Hello, I'm interested in building a small non-invasive battery powered device that wraps around a pressured 1/2" pipe for measuring potable water. From https://en.wikipedia.org/wiki/Magnetic_flow_meter and watching youtube videos, it seems they all use electrodes which contact the liquid (water in my case) to measure the potential difference generated, hence making this an invasive technique. I was wondering if it's possible to just have the electrodes sit outside the pipe and still pick up the electric field? I'm going to be measured flow rates of 0.1 to 20 litres per minute. Quite low. If not what are my realistic options? Is ultrasound the only real way to go? Thanks a lot!