Response to pulsatile flow of a miniaturised electromagnetic blood flow sensor studied by means of a laser-Doppler method.

Abstract

Electromagnetic (e.m.) flowmeter systems are commonly used in physiological experiments. Little information, however, is available about their accuracy in a time-dependent flow field. In miniaturised sensors especially the magnetic flux density cannot be made uniform, which may result in a non-ideal response to axisymmetric flows. The measured flow rate may therefore differ from the actual flow rate owing to the varying shape of the velocity profile. To study this effect unsteady flow experiments were performed to relate the e.m. flowmeter reading to the flow rate deduced from laser-Doppler anemometry. The experiments were performed in a straight circular tube with an internal diameter of 4 mm. The fluid (saline) flow was fully developed, laminar and pulsatile with flow reversal occurring near the wall in certain phases of the cycle. The frequency of the pulsations varied from 0·2 to 2 Hz. The fluid velocity was measured with a single component laser-Doppler system. The velocity profiles obtained were integrated to obtain the instantaneous flow rates and compared with those measured electromagnetically (Transflow 601). The results show no significant differences in the mean volume flow rates (averaged during one cycle). For momentary flow rates the differences are hardly significant. Small but briefly significant differences were found in the instantaneous flow rates, the largest deviation (7·8%) being found at flow reversal. Variation of the pulsation frequency (by a factor 10) or the mean flow rate (by a factor 4) has no significant effect on these differences.