Question

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

Answer 1

Due to mechanical errors, sometimes when the anvil and spindle end are brought in contact, the zero mark of the circular scale does not coincide with the base line of main scale. It is either above or below the base line of the main scale, in which case the screw gauge is said to have a zero error. It can be both positive and negative.

It is accounted by subtracting the zero error (with sign) from the observed reading in order to get the correct reading.

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

Answer 2

If the zero of the circular scale does not coincide with the zero of the main scale (pitch scale), this is known as zero error. There are two types of zero error -
(a). If the zero of the circular scale remains below the line of graduation then it is called positive zero error
(b). If the zero of the circular scale lies above the line of graduation then it is called negative zero error
For positive zero error correction, the zero error should always be subtracted from the observed reading
For negative zero error correction, the zero error must be added to the observed reading.