4. Other sources of error: Further errors can occur due to hardware limitations during signal processing. Hence errors can arise during amplification, differentiation, integration and correlation. Errors can also arise during digitizing of analog data. Lastly, the finite precision of the calibration curve generated by curve-fitting and round-off errors during computer-based data reduction will also result in deviation from the true value of a flow variable. These are classified as software-related errors.

Over and above the factors given above, a host of other causes lead to scatter in the values of velocity, pressure and temperature at a given point in the test cell. This can occur even when the test conditions are nominally maintained constant. Some of these causes are variation in supply voltage and room temperature, air currents, building vibrations and roughness of apparently smooth surfaces. Certain flow phenomena, for example, flow states near the critical points of transition, are intrinsically sporadic, and hence not reproducible.

It is impossible to account for each and every one of the factors that contribute to uncertainty since their effect on the magnitude of a flow quantity is far from predictable. Hence, in practice, the magnitude of a flow quantity is always specified along with the observed uncertainty level arising from the test conditions.