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

Question

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 FCH-m-POM-LC

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.

Answer 1

I guess it's not an electronic problem. The sensor contains a small impeller which is set into rotation by the liquid. If there is air inside the system, the impeller may not start rotating as intended, or not at all.

Also, if the flow stops abruptly (as it does when you use a syringe), the impeller may still rotate for a while, resulting in a too high measured volume. This effect of course is amplified when there's air in the system.

I'm not sure if this sensor is the right thing for your project. May be, a scale below the bucket is better? A household scale is available at pearl for 10€. I'm sure you can still replace the internal electronics by your own, just leaving the sensor and mechanical support.

(greetings from Germany)

Answer 2

Troubleshooting is pretty hard remotely- especially with this sort of thing.

Turbine and paddle wheel flowmeters are not great in repeatability (friction causes errors, especially at low flow rates, and it's not necessarily consistent), and that goes exponentially for cheap ones.

Suggest you test the software with an independent signal source. You can start with a bench-top signal generator at various different frequencies. Go well above the maximum average because the frequency will vary wildly as spurts of 'milk' pass through. Presumably you're using an interrupt-driven architecture or you'll probably be missing edges. There are plenty of things that can go wrong with the software, even so.

Look at the input signal with an oscilloscope and verify it looks reasonable. As @geometrical suggests, check that the power supply for the sensor is within the recommended operational range, and that the output signal is in the proper range for the Rpi. If any of those are not correct, fix them first.

Answer 3

First, there is no sensor in the list at the link you provided which comes close to matching your specs. All of the 3 liter models with pull-down output stages have sensor outputs at roughly 10,000 impulses per liter. Furthermore, none of them have a minimum flow rate of .05 L/min.

There are several units, 971478089, 96103114 and 97138031, which have minimum rates of .05 L/min , but they are 6 liter max rates, not 3. And I strongly suspect that you have a model 971478089 with a nominal output of 5,000 imp/L, which will give you the counts you observe. The other two have outputs of 2500 imp/L.