The foremost attribute associated with vernier calipers is precision and accuracy of measurement. Vernier calipers provide a higher degree of accuracy in measuring lengths than comparable tools with most common metric calipers able to measure values in centimetres up to 2 decimal places i.e an accuracy of 0.01 cm. This is possible due to the vernier scale which is attached to the main scale and provides that extra decimal place in the measured reading.
Vernier calipers can be used to measure a variety of dimensions of an object such as internal and external diameters or length and depth. The key is to understand how to read a vernier caliper in the proper way and that is what we are going to discuss in this article.
We will assume that you have taken the necessary steps before measurement and positioned the calipers correctly according to the required dimension which needs to be measured and now the scales of the caliper have moved a certain distance to indicate the measured value. To read this value the first step is:
Obtain Main Scale Reading:
The Main scale is obviously named because it is the primary scale of a vernier caliper. It is graduated in millimetres or inches. In case of metric units the smallest value that can be measured by the main scale is 1mm. To find out the main scale reading look at the value on the scale directly on the left of the zero mark of the vernier scale. In the example below the value immediately before the zero mark is 2.1 cm.
The main scale has given us a number but it is not the exact value of the measurement. In the example we can see that the exact value is slightly greater than 2.1 but how much? This will be determined by the vernier scale reading.
Find The Vernier Scale Reading:
The Vernier scale is graduated in either 20 or 50 increments. The total length of the scale represents 1 mm. so that gives a least count of 0.02 or 0.05mm respectively. Reading the vernier scale requires a bit of observational skill and since it is the most important step of the process we suggest you do this very carefully. The method is simple. Just look for the line on the vernier scale which is perfectly aligned with mark on the main scale. This value is the vernier scale reading. For example in the above picture the line corresponding to 3 on the vernier scale aligns perfectly so the vernier scale reading is 0.3mm.
The final step is to simply add the two values to obtain the final reading. In this case it is found to be:
An important consideration while dealing with vernier calipers is to check for zero errors and adjust accordingly.
Zero Error Adjustment
The Zero error can be checked by closing the jaws of the caliper without any object in between. When the jaws are in close contact with each other, check the position of the zeros on the main scale and the vernier scale. These should align with each other but if they don’t then you have a zero error. Zero errors can be avoided through proper maintenance and care of the vernier caliper. However if they do occur they can be compensated for easily by just measuring the value of the zero error.
To do this you have to close the jaws of the caliper and note the value of the vernier scale which coincides with the main scale just like when reading the vernier scale. This value should then be either added or subtracted from the total reading.
If the vernier zero is to the right of the main scale zero when the jaws are closed then the error is a positive zero error. This means that the final value will be more than the actual value so the zero error needs to be subtracted from the final value.
In the picture shown above the value of the zero error can be found by looking at the aligned mark on the vernier scale. This is the 6th mark so the value is 0.6 mm. So for a zero error of 0.6mm our final value from the previous example would now be:
If the zero on the vernier scale is to the left of the main scale zero then the error is a negative zero error and the total value will be less than the actual value. So the error will need to be added to the final reading.
The best way to get a firm grip on the method is to look at a few more examples and practice questions and compare your answers. Then do it with a real vernier caliper with objects whose dimensions you already know. With a little practice reading a vernier caliper will become as easy as reading a normal scale.