ohhhh. Yh, the blades rotate in the sensor. That's how I tried testing the whole setup from the beginning, since I did not have running water next to my desk 
BINGO
You are welcome.
Does the sketch counts correctly now?
sorry, wrong formulation from my side.
I blow into the sensor, the blades rotate but no change in counts. I pour water through the sensor, also no change in counts.
also, I had to change accounts because the other account had a limit on number of responses per day...
Hello
check your wiring wrt the data sheet of the sensor.
Yes, dumb mistake on my side. Connected GND with the black cable and 5V with the red cable now. However, that was not the error. Still getting no change in copy_count
Hello
You may have killed the sensor electronics?
oh boy. I will try and connect the sensor to an oscilloscope and see if the "output" signal can be detected when I blow into it.
and study the data sheet for integration notes addional.
connected the sensor to an oscilloscope and no signal could be detected, so the sensor must be broken 
Did you make the circuit just like as shown in the data sheet? The device has to be powered and be connected correctly in order to produce an output signal.
Paul
This thread was hijacked.
WTF?
That's the second time I've seen it. There's nothing to stop people from doing it.
Yes I did. I also connected the sensor directly to a power supply unit because I thought, perhaps the 5V arduino supply is not sufficient. But still, there were no signals to be seen on the oscilloscope.
This is the error that I get: 
Possibly it was the error. As @paulpaulson said, you may have destroyed the circuit in the sensor at the moment you connected it the other way.
@mashallah
It is against the forum rules to have multiple accounts, please do not create any more. I have suspended your other account.
Thanks,
Wouldn't a switch of + and - simply be an "inverted" signal?
You have now flagged 2 posts on here to the moderators, the second one being my reply #37 pointing out that you can't have 2 accounts. In case you missed it I am one of the moderators on this forum and I am trying to politely acquaint you with the rules. Please stop flagging posts that don't need to be flagged and stop creating multiple accounts. If you continue like this you will get a time out from the forum. If you've not already done so please read How to get the best out of this forum
You are very welcome here, but do please try to understand how the forum works.
No. My guess would be that the circuit inside the flow meter contains a hall-effect sensor which detects a tiny magnet attached to the blades. A hall-effect sensor chip could be damaged by reverse power connection. Simpler sensors contain a reed switch instead of a hall-effect sensor. Such a sensor would not be damaged by reverse connection because they have no polarity, they are just switches. Hall-effect sensors are better because they don't wear out and can deal with high flow rates where the blades spin very quickly, but they have the disadvantage that they can be damaged by reverse connection.
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