Choosing measuring instruments

I can choose and explain measuring instruments and methods with suitable range and precision, including the linked quantity-specific determinations.Syllabus
Syllabus K323, 1(e). Choose and explain measuring instruments and methods with suitable range and precision, including the linked quantity-specific determinations.

Make a guess. You are not marked.

You need the diameter of a thin wire, about 0.5 mm across. Which instrument gives the most precise reading?

A micrometer screw gauge, which reads to 0.01 mm.
Vernier calipers, which read to 0.1 mm.
A metre rule, which is long enough to measure the whole wire.

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A micrometer screw gauge, which reads to 0.01 mm.

A micrometer reads to 0.01 mm, so a 0.5 mm diameter is measured to two more figures than a ruler could give. Vernier calipers read only to 0.1 mm.

Choose an instrument that can cover the measurement and distinguish the detail you need.

A metre rule is useful for a book, but its divisions are too coarse to measure the thickness of one sheet of paper well. A micrometer can resolve a small thickness, but cannot directly span a room.

Range: The span of values the instrument can measure, such as 0 to 150 mm for a pair of calipers.
Resolution: The smallest change you can read from the scale or display, such as a 0.01 mm display step.
Precision: How closely repeated measurements agree. Readings with little spread are precise, but may all share the same error.
Accuracy: How close a measurement is to the true or accepted value. Checking the zero and using the instrument correctly help avoid a consistent offset.

A smaller display step does not guarantee an accurate measurement. A digital balance could show the same value repeatedly while its zero is wrong. Its readings would agree closely but remain displaced from the true value.

Match the instrument to the object

These are example ranges and resolutions. Check the actual instrument's markings and specification before choosing it.

Metre rule: 0 to 1 m; 1 mm divisions.
A book or a straight rod. Align the rule with the length and read both ends.
Measuring tape: 0 to 5 m; 1 mm divisions.
A room or a longer object. Keep the tape straight and taut; a sagging tape follows a longer path.
Digital calipers: 0 to 150 mm; 0.01 mm display step.
A small external diameter or thickness. Keep the object square to the jaws and close them gently.
Digital micrometer: 0 to 25 mm; 0.001 mm display step.
A thin wire or sheet. Use the ratchet as instructed so that excessive pressure does not squash the object.

Digital calipers may also have inside jaws for an internal diameter and a depth rod for a hole. Choose the correct contact surfaces. For a diameter, align the measurement across the widest part through the centre; tilting the instrument can give the wrong length.

Ask three questions before measuring

Will it fit? Check the object's expected size against the instrument's range.
Can I distinguish enough detail? Check the divisions or display step against the required measurement.
Can I use it properly? Consider alignment, access, the object's shape and whether contact could deform it.

For a very thin sheet, another useful approach is to measure the thickness of a closely packed stack and divide by the number of sheets. This gives an average thickness; gaps or compressing the stack would affect the result.

Once an instrument is chosen, use sensible measurement methods to read it, check its zero and record the result.

Check your understanding

A rod is about 120 mm long. You need an instrument with a reading step of 0.1 mm or smaller. Which is suitable for measuring its length directly?

Digital calipers with a 0-150 mm range and 0.01 mm resolution.
A micrometer with a 0-25 mm range and 0.001 mm resolution.
A metre rule with a 0-1000 mm range and 1 mm divisions.

Show the answer

Digital calipers with a 0-150 mm range and 0.01 mm resolution.

120 mm is within the 150 mm range, and a 0.01 mm reading step is smaller than 0.1 mm. You must still check the zero and align the jaws correctly; the display step does not guarantee accuracy.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

Choose the most suitable instrument (metre rule, vernier calipers or micrometer screw gauge) to measure each of these: (a) the length of a laboratory bench, about 1.5 m; (b) the internal diameter of a test tube, about 1.6 cm; (c) the diameter of a copper wire, about 0.5 mm. Give one reason for choice (c). [3]

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