Accuracy class

In ISO 4064 and OIML R 49, accuracy class 1 or 2 sets a meter's maximum permissible error; class 1 is the tighter. Older tenders still ask for Class B or C, wording from ISO 4064-1:1993, where the class fixed the low-flow range against nominal flow. The R ratio has replaced it.

Old Class B or C wording has no single R-ratio equivalent across meter sizes. Writing the accuracy class and the R ratio into a tender lets every bidder quote against the same requirement.