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.

What is the difference between accuracy of reading and accuracy of full scale? 

Accuracy of Reading (%RD) ensures that the error remains a consistent percentage relative to the actual flow rate. 
This means that the precision is proportional across the entire flow range.

Accuracy of Full Scale (%FS) guarantees that the error is a fixed value, unaffected by the flow’s position within the range. 
The variation can be expressed as a constant flow rate (for example, l/min), rather than as a fluctuating percentage.

Accuracy %RD from our MASS-VIEW calibration certificate