Design and characterization of a bidirectional, low cost flowmeter for neonatal ventilation

In this paper, we propose a novel, low cost flowmeter suitable for application in disposable breathing circuits. The sensor consists of two nominally identical transistors employed as hot sensing elements, placed into a pipe where the fluid flows. The working principle is based on the convective heat transfer between the transistors, heated by Joule effect, and the colder hitting gas. The proposed design allows the sensor to discriminate flow direction. The sensor response has been numerically simulated, and the results validated by experimental trials varying the pipe diameter (i.e., 10 and 30 mm), the flowrate values (ranging from -8 to 8 L/min), and the collector current (i.e., 100, 300, and 500 mA). Experimental results show that the configuration with a pipe diameter of 10 mm at the highest collector current guarantees the highest mean sensitivity (1143 mV/L . min(-1)) at low flowrate (i.e., +/- 1 L/min); in addition, this configuration ensures the minimum dead space (0.5 versus 5 mL for 30 mm of diameter). However, the 30-mm pipe diameter allows extending the range of measurement (up to +/- 8 versus +/- 3.5 L/min at 10 mm), and improving both the discrimination threshold (< 0.1 L/min) and the symmetry of response. The response time of the sensor is 340 ms. These characteristics together with the low dead space and low cost foster its application to neonatal ventilation.