PNEUMOTACHOMETERS

Differential manometer: Small resistance Pressure drop Change in pressure is measured using differential pressure transducer.

Hot wire anemometer: Small heated element in flow path current maintain the element at constant temperature Output proportional to the gas flow that cools the element

FLEISCH PNEUMOTACHOMETER: Rolling a thin sheet, corrugated metal with a plain strip of metal. Inserted within a metal cover. Small, parallel metal channels O/p : pressure drop across the element is proportional to the flow rate of the gas.

Fleisch pneumotachometer:

Transducers: Capacitive type pressure transducer More stable Less vibration sensitive The viscosity of a mixture of gases, X =fraction of gas = viscosity

Calibration: Volume flow rate can be done by passing a known gas flows through it. Reported as BTPS Body temperature Ambient pressure Saturated with water vapour 37o C , to prevent condensation Warmer is electrically isolated

Venturi type pneumotachometer: Replaces the linear resistance element Pressure drop is square of volume flow Drawbacks: non-linearity of calibration Requirement of laminar flow

Turbine type pneumotachometer: Air flowing through the transducer rotates a very low mass (0.02 g) turbine blade Rotation of the turbine interrupts the light beam from LED. Phototransistor produces a train of pulses.

Turbine type pneumotachometer:

Measurement of volume: Volume in single breath Total volume in a given time Output voltage is proportional to the pressure difference at the manometer input The output

Measurement of flow and volume:

Flow volume curve: Instantaneous maximum expiratory flow rate versus volume Shape of the curve obstructive airway disease Time consuming and expensive

Area of the flow volume: Area under the maximum expiratory flow volume curve (AFV) is a sensitive indicator of lung function impairment. VT = total volume exhaled during the time.

Nitrogen washout technique: Indirect determination Nitrogen concentration is plotted versus volume during the expiration.

Pulmonary function analyzer: Nitrogen analyzer Vacuum pump X-Y recorder Digital displays Other electronic circuits

Pulmonary function analyzer:

Impedance pneumography: Indirect measurement Measurement of thoracic impedance change signal

Impedance pneumography:

Impedance pneumography: Disadvantages: Electrode position, body posture Accuracy is still inferior to other methods