We have already introduced the vernier caliper as a simple but brilliant tool for taking precise measurements which a normal ruler is incapable of. The error is commonly about 0.05 mm which is pretty accurate. In this article we will be taking an in depth look at the main features of a caliper and how to use one to take a reading. If you’re looking for a shorter version check out this article.
Understanding the Caliper:
A Vernier caliper consists of two sets of jaws. The larger ones are used to hold objects between them and measure their outer diameter while the smaller ones on top are inserted into an opening to measure internal diameters. There is a fixed main scale and a secondary, sliding scale attached to the jaws which is known as the vernier scale.
The scales on a vernier caliper are marked and look similar to any ordinary ruler. The main scale is marked in inches or centimetres and the vernier scale is usually graduated in divisions representing 1/10 of the smallest main scale division. So for a caliper with 1 mm as the smallest reading on the main scale, the vernier scale’s divisions would represent 0.1 mm.
Once you have determined the value of one division, you can also find out the distance represented by the smallest divisions on the vernier scale. This can be done by dividing the value of one division by the number of lines present in between. For example if there are ten divisions per marking in the vernier scale then the smallest division will represent 0.1/10 = 0.01 mm.
Preparing the Object to be Measured:
The first thing to do before measuring anything is to first clean the object that needs to be measured and ensure that there is no dirt or anything that can touch the jaws and interfere with the reading.
Preparing the Instrument:
In order to ready your vernier caliper for measurement, first loosen the lock screw positioned on the upper side of the caliper by turning it counter clockwise.
The next and most important step then is to close the jaws and check if the zero mark on the vernier scale aligns with the zero on the main scale. If not then you have to account for the zero error. If the zero mark on the vernier scale is aligned with a mark after zero on the main scale it is known as a positive zero error and if it is aligned before the zero it is known as a negative zero error.
For example if the zero mark aligns with 1 mm on the main scale then you need to subtract 1 mm from the final reading. However if the zero lines up before the main scale zero then you will need to note the position of another mark that aligns somewhere after the main scale zero. Then slide the jaws so that the two zeros align. Next note the distance travelled by the other mark. This will be the value of the zero error. For example if the other mark was previously on 0.4 and ended up at 1.2 then the zero error will be 1.2-0.4 = 0.8 mm. This value should then be added to the final reading.
Taking A Measurement:
Place the object in between the two jaws such that they touch opposite ends of the object making sure the object is held firmly but don’t press too tight. If you need to measure an internal diameter, then insert the upper jaws in to the cavity and open them till they touch the sides. Tighten the locking screw to hold the jaws in position.
Note the position of the vernier scale zero on the main scale. The main scale reading is the division just before where the zero mark of the vernier scale is aligned. So is the zero mark aligns just after the fifth division between 3 and 4 the main scale reading is then 3.5.
The next step is to take the vernier scale reading. To do this find the mark on the vernier scale which lines up perfectly with a mark on the main scale. The vernier reading can then be found by multiplying the least value of the vernier scale with the number of divisions till that mark. For example if the least value is 0.01 mm and the 7th mark of the vernier scale is lined up perfectly then the vernier scale reading is 7 x 0.01 = 0.07.
The final step is to add the main scale and vernier readings to get the final measurement. For example 3.5 + 0.07 = 3.57 mm.
