GB1439598A - Fluid flow measuring apparatus - Google Patents

Abstract

1439598 Measuring fluid flow-rate electrically D E ABBOTT and C L SMITH 4 June 1973 [6 June 1972] 26395/72 Heading G1N A fluid flowmeter, for use in spirometry, includes a sensor tube 12; a vane 14 mounted within, and rotatable about an axis 16 of, the tube; and an electrically conductive sensing element 18 mounted in the tube and encapsulated or enclosed in a moisture-impervious material, the element being positioned adjacent the periphery of the area swept by the vane on its rotation about its axis, the arrangement being such that movement of the vane relative to the element as a result of passage of fluid through the tube produces a change in the capacitance between the sensing element and the sensor tube. In a preferred embodiment the vane is of a plastics material coated with aluminium and weighs less than one-tenth of a gram. The encapsulating material may be silicon varnish. The element is connected to a transducer unit 30 to amplitude modulate the output of a 2MHz oscillator connected to a demodulator to provide a single pulse output each time the vane passes the element or probe 18. The pulse output from the transducer is fed to a pulse shaper e.g. a Shmitt trigger set to provide a constant amplitude pulse of frequency dependent upon vane rotation speed. This pulse is differentiated and fed to a terminal 40 of a circuit, Fig. 3, for determining the 'tidal volume' or 'vital capacity'. The signal at terminal 40 triggers a monostable mv. 42 so that each of its output pulses enables transistor 44 whereby a reference current from a source +V flows to charge capacitor 52, the monostable pulse width and time constant 50, 52 being chosen so the rise in voltage across the capacitor is linear. The capacitor is maintained at zero voltage by FET 76, until the vane starts to rotate when the pulses from terminal 40 act with transistors 66, 70 to apply a negative voltage to the gate of the FET 76 whereby it goes OFF and capacitor 52 starts to charge to represent the tidal volume being measured. At the end of a breath when vane rotation ceases the FET 76 again short circuits the capacitor. The 'tidal volume' is indicated by meter 58 at the output of amplifier 54 which may also be coupled to a peak sample and hold circuit. If the measurement is taken when the person has inhaled and exhaled fully then the reading on meter 58 or in store (78), Fig. 5 (not shown), is the 'vital capacity'. If a timer is substituted for control circuit 64 a succession of readings can be accumulated for a given time, e.g. one minute, and the total is the 'minute volume'. Fig. 4 (not shown), embodies a circuit including a threshold detector and alarm (92) for patient monitoring. Fig. 5 (not shown) allows the measurement of 'forced expiratory volume', 'maximum forced expiration rate' and 'lung efficiency'.