What is the difference between “Accuracy of reading” and “Accuracy of full scale”?

These two claims are only equal at the maximum capacity of the measuring device. 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 gal/min 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. On the other hand, 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.