Laboratory conditions have shown thin-plate weirs to be accurate to within +/-2%.  However, unlike flumes, which are more forgiving to variances in upstream conditions and device conditions, weirs are quite susceptible to these variations.  Also, weirs require more maintenance than flumes.  Under field conditions, accuracies of +/- 5-15% are more common.

In particular, a weir's crest must be in good condition – without nicks or abrasions and free of clinging debris.  Since Portable Weir Plate sets are usually moved from site to site, it is important to check each plate's crest before use as they may have been nicked or damaged during transit (probably rolling around in the back of a pick-up bed).

The upstream weir pool must also be of the proper size and kept free of sediments, debris, and trash that may change the weir pool's depth.