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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