I worked with the good folks at Colorado State University's Hydraulics Lab (https://hydraulicslab.engr.colostate.edu/) on the calibration of a series of flow velocity sensors including the DN-80 sensor used for this project. We ran their fume from near-still water conditions up to 0.4 m/s flow rate and hung a Marsh McBirney flow meter along with the DN80 for an open channel calibration. The lowest stream velocity the DN-80 could record using a 5 sec integration time was 0.025 m/s but above that limit, a linear regression between the DN80 and MM estimates had an r2 of 0.9955, with a bias of -0.004 m/s and RMSE (aka standard deviation of residuals) of 0.009 m/s. Will try to get larger range of observations in the future but this is a very good start, IMHO.  Photo of DN80 in the flume is at 

https://photos.app.goo.gl/iHZ2Cd89UM8E9uDn9

That's really interesting, thanks for telling me about your work and do update on any further testing or developments.

Also Sorry! For some reason I did not receive the email I normally get when people comment, so my reply is months late!

Wow- this is great. So many issues I hadn't thought of! The one thing I could still use is any suggestions on soldering to the existing solder pads as I've never done this before. Is it done with a plain soldering iron or some other tool? What temperature did you use?

Thanks so much for the help.

You can look round YouTube for tutorials on soldering, there are many. Any soldering ion that is small enough to independently melt the solder on each pin will be ok. Better to practice on something else first but it's not that difficult. I used a pare of 'helping hands' to hold the wires and PCB of the sensor as that is very small.

I'm assembling the flow meter now but I have a number of questions that I can't find the answer to. But one of those questions is: is there any documentation other than the youtube videos?

Um! I keep meaning to sit down and write instructions on assembly. But I have not done it yet!
Do you have all the printed parts?
I think they fit together quite easily, but I need to upload the wiring diagram, I'm sure without that it will be very hard to complete!

I have completed the sensor assembly and have put together the Arduino/touch screen combination which is running the program fine. I just need to attach the two. The info I need is: 1) better information on the switch and aviation connector so I can purchase the correct ones, 2) the wiring diagram, 3) suggestions on how to best solder the wires from the aviator connector to the touchpad screen (like where to apply heat)

Edit: Also could use information on what the smallest piece (rectangular with two complete holes and a semicircle take out of it)

OK, I will see what I can do, I will get a full wiring diagram made in the next few days and upload it.
I will post the dimensions of the connectors, switch and battery mount I use along with photos.
That smallest pace is for the sensor end, it holds the wire in place with two 3mm bolts. Without this the wire would get pulled out of the sensor and or the movement may let water inside.

Have a look, I added some files, updated the components list and wrote some build instructions. Let me know how it goes and do send me a photo when you finished building it. Good luck.

Yes, my sensor is a smaller version made for 3/4" pipe, and I calibrated it while it was plumbed up to such pipe. In my case, the exact volume needs to be known, but it seems like the number needs to be more relative in your application.

I'm curious how you calibrated the flow sensor. I'm working on a similar project (https://hackaday.io/project/175782-water-flow-monitor) and managed to get my sensor calibrated gravimetrically. Wondering what method you used.

Thanks for sharing. Am I right that your system is to measure flow volume (litres of water pumped per unit of time)?

I'm working on a sensor that can be put on a poll to measure flow speed in a river. In your case it should be easier as the pipe sensor you have is designed for measuring volume. I brought quite a few different pipe sensors to test out but settled on the largest one I could find as that was able to give better readings in slow moving water.
I'm not aware of anything to calibrate on the sensor. It just pulses once for every rotation of the prop and you can use that combined with knowing the width of the pipe to measure how much water passes through.
I will update my project here once I get the hardware sorted out a bit more about the accuracy of the unit. However for my use case it dose not need to be very accurate, it's just for teaching children how to do data collection in the field (the data they collect is not going to be used for anything).