Making measurements, Measuring, Length

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Length

In the previous section we talked about the metric system, and used the units of length to show how the different prefixes relate to each other - such as kilo-, deci-, centi- and milli-.

In the building, furnishing and manufacturing industries, measurements are generally recorded in millimetres.

Audio for slide 2 (mp3 |6|KB)

In cases where thicknesses or tolerances of less than a millimetre need to be measured, such as in metalwork and machining, the unit of measure often used is the micrometre.

Audio for slide 3 (mp3 |6|KB)

One micrometre is 1/1,000,000 (one millionth) of a metre, or 1/1,000 (one thousandth) of a millimetre.

Its symbol is µm', which is sometimes written as 'um'.

Audio for slide 4 (mp3 |6|KB)

Many people prefer to use the old fashioned term micron instead of micrometre.

This is designed to avoid confusion with the measuring device called the micrometer, although strictly speaking 'micron' is no longer officially recognised as a term under the SI system.

Audio for slide 5 (mp3 |6|KB)

In practice, microns are often expressed as millimetres to several decimal places.

For example, 500 µm might simply be referred to as 0.5 mm, and 50 µm would be 0.05 mm.

Audio for slide 6 (mp3 |6|KB)

Measuring devices

The most common general-purpose measuring device is the tape measure. Its main advantage is that the long tape can wind up into a pocket-sized unit.

Spring loaded retractable tape measures range in length from tiny 1 metre tapes to the standard 7 to 8 metre tradesperson's tape. Open reel and closed reel tapes, used by surveyors and builders, typically range from 30 metres to 100 metres.

Audio for slide 7 (mp3 |6|KB)

When you look closely at the end of a normal retractable tape, you'll notice that the steel hook is secured with rivets, with a bit of play in the holes allowing it to move back and forth.

The amount of movement allowed is the same as the thickness of the hook.

Its purpose is to compensate for the hook thickness when you either push the tape up against an object for an inside measurement or hook it over the object for an outside measurement.

Audio for slide 8 (mp3 |6|KB)

Other common measuring devices are as follows:

Steel rule - very rugged, good for fine measurements, able to be used as a straight edge.

Audio for slide 9 (mp3 |6|KB)

Folding rule - not so popular these days, but still sometimes used by carpenters.

Audio for slide 10 (mp3 |6|KB)

Vernier caliper - used for measuring thicknesses and diameters very precisely, in some cases to an accuracy of 10 micrometres, or 0.01 mm.

Audio for slide 11 (mp3 |6|KB)

Laser distance meter - measures digitally with a laser beam; can be either hand held or combined with a laser level.

Learning activity

Audio 12 (mp3 |6|KB)

Below are five segments of a tape measure, together with an arrow pointing to a particular length. See if you can read the correct length in millimetres for each one.

Don't be fooled - some of these are harder than they look! Remember, you'll need to take into account the units of measure, and on two of the segments, the previous markings on the left hand side that aren't visible.

This is good practice for the times when you're actually using a tape measure on the job, especially when you're measuring long lengths. It takes concentration to read off the correct measurement without misreading the position of the graduations on the tape.

Enter your answers in the cells on the right hand side, and click on the 'Check your answer' button for each one as you work through.

Ruler	Length indicated	Unit of measure
	2200	mm
	3110	mm
	7328	mm
	1780	mm
	4585	mm

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Unit: Making measurements

Supporting: MSAPMOPS101A Make measurements

Section 2: Measuring

Length

Measuring devices

Learning activity