By David W. Spitzer

A Coriolis mass flow meter with an accuracy of 0.1 percent of the flow rate, an upper range limit of 300 kilograms per minute (kg/min) and a zero stability of 0.01 percent of upper range limit is calibrated to operate between 0 and 100 kg/min. What is the approximate performance of the flow meter at 30 percent of its full scale flow?

A.    0.1 percent
B.    0.1 percent of actual flow rate
C.    0.2 percent
D.    0.2 percent of actual flow rate
E.    0.3 percent
F.    0.3 percent of actual flow rate

Answers A, C and E are incomplete performance statements and can be deemed inappropriate. The zero stability of the flow meter is 0.01 percent of 300 kg/min, or 0.03 kg/min  independent of the flow rate. The flow meter is operating at 30 percent of its 100 kg/min full scale, or 30 kg/min. The overall flow meter performance is the sum of the accuracy and the zero stability  in like units.

One way to calculate the overall performance is to add the performance specification expressed as a percentage of flow rate to the zero stability similarly expressed as a percentage of flow rate. In this application, the performance specification is 0.2 percent of flow rate, and the zero stability is 0.1 percent of the flow rate, as calculated by

100 ∙ 0.03 = 30

Answer D is correct.

### Additional complicating factors

The performance of this Coriolis mass flow meter varies with the flow rate. For example, operating the flow meter at 15 kg/min will increase the zero stability to 0.02 percent of the flow rate — degrading the overall performance to 0.12 percent of the flow rate. This may not appear to be much of a concern, but operating the flow meter at much lower flow rates can significantly degrade its performance. In many gas applications not considered here, flow meter performance can be well above 1 percent of the flow rate.