Accuracy is a measure of how close to true or actual flow the instrument indication may be. It is often expressed as a percent of full scale (FS) and/or of reading (“rate”). Accuracy at a particular flow rate may be an order of magnitude better than rated flow range accuracy. Accuracy is specified for a given turn-down, r flow range. Generally speaking, the accuracy of a flow meter decreases with increased turn-down.
Unless a flow meter can be proved by another device, such as a “master meter” or a “prover” traceable to the National Bureau of Standards (NBS), now called National Institute of Standards Technology (NIST), accuracy is often difficult to prove. Furthermore, improper installation may adversely affect the accuracy of a flow meter.
The term “accuracy” may not always appear on a vendor’s performance specification sheet but may be expressed as “repeatability” and “linearity.” These two terms are discussed below.