As an instrumentation and flow meter supplier, we've provided magnetic flow meters or paddlewheel flow sensors for such applications; both options can offer good accuracy if sized correctly
Magnetic flow meters typically require a pipe full of flowing liquid. This is probably where your problem lies. Magflow meters are also restricted to liquids where conductivity is at sufficient levels to induce measurable voltage but hopefully thisis not an issue
For aggregate mass flow rather than individual components, if the velocity is not too high you may be able to do this with a Coriolis mass flow meter or perhaps industrial mass meter depending on pressure, temperature etc
For an economical option, if your flow rate is no lower than approx 1.0 l/hr then you could consider using a micro oval gear flow meter with a pulse output. An example of such a meter can be found at http://www.icenta.co.uk/product/Low-Flow-Meter-OVAL-Flowmate. Lower flow rates are possible...
