I'd like to read value from gear flow meter (attached specifications - model type V2). I have never done it before that why I'm asking what is the best approach to program the code for this type of meters.
As I understand right I have got 2 outputs in quadrature and the output voltage = voltage apply -1. So, I will have to provide power source between 10V-28V. If I will go for 10V I would get 9V output voltage. Is it possible to use NI9215 or NI9375 to acquire and properly calibrate the signal?
This sounds like you are still figuring out the correct hardware configuration for your set-up.
In order to make a recommendation we would need to look much deeper into your application and specification (also think about channel count, accuracy, sampling frequency, do you need to expand in future etc.)
I believe our technical sales team would be the best address to help you and it is best if you give a call to your local NI office.
Between the two modules you mentioned, the 9375 is the more appropriate choice since it's a mixed digital I/O module and not an analog input. I have never used an AI module for a flow meter application; I suppose it's possible, but it would seem mismatched, especially since analog sampling modules are typically a little more expensive.
The 9375's digital logic ON state is specified in the datasheet as >=10V, so 9V would not be within spec, and you would probably want it slightly higher than the 10V since the input impedance ratings are +/-5% tolerance resistances. I would find a way to increase the voltage or find a more flexible module, then start building your tick count algorithm from there.