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 Sizing

• Weir notch undersized/oversized for flow rate

Weir Manufacture

• Out of dimension notch opening
• Weir crest too thick (crest thickness should be no more than 1/8-inch [3.175 mm]).

Weir Installation

• 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

Weir Use

• Ice buildup
• Crest ragging
• Crest damage, nicks, abrasions
• Weir pool sedimentation
• Crest rounding
• Incorrect zero reference elevation
• Incorrect point of measurement