Design and Construction of a Pulsed Ultrasonic Air Flowmeter

The construction and specific function of a new ultrasonic flowmeter are described. The mean velocity of the respiratory air flow is calculated by measuring the transit times of short ultrasonic pulse-trains simultaneously transmitted upstream and downstream at a 650 Hz rate. The flowmeter system consists of a control unit and a separate flowhead. The former includes the power supplies, a controlling microprocessor, most of the signal processing circuitry, and three analog outputs for flow, volume, and temperature, respectively. The flowhead contains the respiratory tube with a constant circular cross section (length 90 mm, diameter 20 mm, dead space 35 cm3), a fast temperature sensor, two electronic circuits for processing of flow and temperature data, and a sound transmission channel with two capacitive ultrasonic wide-band transducers. This respiratory air flowmeter is extremely fast (response time 1-2 ms) and accurate, with low noise (below 9ml/s), with a wide flow range (bidirectional from 0 to 9 l/s) and with a linear frequency response up to 70 Hz.