1. Meaning of measurement error

The purpose of measurement is to determine the true value of the measured object. However, in the measurement process, due to the error of the measuring instrument itself and the constraints of the measurement method and measurement conditions, the measured value is often not the true value of the measured object. There must be a difference between the two. This difference between the measured value and the true value of the measured object caused by the imperfection of the measurement is called measurement error.

2. Sources of measurement error

The sources of measurement error are multifaceted, and the main sources are as follows:

Standard instrument error

Standard instruments such as linear scales and gauge blocks also have errors, such as the line error of the linear scale, the manufacturing error of the center length of the gauge block, and their verification errors. The error of standard instruments is one of the main sources of measurement error.

Measuring instrument error

The error of measuring instrument refers to the inherent error of the measuring instrument.

Principle error The error caused by replacing the theoretical working principle with the approximate actual working principle when designing a measuring instrument is called principle error. For example, the error caused by replacing the theoretically required nonlinear scale with a linear scale. The principle error is often corrected during the design, so it can be ignored.

Abbe error The error caused by the structure of the measuring instrument or the placement of the workpiece violating the "Abbe principle" is called Abbe error. The "Abbe principle" is the principle that the measured length and the reference length are placed on the same straight line.

Manufacturing and adjustment errors The manufacturing and adjustment errors of each component of the measuring instrument will cause measurement errors. For example, the manufacturing and installation errors of the dial, the pitch error of the micrometer screw, the magnification and discrimination errors of the optical system, and the errors of various parameters of electrical components.

Reading method error The reading method error caused by the device that uses a pointer or an indicator line to align the scale for counting consists of three parts: alignment error, parallax, and estimation error.

Measurement method error Different measurement methods have different sources of measurement errors.

Function error of indirect measurement When using indirect measurement, the measurement error needs to be calculated according to a certain functional relationship.

Error caused by measuring force When using contact measurement, in order to ensure reliable contact, a certain measuring force must be applied to the probe. The measuring force will cause elastic deformation or other state changes (such as changes in clearance, friction, etc.) of the workpiece and the parts of the measuring instrument, causing measurement errors. This error can be ignored in general measurements, but it should be analyzed, estimated, and corrected when necessary in precision measurements or measurements of small sizes and soft materials. The calculation formula for the indentation amount in contact measurement.