Flow rate starts at non-zero value when machine is switched on.

[hallboyone]

Probably something is not being reset correctly.

[hallboyone]

After a little testing, the problem has been traced to noise in the flow sensor's power supply when the machine is switched on. This is read as a pulse by the pico. I tried a software fix (zeroing the flow sensor when the machine is powered on), but this led to corner cases where the sensor would be zeroed and then pulsed, leading to an occasional very large spike. It will be best to just clean the 5V power in the next version of the PCB. Until then, there will be some phantom flow rate now and then when the machine is turned on.