For low flow rates a positive displacement sensor is required normally.
Water co meters will accurately measure the rate of a bath being filled, they will also accurately measure a dripping tap.
Unfortunatley they are not cheap compared to turbine meters.
http://www.sailsmarine.com/ItemDetail.aspx?c=220039&l=g&cc=GB#.VoW1F6btmko
Using one of these "backwards" with a rotary sensor may work but i suspect only for relatively high pressure as they are rather "torquey"
BTW this thread is 2 years old better to start a new one.