When using a thin-plate weir to measure flow rate, a number of errors can occur which affect overall measurement accuracy.
The errors can be grouped into: Sizing, Manufacture, Installation, and Use.
- Weir notch undersized/oversized for flow rate
- Out of dimension notch opening
- Weir crest too thick (crest thickness should be no more than 1/8-inch [3.175 mm]).
- Weir not vertical
- Weir not horizontal
- Weir pool not wide enough
- Weir crest not set at least 2Hmax above weir pool floor
- Insufficient upstream straight run
- Poorly formed approaching velocity profile
- Submergence due to downstream conditions
- Weir notch not centered in flow stream
- Ice buildup
- Crest ragging
- Crest damage, nicks, abrasions
- Weir pool sedimentation
- Crest rounding
- Incorrect zero reference elevation
- Incorrect point of measurement