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In flow measurement, flow meter error is typically reported by a percentage indicating non linearity of the device. This can be expressed as either a +/- percentage based on either the full range capacity of the device or as a percentage of the actual indicated flow.
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Accuracy of full scale
If a meter’s accuracy is based on a percentage of its full reading capability, the error is a fixed value. For example; an error of 0.5% of full scale, in a 100-gallon per minute (gpm) device is +/- 0.5 gpm. This is the uncertainty all of the time, so as you move away from the full scale capability, the 0.5 gpm error becomes a much larger percentage. At 50 gpm, you are risking a 1% error. At 10 gpm you have a potential 5% error.
Accuracy of reading
If the device has an error expressed as a percentage of the actual flow, then a 0.5% error of 10 gpm is only +/-0.05 gpm; a 10 times better result.