Guaranteed accuracy and limiting error: A 0–25 A ammeter has ±1% of full-scale accuracy. If it reads 5 A, what is the limiting error expressed as a percentage of the measured value?

Introduction / Context:Guaranteed accuracy is often specified as a percentage of full-scale (FS). When the reading is far below full-scale, the same absolute error corresponds to a larger percentage of the reading. Understanding this distinction is crucial for correct error reporting and for deciding appropriate instrument ranges.

Given Data / Assumptions:

• Ammeter range: 0–25 A.
• Accuracy: ±1% of full-scale.
• Measured current: 5 A.

Concept / Approach:

Absolute error at any reading = (accuracy% of FS) * FS. Limiting error as a percentage of the measured value = (absolute error / reading) * 100%. This converts a full-scale-based tolerance to a reading-based percentage, which is commonly requested in test reports.

Step-by-Step Solution:

Verification / Alternative check:

If the meter read 25 A, the same ±0.25 A would be ±1% of reading, matching the specification. Lower readings inflate the percent-of-reading error, which is a key reason to select the smallest meter range that safely covers the expected value.

Why Other Options Are Wrong:

• 2%, 2.5%, 4%: underestimate the impact of applying a full-scale accuracy to a sub-range reading.
• 1%: confuses accuracy-of-FS with accuracy-of-reading.

Common Pitfalls:

• Reporting “±1%” at all readings without converting to percentage of the actual reading.
• Mixing up full-scale and of-reading specifications.

Final Answer:

5%