A pulsed-wire gas flow meter using a singaround system and a constant temperature heating method was developed for the spirometry with a wide range of flow rate.
A wire located at the upstream was heated by a pulse current and a traveled heated air was sensed by the other wire. The sensed signal triggered the next pulse current so that the system could selfoscillate. The repetitive pulse was period was determind by the traveling time of heated air from the heated-wire to sensor-wire.
The current of pulsed-wire was electronically controlled so that heating wire was kept in constant temperature.
The transducer was constructed in a rigid pipe of inside diameter of 35mm. The heated wire was tungsten wire of 5μm and placed a metel mesh screen with gauze to make flow velocity uniform.
Two wires of 5μm tungsten wire used for seneing, one was for detecting temperature compensation.
The range of flow rate was from 0.05l/s to 12l/s with 12mm wire spacing and from 0.02l/s to 0.5l/s with 4mm wire spacing.
The stability of the pulse frequency in ambient temperature air gas was within 2 percent. The influence of each gas composition (CO
2, N
2, O
2, of 100%) was within 5 percent to the air.
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