Measurement Errors and Standards Questions and Answers

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The class figure fixes the permitted error as a fraction of full scale, so it is a constant number of volts or amperes wherever the pointer sits. Expressed as a fraction of the reading it therefore grows as the deflection falls - 1 per cent at full scale, 5 per cent at a fifth of it.

Answer: C

Shunting a high-impedance circuit alters the very quantity under measurement, and the error can be tens of per cent. The reason is the mechanism, and it is why input impedance appears on every voltmeter's specification.

Answer: D

Correct answer: False, since accuracy depends on calibration and on suitability for the task

Price correlates loosely with quality but guarantees nothing: an expensive instrument out of calibration, or applied where it loads the circuit, gives a worse result than a cheap one used properly. What determines accuracy is a valid calibration, an appropriate range, and a measurement method that does not disturb what it measures.

Answer: A

Correct answer: False, since resolution and accuracy are different properties

Extra digits let the instrument discriminate finer changes, which is resolution. Whether those digits are correct is accuracy, and that depends on calibration. A five-digit meter that is 1 per cent out displays its error to five figures - the additional digits make the wrong answer more precise, not more right.

Answer: B

Correct answer: a best estimate with a stated uncertainty

A result without an uncertainty cannot be used: the reader has no way to judge whether it agrees with another figure, meets a specification, or supports a conclusion. Reporting the best estimate together with its uncertainty, and the basis on which that uncertainty was arrived at, is what makes the measurement usable by someone else.

Answer: C

Correct answer: throws away information the measurement actually contains

Over-rounding is as much a fault as quoting spurious figures, though a less conspicuous one. If the uncertainty is 0.02 then quoting 12.3 discards a digit the measurement genuinely determined. The convention is to round the uncertainty to one or two significant figures and match the result to the same decimal place.

Answer: D

A pointer resting off zero shifts every reading by the same amount, which is the definition of a systematic error, and the adjuster exists to remove it. Checking it before and after a set of readings is standard practice.

Answer: A

Correct answer: a drift during the readings would otherwise go unnoticed

Checking the zero at both ends brackets the readings: if it has moved, the readings taken between are suspect and can be corrected or repeated. Checking only beforehand assumes stability over the whole session, which for a sensitive instrument warming up or in a changing ambient is not a safe assumption.

Answer: B

The correction from the certificate is itself uncertain, and that uncertainty is inherited by every measurement made with the instrument. It is one link in the traceability chain and has to be combined with the others.

Answer: C

Correct answer: Thermal noise in a measuring circuit

Thermal noise is a fluctuating quantity of unpredictable sign, so it scatters readings about a mean and is reduced by averaging. The other three all shift the reading consistently in one direction - an uneven scale, a persistent temperature offset, a pointer off zero - and are systematic errors, correctable but not averageable.