A transistors-based, bidirectional flowmeter for neonatal ventilation: Design and experimental characterization

A bidirectional, low cost flowmeter for neonatal artificial ventilation, suitable for application in mono-patient breathing circuits, is described here. The sensing element consists of two nominally identical bipolar junction transistors employed as hot bodies. The sensor working principle is based on the convective heat transfer between the transistors, heated by Joule phenomenon, and the colder hitting fluid which represents the measurand. The proposed design allows the sensor to discriminate flow direction. Static calibration has been carried out in a range of flowrate values (from -8 L·min^-1 up to +8 L L·min^-1) covering the ones employed in neonatal ventilation, at different pipe diameters (ie., 10 mm and 30 mm) and collector currents (i.e., 500 mA, 300 mA, and 100 mA) in order to assess the influence of these two parameters on sensor´s response. Results show that the configuration with a pipe diameter of 10 mm at the highest collector current guarantees the highest sensitivity (i.e., 763 mV/Lmin1 at low flowrate ± 1 L-min^-1) and ensures the minimum dead space (2 mL vs 18 mL for 30 mm of diameter). On the other hand, the 30 mm pipe diameter allows extending the range of measurement (up to ±6 L-min 1 vs ±3.5 L· min^-1 at 10 mm), and improving both the discrimination threshold (<;0.1 L·min-¹) and the symmetry of response. These characteristics together with the low dead space and low cost foster its application to neonatal ventilation.