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Table 3: Types of flow spirometers.1,27 |
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Type of spirometer |
Principle of action |
Advantages |
Disadvantages |
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Pneumotacograph (differential pressure) |
Measure the pressure difference generated by passing a laminar fluid through a known resistance, where flow = Δpressure/resistance The resistance may be a mesh or a tube formed by a set of capillaries; it is usually heated to 37 oC to prevent condensation of water vapour from the exhaled gas |
• Highly accurate at different flow rates • Portable • Automated • Available with disposable sensors |
• Requires recalibration during the same day if ambient conditions change significantly • Accumulation of secretions or condensation of exhaled vapour changes the resistance and hence the flow measurement • Susceptible to resistance contamination if used without a filter. Change in gas composition requires calibration |
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Electronic turbine |
It consists of a helix inside the tube that receives the flow. A light emitting diode (LED) is mounted on one side of the propeller and a photodetector on the other side. Each time the propeller rotates, it interrupts the light from the LED reaching the detector. These pulses are counted and summed to calculate the gas flow |
• Portable • Useful in cardiopulmonary exercise testing (CEPPT) • Automated • Available with disposable sensors |
• At high flows, the propeller is subject to distortion • At low flows, inertia may lead to misestimation of the flow rate • Susceptible to turbine contamination if used without a filter • Fragile moving parts with a tendency to accumulate dirt that impedes free rotation of the turbine |
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Thermistor (or hot-wire thermistor) |
It consists of two metal filaments (usually platinum) heated by an electric current. The flow of gas through the filaments causes them to cool. In one filament, the current increases to maintain a constant temperature; the other filament acts as a reference. The change in current is proportional to the gas flow |
• Portable • No moving parts • Measurement not susceptible to ambient temperature and pressure or fluid viscosity • Automated |
• Sensor resistance connected in series, any modification to the components could be erroneously measured as a flow |
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Pitot tube |
Based on the measurement of the fluid pressure at a given point in the pipe and using the relationship between the pressure and the area the fluid passes through, the fluid flow can be calculated |
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Ultrasonic |
Ultrasound waves travel through membranes on both sides of a tube at an angle to the gas stream. The sound waves speed up or slow down depending on the direction in which the gas is flowing. By measuring the transit time of the ultrasonic waves (which is modified by the passage of the gas) the flow can be accurately measured |
• Portable • Highly accurate • Measurement not susceptible to ambient temperature and pressure or fluid viscosity • Air exhaled by the subject is not in contact with the sensor • Measures molar mass, with several additional applications possible |
• Piezoelectric material very sensitive to shocks or falls • In absence of HEPA filter (high efficiency particle arrester) favours aerolisation • High cost sensors |