· Principles

· Types

· Sources of error

Definition |
· Device that measures flow rate · Fixed orifice device ·
Flow: rate of passage of fluid from one point
to another, units m |

Use |
· Measure insp and exp flow rate · Measure volume (integrate area under curve) |

Equations |
· Ohm’s law: Flow rate = pressure drop / resistance ·
Poiseuille equation: Resistance = (8 x length
x viscosity) / (π x radius · Reynolds number: Re = (diameter x velocity x density) / viscosity |

Physical basis |
· Fixed resistance, hence pressure drop directly proportional to flow rate · Low Re -> laminar flow ·
Note actual pressure drop usually small,
1-2cmH |

Fleisch |
· Multiple narrow tubes · Narrow diameter -> laminar |

Screen |
· Mesh with large number of narrow apertures · Narrow diameter -> laminar flow |

Pitot tubes |
· One upstream facing and one downstream facing (measures static pressure) · Pressure difference directly ∝ flow rate |

(Hot wire anemometer) |
· Two heated wires in the stream at right angle to each other · Rate of flow is proportional to the degree of cooling of a heated wire. · Cooling affects electrical resistance (increases if semiconductor, decreases if conductor) |

Extremes of flow rate |
· ↑↑ flow rate: ↑Re -> ↑risk turbulent flow -> falsely ↑rate |

Temperature change |
· ↑ viscosity -> ↑ resistance to laminar flow · ↓ viscosity or ↑ density -> ↑Re -> turbulence · FIX: heating for temp control |

Condensation |
· Irregular conduit -> ↑risk turbulence ·
FIX: heating -> ↑ SVP-H |

Changing gas mixture |
·
(e.g. N · ↑ viscosity -> ↑ resistance -> falsely ↑rate · ↑ density -> ↑increasing likelihood of turbulent flow |

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