CHAPTER 13 - SPECIAL MEASUREMENT METHODS IN OPEN CHANNELS
3. Deflection Meters
These meters are out of production for irrigation use. However, some are still in use. These meters have some advantages, and they may come back into production. Deflection meters consist of a shaped vane(s) that projects into the flowing water to sense velocity. A secondary device measures the deflection caused by the force of the flow. These meters can be installed permanently or may be easily moved from one location to another. In use, they hang into the flow and are supported on pivots. Vanes can be shaped to match the flow section geometry to make them deflect the same amount for any given discharge regardless of the depth of flow in the flow section.
This attribute is a considerable advantage where the head-discharge characteristic of the channel is unstable. Flow sections with permanent pivots can be installed at various locations in an irrigation district, and the vanes can be transported from measuring station to measuring station.
Under ideal conditions, deflection meters have been found to be accurate within 2 percent. Generally, this accuracy will not be attained because field conditions are seldom ideal. For example, wind can produce errors up to 100 percent. However, a windbreak made from a piece of plywood will substantially reduce this kind of error.