The Review of scientific instruments
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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 differential nature of the output allows to obtain repeatabilities in the order of 2% for fluid temperatures between 20 and 25 degrees C and of about 6% if the fluid temperature lies in the range of 15-35 degrees 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.