3. Frequency response: In commonly occurring problems, the flow variables exhibit fluctuations consisting of a variety of frequency components. The probe will however have an upper limit of frequency beyond which it does not recognize signals. The information content of these signals is hence ignored during the measurement. For practical reasons there is also a lower limit of frequency below which measurement is not performed. This enables data acquisition to be carried out in a reasonable period of time.

Consider a flow whose frequency spectrum in the measured variable is as shown in Figure 1.5. A probe is required to be designed or selected that has an upper frequency out-off Hz. Each measurement process must be continued for a duration of seconds so that at least 5 cycles of the slowest harmonic is included in the averaging process. Also, the data sampling rate of the measurement system must be at least . This step ensures that the highest harmonic is represented by at least two points. This requirement is also applicable to the FFT algorithm, that faces errors due to aliaising. With and suitably chosen most of the energy in the spectrum will be included in the calculations and measurement proceeds at a convenient speed.