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Wednesday, 18 February 2015

How To Read Vernier Callipers ?

Trained Engineers - 02:55

A vernier caliper is used to precisely measure dimensions to within thousandths of a centimeter. We will use the vernier caliper frequently throughout the year, so it is important that you learn to read the scale properly; a small error in measurement may greatly affect subsequent calculations.

below image is a sketch of a typical vernier caliper, similar to the type you will use in this course. Most frequently you will use the larger “jaws” (A) to measure outer dimensions, such as the width of a block or circular rod . This instrument can also be used to measure inner dimensions, such as the diameter of a hole, using part (B) , as well as the depth of a hole, using part (C). The vernier scale is read the same in each case no matter which portion of the caliper is used take your measurement.

Measurements are taken as follows:
1. Loosen the thumb screw clamp (F), if necessary, and close the sliding jaw so that it fits snugly on the object to be measured. If the object is circular or spherical, make sure you’re measuring at the widest point. When measuring outer dimensions, be sure to use the flat section of the caliper jaws; don’t use the region marked in Figure 1.

2. Gently tighten the thumb screw clamp, and remove the caliper from the object; the screw will allow you to move the caliper without changing the position of the sliding jaw. Hint: You can make a first approximation of the measurement – within 0.1 cm – by laying the caliper on top of a ruler or meterstick and measuring the distance between the jaws.

3. Now look at the scale on the sliding jaw, as shown in the magnified image of this region below:

The measurement up to the first digit after the decimal point is obtained by looking at the mark below the first zero that appears on the sliding scale, as indicated by point (a) in Figure 2 above. In this example you will note that this mark falls between the 5.6 and 5.7 cm mark on the fixed scale, so we know our measurement falls within this range. Therefore, we’ll start our measurement with 5.6.
Note: A common mistake is to begin reading from the edge of the sliding scale; make sure your measurements are taken from point (a), the line directly below the first zero on the sliding scale, or your reading will be short by several tenths of a centimeter!

4. The next two digits are read by carefully finding the mark on the sliding scale (E) that is best aligned with a mark on the fixed scale (D). Several may look as though they line up, but only one will match best (Hint: Hold the caliper at a slight angle away from you, and close one eye!).
In the example above, we see that the second small mark to the right of 4 on the sliding scale is aligned with a mark on the fixed scale, at point (b). The last two digits of your measurement come from the sliding scale, not the fixed scale! Since the marks on the sliding scale are 0.002 cm apart, the mark at point (b) represents 0.044 cm. Adding our total measurement together, we get 5.644 cm as our final reading.

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