A transistor based air flow transducer for thermohygrometric control of neonatal ventilatory applications

An air flow transducer for controlling heated humidifiers used in neonatal artificial ventilation, suitable for in-line application in monopatient breathing circuits, is described here. The sensor is built with two nominally identical bipolar junction transistors, with different packages, as hot elements operated at a regulated constant voltage. The operation principle is based on the differential convective heat power exchanged with the fluid stream due to the different thermal conductivities of the transistors' packages. The underlying theory is described in mathematical terms and the theoretical model is validated with experimental data in the mass flow rate range from 4 to 215 mg s-1. The nonlinear behavior allows sensitivities from -5 mV/ (mg s-1) at flow rates in the range of 4-130 mg s-1 to -2 mV/ (mg s-1) at higher flow rates up to 215 mg s-1. The linear range extends from 40 to 130 mg s-1, with constant sensitivity equal to -5 mV/ (mg s-1). The differential nature of the output allows to obtain repeatabilities in the order of 2% for fluid temperatures between 20 and 25 °C and of about 6% if the fluid temperature lies in the range of 15-35 °C. The relatively long time constant, in the order of 20±5 s, makes the sensor suitable for average flow rate measurements. Using the sensor's output as a control variable of a heated humidifier for artificial ventilation, the relative humidity of gases varies by only 20% in the flow rate range of the sensor (from 95% to 75%), whereas the same parameter shows a variation of about 40% (from 100% to 60%) with the same humidifier without flow control.