The stated accuracy of RBC flumes is +/-2% as stated by Replogle, Bos, Clemmens, and Wahl (R, B, and C respectively). However, practical considerations such as approach flow, installation, and dimensional tolerances make it free-flow discharge precision to be +/-5% - similar to that of other flumes.  

As with all primary device applications, actual site accuracy is dependent upon proper approach, site, and downstream conditions.  Flow meter accuracy is in addition to the primary accuracy listed above.