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.