What do you mean by zero error of a screw gauge? How is it accounted for?

In an ideal case, when the flat end B of the screw is in contact with the stud A, and if the zero mark of circular scale coincide with the base line of main scale, the screw gauge is said to be free from zero error.

But sometimes, due to the mechanical error, on bringing the stud A in contact with stud B, the zero mark of the circular scale is either below or above the base line of the main scale, then the screw gauge is said to have a zero error.

There are two types of zero error —

1. Positive zero error and

2. Negative zero error.

The zero error is accounted by subtracting the zero error with its sign from the observed reading.

Correct reading = Observed reading - Zero error (with sign)