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Accuracy: % Full Scale vs. % Reading

The accuracy (really inaccuracy) of mass flow instruments is specified in one of two ways, either accuracy as a percentage of full scale (% FS), or accuracy as a percentage of reading (% RD).

If an instrument has accuracy specified as % FS then the error will have a fixed value no matter where the flow is in the flow range. Take, for example, an instrument calibrated for a flow of 100 ln/min with stated accuracy 1.0% of FS. At a flow of 100 ln/min (full scale) the error will be 1% of full scale, or +/- 1 ln/min. As the flow moves way from full scale the error will still be 1% FS (+/- 1 ln/min), so at a flow of 50 ln/min that error of +/- 1 ln/min becomes a larger percentage (+/- 2%) of flow. Going further away from full scale flow further increases the error as a percentage of flow; at a flow of 10 ln/min the +/- 1 ln/min error is +/- 10% of the flow.

If, however, an instrument has accuracy specified as % RD then the error will always be the same percentage of the actual flow. Using the 100 ln/min instrument again as the example, but this time with a stated accuracy of 1% RD, at 10 ln/min of flow the error is only +/- 1% of the flow, better by 10 times.