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FRCA Flow (Principles (Venturi effect (Introduction of a constriction in a…

- - - - Wright's vanemeter/respirometer
        
        Air flow directed to one side, spins a vane and a light counter measures number of oscilations
        
        Problems
        
        Inertia means it will under-read at low flow
        
        Momentum will mean it will tend to over-read at high flows
      - Peak Flow meter
        
        Variable orifice to measure movement of a vane at maximum velocity
      - Mechanical flow transducers
        
        Flow on a disc will cause bending of its pin which is read on a strain gauge
    - - Velocity of USS signal is increased by gas flowing alongside in same direction.
        Also used in echocardiography as some signal is reflected by some is refracted forwards (Doppler probes)
      - Time difference between signal detection is calculated
    - - Pneumotachygraph
        
        Based on Hagen-Pouseille
        
        Small tubes create fixed known resistance. Pressure difference sensed by transducer
        
        May incorporate heating element to prevent moisture buildup
        
        Problems
        
        Increased WoB
        
        Tubes may block and increase resistance
    - - Gas flowing past a heated element acts to cool it. Processor converts this into velocity
    - - Rotameter/Flowmeter
        
        Variable orifice tube, calibrated for gas
        
        Top of tube: high velocity, Reynolds number high, more turbulent
        
        Bottom of tube: low velocity, Reynolds number low, laminar flow
      - Potential failures
        
        May be blocked by dirt in tube
        
        Static electricity may cause bobbin to stick
        
        Tube may be cracked or not completely vertical
        
        Back pressure from ventilator
    - - Variable pressure, variable orifice: Water-sight flowmetr
        The flow meter system to the left would have had a plain glass jar, filled with water to the level of the bar. Below that bar, each flow meter down-tube is perforated at intervals with holes. In use, increasing gas flow would depress the water in the tube causing gas to escape from increasingly low holes. These levels were calibrated in units of flow which would be read from a card attached to the bottle
      - Constant pressure, constant orifice: Bubble flowmeter
        The most basic configuration of a bubble flowmeter is a calibrated tube, a stopwatch, and a good eye. The bubble is timed as it moves up the calibrated tube between two markings.