ADC input protection
Because the stages, which create the input for the ADC run partially at 24V, a solid input protection is needed. Otherwise the input might get destroyed.
using a 1kOhm + 3V3 Zener diode in series.
This causes a strange behaviour in current measured by the Flow Meter itself. Also the voltage shows the same behaviour.
The 3V3 Zener diode has some significant current before the voltage reaches 3V.
By this, it forms a voltage depended voltage divider, which causes the observed behaviour.
Removing the 3V3 Zener diode and increasing the resistor to 56k. This is a bit more dangerous regarding ADC input protection, but on the other hand the non linearity is gone.
