Influence of the length of lead lines on the response of a variable orifice meter: analysis of sensitivity and settling time

Gas flow measurements are pivotal in several medical applications. For instance, mechanical ventilators and applications for respiratory monitoring need flowmeters with strict requirements: they must be accurate, with adequate dynamic response, high sensitivity (especially if used for neonatal purposes), and they must be almost insensitive to the composition of the gas Only few types of flowmeters are may be used in these applications. Among differential pressure flowmeters, characterized by good static and dynamic responses, variable area orifice meters (VAOMs) are gaining large acceptance in applications related to respiratory monitoring, estimation of respiratory function, and mechanical ventilation. VAOMs consist of two main parts: a primary element (basically a restriction), and a secondary element (i.e., a differential pressure transducer). The installation of the primary and the secondary elements can strongly influence the input-output relationship of VAOMs and can introduce relevant bias error. The aim of this study was twofold: i) the experimental assessment of the influence of the lead lines length (L_L) on the calibration curve of a variable orifice meter; ii) the experimental analysis of the influence of L_L on the step response of the flowmeter in terms of settling time. Results show that the value of L_L influences both the static response and the step response: regarding the static response, the sensor sensitivity significantly decreases with L_L (e.g., the sensitivity decreases from 5.3 Pa/L·min^-1 to 4.0 Pa/L·min^-1, when L_L increases from 4 cm to 182 cm); concerning the step response, the flowmeter increases the settling time from approximately 20 ms up to 60 ms using L_L values of 4 cm and 182 cm, respectively. The findings of this study can be useful to figure out the impact of the L_L value on the sensor response; in addition may provide useful information to correct the sensor response if it is used in condition of installation different from the one used during the calibration.