There are many uncertainties in a measuring instrument. These uncertainties restraint the precision of the instrument. The measuring instrument consists of many mechanical, electrical and/or optical components. Each component will contribute to the total random error, and therefore affecting the precision of the instrument. For example, the factors in mechanical components leading to the random errors include manufacturing tolerance, assembly tolerance, and backlash. Similarly, there are some error sources such as heat noise in electrical components.

In addition, the measurement environment also affects the instrument precision. The environment may be subject to temperature change, structure vibration and air disturbance.

Thus, the uncertainties of the measuring instrument and environment factors make the instrument not have sufficient precision.