What is the lowest flow rate I can measure with open channel flow techniques?
As a practical matter it is difficult to resolve open channel flows below about 5 gallons per minute. Most depth sensing techniques have a built in resolution of about +/- 0.01 feet and the most sensitive systems can only resolve depths to around +/- 0.005 feet. In addition the relationship between depth and flow through flumes begins to deviate from the theoretical rating when there is less than 0.1 foot of head available in the flume. When we apply these two facts to the smallest flumes in the various families of flumes we consistently come up against a practical lower limit around 5 gallons per minute.
