Answer: Most accuracy specifications for Test Equipment are listed as % rdg + % fs.
Example: 0.1% rdg + 0.1% fs
This is a percentage of the reading or measurement + the percentage of the Full Scale for the range you are in.
If a DC Electronic Load has only one range, say 80 amps and the Current Measurement Accuracy rating in the Spec is 0.1% rdg + 0.1% fs. Let’s say you are only reading 8 amps. The accuracy would be as follows:
(8 amps reading x 0.1%) + (80 amps range x 0.1%) = a tolerance of +/- 0.088 Amps.
If the same tolerance is applied to a Load with Multiple Ranges (like the Chroma 63640-80-80, which has 3 ranges, 0.8 amps, 8 amps and 80 amps), and you set the current to 8 amps, you are in the 8 amp range. The accuracy of the current measurement would be, (8 amps reading x 0.1%) + (8 amps range x 0.1%) = a tolerance of +/- 0.016 amps.
You can clearly see that having multiple ranges greatly improves the total accuracy of the measurement. 0.088 amps for the single range load versus 0.016 amps for the multi range load.
The Chroma Load Accuracy is 5.5 times greater than the load with a single range.