I discovered this article about CamelBak Flow Meter: http://www.trailspace.com/blog/2009/07/22/outdoor-retailer-camelbak-flow-meter.html
First I thought what a brilliant idea, since as everybody I sometimes asked myself : "how much water left ? could I take one more sip if I want to have a soup tonight ?".
Then I remember calibrating pace lenght to calculate distance…
It is very simple to calibrate your water bladder. I did the exercice on a evening, drinking between 10 and 5 sips by series and calculate the weight difference between each serie of drinking.
The result is quite precise. For me an average sip is 30 ml, 10 sip = 300 ml and 5 = 150 ml, with precision of 10 ml on these series.
Very simple trick when water is on shortage and you need to monitor. You need to do the calibration before but it is pretty reliable.
I don't think I will buy the flow meter ;-)
