Section 2 Measuring Overview Now that we’ve covered the main terms used in the metric system and looked at the relationship between fractions, decimals and percentages, it’s time to apply these principles to some workplace calculations. In this section, we’ll start with measurements of length, and work up to volume calculations for three dimensional shapes. We’ll also talk about methods of checking the angles and squareness of manufactured items. And we’ll introduce some of the measuring instruments used in the manufacturing industry and discuss ways of making sure that the measurements you take with them are reliable and accurate.
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. Write your answers in the boxes provided. When you’ve finished, check your answers in the Answers section at the back. 1. 2. 3. 4. 5. Area If you think of length as being one dimensional, that is, going in one direction only, then area is two dimensional, because it has length and width. Let’s have a look at the area of some common shapes. Squares and rectangles The area of any square or rectangle is simply its length times its width. For example, if a rectangle is 3 metres long and 2 metres wide, its area is: Length x width = 3 m x 2 m = 6 square metres (m2) What if you had a sheet of particle board measuring 3.6 m x 1.8 m? Its area is simply: Length x width = 3.6 m x 1.8 m = 6.48 m2 Triangles Let’s say you cut the sheet of particle board in half diagonally, forming two equal triangles. The area of each triangle is exactly half of the original rectangle. That is:
reading the numbers and graduations carefully on a tape measure, to make sure that you are taking off the correct measurement from the correct section of the tape. Below are some more suggestions for making sure that the measurements you take are accurate and correct. Reading dials and gauges A common error when reading dials or gauges is to look at the pointer from an angle to the scale. This means that instead of reading off the mark immediately behind the pointer, you read a mark that is either on the left or right hand side of the pointer. The problem is called parallax error, because ‘parallax’ refers to the way an object seems to change its position when your own point of observation changes. Some instruments have a mirror on the dial so that you can line up the pointer with its own reflection, to make sure that your reading is exactly at right angles to the scale. Even if there isn’t a mirror, the simple solution to the problem is to use one eye only and make sure you’re looking at the scale at exactly 90 degrees. It is even possible to get a parallax error with a ruler or tape measure if the graduations are not hard against the mark or edge that you want to measure. Again, always make sure that your eye is at 90 degrees to the scale when you take the measurement. In the case of the tape measure, you should also push down the steel blade so that the top edge is flat against the object you’re measuring. Calibrating instruments Some measuring instruments need to be calibrated before they are used. This often means adjusting the zero mark on the gauge to correspond with a true zero reading. Weighing scales are an example of an instrument that must be set to zero before you start taking measurements. Some instruments also need to be adjusted differently for particular types of materials. For example, moisture meters for timber use different scales depending on the species of hardwood or softwood being measured. The simplest way of making these sorts of allowances is to have a conversion chart that enables the user to add or subtract amounts to give a ‘corrected’ reading. ‘Measure twice, cut once’