In an ideal case, when the flat end B of the screw is in contact with stud A, and if the zero mark of the circular scale coincides with the baseline of the main scale, the screw gauge is said to be free from zero error.
But sometimes, due to mechanical error, in bringing stud A in contact with stud B, the zero mark of the circular scale is either below or above the baseline of the main scale, then the screw gauge is said to have a zero error.
There are two types of zero error —
- Positive zero error and
- Negative zero error.
The zero error is accounted for by subtracting the zero error with its sign from the observed reading.
Correct reading = Observed reading – Zero error (with sign)
