Linearity and frequency response of Fleisch type pneumotachometers.

Some aspects of the linearity and the frequency response of flow measurement with Fleisch type pneumotachometers are considered. The transition from a uniform velocity distribution at the entrance of the capillary tubes of the transducer to a fully developed flow profile is discussed as one of the phenomena influencing the linearity of the set-up. Besides, an expression is derived for the frequency response of the Fleisch pneumotachometer. The frequency responses as derived from measurements show good agreement with theory. In the range of the lower frequencies the frequency response of the flow measuring set-up is proved to be largely determined by the frequency response of the pressure transducer with the connecting tubes. A simple way is indicated to measure the latter. This, in combination with the calculated frequency response of the flow transducer, makes it fairly easy to determine the frequency response characteristics of a flow measuring set-up containing a Fleisch type pneumotachometer.