I'm new to Arduino and I'd like to build a something to measure water flow with a hall effect sensor which I have. Mostly measure cumulative amount. Do I use a Uno R3 or is there a smaller Arduino that will work? Also, what shield do I need? This is going to be used in my RV to track water usage .
The Arduino Uno board is the easiest board to start with. The Arduino Nano is smaller and still has a usb connector. The Pro Mini is very small, but has no usb connector.
There are prototyping shield. So you can solder connectors and some components on that shield.
What kind of water flow sensor are you using ? I don't understand how you want to use the hall sensor.
It's a hall effect sensor. Put out pulses as water flows through it. Want to count pulses to measure flow.
The best way is to connect the hall sensor to an Arduino input. Use that input as interrupt. http://arduino.cc/en/Reference/attachInterrupt Count the pulses in the interrupt routine. That way no counts will be missed.
This is such a project: http://www.freetronics.com/pages/jaycar-water-flow-gauge You can use that sketch as a guide. The sketch is good.
It looks the same as this project: http://www.practicalarduino.com/projects/water-flow-gauge