A bidirectional respiratory flowmeter using the hot-wire principle.

We have devised a bidirectional respiratory flowmeter using the hot-wire principle. The flow-direction sensor consists of a pair of tungsten wires strung parallel to the platinum hot-wire one on each side of the platinum wire. When the gas stream passes through the transducer, the upstream wire is cooled and the downstream wire is heated by the gas stream producing a temperature difference between the two tungsten wires. The difference in resistance thus produced between them is detected and amplified by a differential amplifier whose output serves as a triggering signal of flow inversion. The switching times of the flow inversion of the present instrument are 3 ms during panting and 10 ms during quiet breathing, when the distances from the platinum wire to the tungsten wires are 1.6 mm. Artifacts produced by the delay in switching are practically negligible. The flowmeter can be adapted for many kinds of respiratory flow measurement, except under the condition when inflammable gases are used.