exact format is not too important at this stage, other than every line should be the same format.
Say something like:
and so on.
Be it has PSI at the end doesn't matter at this stage.
These data packets are sent to NR and processed.
If that isn't working, you are in trouble before you get started.
If they are all good, maybe then you need to get the
serial input node and connect it to a
This should give you the same results as you did in the Arduino's IDE.
Given that, then you need to look at the flow.
But until then.......
I hope that helps you with finding where the data is going wrong.
All the best.