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Accuracy & Repeatability

What is accuracy of a mass flow meter, what is repeatability of a mass flow meter, and what difference does the difference make?

First let’s define the terms. Accuracy of a mass flow meter is the expression of error (%) as compared to a primary standard. In practical terms it is the degree to which the actual output of the flow meter matches the calibration curve of the flow meter.

Repeatability of a mass flow meter is the amount (%) that any single device can repeat the value previously delivered at the same conditions. In practical terms this means that the output seen at a given set of conditions may, or may not, match the calibration curve of the instrument but it will regularly be the output seen at that same set of conditions.

A highly accurate instrument requires high repeatability, but a highly repeatable instrument does not require high accuracy.

So, why does it matter? Increasing accuracy tends to come at an increasing cost which can be justified if required in the application. High repeatability with lower accuracy may be a cost effective solution for a given application. Clearly identifying the requirements of an application will allow the best instrumentation recommendation to be made.