Experimental Assessment of a Variable Orifice Flowmeter for Respiratory Monitoring

Accurate measurement of gas exchanges is essential in mechanical ventilation and in respiratory monitoring. Among the large number of commercial flowmeters, only few kinds of sensors are used in these fields. Among them, variable orifice meters (VOMs) show some valuable characteristics, such as linearity, good dynamic response, and low cost. This paper presents the characterization of a commercial VOM intended for application in respiratory monitoring. Firstly, two nominally identical VOMs were calibrated within +10 L.min(-1), to assess their metrological properties. Furthermore, experiments were performed by humidifying the air, to evaluate the influence of vapor condensation on sensor's performances. The condensation influence was investigated during two long lasting trials (i.e., 4 hours) by delivering 4 L.min(-1) and 8 L.min(-1). Data show that the two VOMs' responses are linear and their response is comparable (sensitivity difference of 1.4%, RMSE of 1.50 Pa); their discrimination threshold is <0.5 L.min(-1), and the settling time is about 66 ms. The condensation within the VOM causes a negligible change in sensor sensitivity and a very slight deterioration of precision. The good static and dynamic properties and the low influence of condensation on sensor's response make this VOM suitable for applications in respiratory function monitoring.