The components of measurement uncertainty are conventionally [VIM 93] grouped into two categories. The first one, type A, is estimated using statistical methods, and the second, type B, is estimated using other methods. It is a priori based on laws. In fact, the person operating is responsible for assessing the sources of errors. Although the manufacturers provide data such as the class of the device, the standard, and the resolution, we should have sound knowledge based on experience. Combination of both categories A and B gives the compound uncertainty