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Liquid throughput hall sensor giving odd number of pulses plus only works in horizontal level

I'm an IT guy, but have lately started working more and more "on the edge" as we say, so close to the real world and with low level sensors. I'm currently building a liquid throughput sensor and reading it with a raspberry pi to send it to a server.

These are the specificiations for the sensor:

2500Imp/L at 20°C
0,05 - 3L/min
using a Hall sensor principle
The link to the product would be this OHS3040U: http://www.kynix.com/Detail/64233/OHS3040U.html.

It is from the TT electronics: http://www.ttelectronics.com/en/home
I can read the pulses using a pull up resistor at the signal with 3.3V and 2kOhm. But there are two problems:

when I squeeze 60ml through a syringe into the sensor I get about 300pulses == to 120mL with a deviation of about 10ml (tried 10 times). Did I calculate something wrong? I'm listening for "falling edges" so I count every time the voltage drops to 0V.

It often doesn't work, and if it works, it does so best when in a horizontal axis, so reading pins up, liquid in/out on one level. Is this normal for these flow sensors?

as you might be able to tell I'm used to software, not hardware so treat me like a first year engineering student if you want.

I'd greatly appreciate your help! Greetings from Germany

P.S. its for a cow milking battle where we want to have several participants milk a fake cow and see who's the fastest. We want to measure on a 1/10sec precision and visualise the data throughput/second for 120seconds

I created an image of my setup. however I used 5x 10k Ohm resistors in parallel since I don't have 2.2kOhm Resistors. the image description is here:
http://www.kynix.com/BlogImage/11.14.5.png

1 out of 1 members thought this post was helpful...

300 pulses for 120 mL is right on 2500/Liter. I don't see what your problem is.

The value of the pullup resistor will not be critical.