When the OMR examination pattern was introduced, it permitted to use Pencils in order to change the answers. Use of pencil allowed the candidate to erase the marked wrong answers.
On referring the above table, it is clear that optical devices and scanner detect the darken part from HB onwards. Therefore the use of HB pencils was compulsory. Use of 2B-4B and 6B will create smud and spilling of graphite dirt over the sheet leading to unnecessary marks overte sheet.
The earlier system was newly introduced, the system was not much developed and candidates were allowed to change the answers by erasing wrong and mark the right one. But some candidates claimed tampering of theiranswer sheet by changing their answers marked with pencils. Nowadays the number of candidates appearing for the exam has increased. Now the candidates are not allowed to change the answers as the examining body wants everything to be simple and safe for everyone. Now the candidates are well instructed about the rules before the exam. The candidate should be confident about the answers marked. Therefore, it is suggested to use black or blue pen and use of pencil is not allowed.
The above diagram will clear the concept of the use of HB pencil. The range of darkness of pencil varies from 6H to 6B or it can be understood as darkness level -6 to +6 with HB being at the zero point or standard optimal darkness.
So when a 6B pencil is made, it involves the use of pure graphite which is a natural element or mineral and is obviously costlier than its synthetic replica. It’s same as the case between original diamonds and American diamonds. So a 6B pencil is costlier than HB pencil and a HB pencil is costlier than a 6H pencil. Now these pencils are made in small scale industries and there is no standardization as such.
So as to lower the selling price many companies sold 2H synthetic pencil lead and labelled it as HB. The students are unaware of this thing and they buy this type of fake HB pencils. The answers marked with such pencil create problem while scanning the OMR sheet.
Now if the candidates change the answer by erasing the wrong answer, in such case there is a difference between completely erased answer and partially erased.If a mark has been erased it should be erased completely such that the scanners and optical devices set on high sensitivity and looking for darkness are not able to pick it up.
The improperly erased mark is detected as a mark by the scanner. So when an optical mark recognition system is set to respond to lighter HB pencil marks. Then the marks left after improper rubbing of dark pencil marks are also enough to be detected by the scanner. If we set the sensitivity to skip the improperly erased marks, then even the light pencil marks will get skipped.
To avoid this situation, the examining body suggested to eliminate the use of Pencil and the candidates are instructed to be sure when they mark the circles with Pen.