1. The materials to be measured by the thickness gauge have different structures and shapes. On workpieces with different structures, the magnetic field distribution will vary with the structure and shape, resulting in measurement errors.

2. The material being tested itself contains magnetism. During the processing of some materials or certain process requirements, there is a residual magnetic field in the material being tested. Due to its uneven distribution, the resulting measurement errors are also inconsistent, and the measurement values of certain parts of the same workpiece suddenly become larger or smaller.

3. Different parts of the same material may also produce changes in the magnetic field. For example, the magnetic field distribution at the edge and middle area of the material is different, which may cause measurement errors.

4. Depending on the properties of the material being measured, the magnetic flux will be different, which is also one of the reasons for the error.

5. Different sizes and thicknesses of materials can also lead to measurement errors.

6. The surface of the material being tested is not smooth enough, which is also the cause of the error.