Sources of Error in Measurement

The process of measurement involves locating the probe in the flow field, an act that invariably changes the flow itself. Hence the measured quantity is only an approximation of the local value of interest. Besides this, other sources of error are:

1. Spatial resolution: The probe is never a point and the probe response is always an average over a finite volume. The issue of probe size becomes crucial in micro-environments; examples are flows set up in electronic circuit boards for the purpose of cooling, and micro-electro-mechanical systems.

2. Temporal response: With a step change in the velocity it would take an asymptotically large time for the probe to fully sense the new state. Hence any reading taken in finite time is really a cut-off value. The transients depend on the inertia and damping present in the probe and the measurement system. They could be monotonic or oscillatory. Typical probe responses are shown in Figure 1.4.