The standard needs to be _____ times more accurate than the tool being calibrated.

• A. 2
• B. 4
• C. 6
• D. 8

Answer: (B)

When calibrating a tool, you want the reference standard to be significantly more accurate than the device under test. A common guideline is that the standard should be four times more accurate. This keeps the standard’s own measurement uncertainty from dominating the calibration results and helps ensure reliable traceability.

Think of it in terms of uncertainty: the overall uncertainty in a calibration combines the uncertainties of the instrument being calibrated and the reference standard. If the standard’s uncertainty is four times better (one quarter the error) than the instrument’s, its contribution to the total uncertainty is relatively small. For example, if the tool has an uncertainty of about 1 unit, a standard with an uncertainty of about 0.25 units makes the combined uncertainty only a bit more than 1 unit, rather than being strongly affected by the standard’s error. If the standard were only two times more accurate, its bigger uncertainty would contribute more to the total, reducing the precision gained from the calibration.

So, four times more accurate is the best general rule for the standard in calibration.