11.2 Zero error of vernier calliper

  Basic Mathematics and Measurements

If the zero of the vernier scale does not coincide with the zero of the main scale when jaw $Q$ touches $P$, then the instrument has an error called zero error.

Note: Zero error is always algebraically subtracted from the measured length.

Types of zero error.

1. Positive zero error: If the zero of the vernier scale lies to the right of the main scale, then the zero error is called as positive zero error. (As shown in fig. (b)).
$${\text{Positive zero error}} = \left( {M.S.R. + n \times L.C.} \right)$$
where,

$n:$ vernier scale division which coincides with any main scale division.

2. Negative zero error: If the zero of the vernier scale lies to the left of the main scale, then the zero error is called as negative zero error. (As shown in fig. (c)).

Illustration: Calculate the positive and negative zero as shown in the figure given below.

Process of taking zero error reading is same as the vernier scale reading.

(a) For positive zero error:
$${\text{Positive zero error}} = M.S.R. + n \times L.C.$$$$R_+ = 0 + 7 \times 0.1\,mm$$$$R_+ = 0.7\,mm$$

(b) For negative zero error:
$${\text{Negative zero error}} = - \left( {M.S.R. + n \times L.C.} \right)$$$${R_ - } = - \left( {0 + 6 \times 0.1\,mm} \right)$$$${R_ - } = - 0.6\,mm$$

Correction of zero error

True reading $=$ Observed reading $-$ zero error