2012A10 Outline the principles of a pneumotachograph. What factors affect the accuracy of this device?

 

List:

·      Principles

·      Types

·      Sources of error

 

Principles:

Definition

·   Device that measures flow rate

·   Fixed orifice device

·   Flow: rate of passage of fluid from one point to another, units m3/s

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 radius4)

·   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, 1-2cmH2O

 

Pneumotachograph types:

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)

 

Sources of error:

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-H2O -> ↓condensation

Changing gas mixture

·   (e.g. N2O, volatile, heliox)

·   ↑ viscosity -> ↑ resistance -> falsely ↑rate

·   ↑ density -> ↑increasing likelihood of turbulent flow

 

 

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