Thanks for the posts, I was originally going to use a water flow meter (with a rotary sensor in it) but I think the tape will be much better at measuring water in the bladder. Quick question though, how quickly will the tape respond to changes in the water level?
Or alternatively should I use this:http://www.seeedstudio.com/depot/g12-water-flow-sensor-p-635.html
Could this be used as both a valve and a water flow sensor, and is it sensitive enough to detect changes in the water flow accurate to a ml, also would inertia effect it?
As for a valve, I have been reading around and I wouldn't mind trying a pinch valve as a cheap and not power hungry alternative, but how effective are these kind of valves at not letting drips through (because this will really be on a drip by drip basis)? If the pinch valve is the way to go, can anyone recommend a clamper?