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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.

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: % Full Scale vs. % Reading

MASS-VIEW Accuracy

Accuracy %RD from our MASS-VIEW calibration certificate

Digital Variable Area Flow instruments 

Our MASS-VIEW instruments have an Rated accuracy of ±(1.75% Rd + 0.25% FS)
Optional Calibration certificate is available for the MASS-VIEW instruments.
On the product page of the instrument you will find all the specifications: MASS-VIEW

Magnetic Inductive flow meters

Most MAG-VIEW instruments have an accuracy ±0.7 % of reading (RD) and ±0.3 % of range (FS)

The low flow models 0.0083 .. 1 l/min (MVM-001-PA/PN) and 0.05 .. 2 l/min model (MVM-002-PA/PN)
Have an FS accuracy depending on the measuring range:
±1 % of 0...50 % of measuring range.
±2 % of 50…100 % of measuring range.

See the product page of the MAG-VIEW to see the accuracy of all specifications.

Preview Calibration Certificate