Gas Flow

Describe the measurement of flow, pressure and volume of gases

Types of Flow:

  • Laminar flow
    Fluid moving in a steady manner without turbulence.
  • Turbulent flow
    Irregular fluid movement in radial, axial, and circumferential axes.
    • Laminar flow is more efficient than turbulent flow, as it requires a smaller pressure gradient to generate the same flow
    • For two fluids moving at the same speed, the velocity of individual particles in laminar flow will be both higher and lower
  • Transitional flow
    Mixture of laminar and turbulent flow. Flow is typically turbulent in the centre, and laminar at the edges.

Devices used to measure gas flow include:

  • Variable-Orifice Flowmeters
  • Fixed-Orifice Flowmeters
    Pneumotachograph.
  • Hot wire flowmeter

Note orifice based flowmeters rely on the Hagan-Poiseuille Equation:

  • Viscosity () and length () are fixed by both devices
  • Fixed orifice flowmeters also fix radius (), such that the change in pressure must therefore be proportional to flow:
    , where is a constant
  • Variable orifice flowmeters also fix pressure (), such that flow can be calculated from the radius:

Flowmeters

Constant pressure, variable orifice flowmeters are found on wall and cylinder gases. They consist of:

  • An inverse conical tube (i.e. narrower at the bottom, and wider at the top)
  • A needle valve
  • A bobbin
    May have a groove which causes the bobbin to spin, confirming it is not stuck.

Method:

  • Gas flows from the bottom to the top of the tube
  • The bobbin obstructs flow
    Therefore there is a pressure difference across it.
    • Remember:
    • At equilibrium, the pressure exerted by the bobbin on the flow of gas () is equal to the pressure exerted by the gas on the bobbin
  • As flow is increased, the bobbin is pushed further up the flowmeter due to the increased pressure
  • The bobbin will reach a new equilibrium position when the orifice of the flowmeter has become wide enough for the pressure on the bobbin to equal the pressure of gravity

  • Flowmeters are calibrated for individual gases as:

    • Laminar (typically low flows) flow is proportional to viscosity
    • Turbulent (typically high flows) flow is proportional to density

Pros

  • Cheap
  • No additional power supply required
  • Accurate
    Readings may be altered by:
    • Change in temperature affects viscosity and density of gas
    • Change in pressure affects density of gas

Cons

  • Must be vertical
  • Bobbins can become stuck

Pneumotachographs

Constant orifice, variable pressure flowmeter. Several different designs exist, and include:

  • Fleisch pneumotachograph
    • Consists of several fine bore parallel tubes placed in the gas circuit
      Decreased radius and increased resistance reduces gas flow velocity, improving laminar flow.
    • A differential pressure transducer is placed at either end of the tubes
    • The pressure drop across the tubing is directly proportional to flow
  • Pitot tubes
    • Consists of two tubes placed into the gas circuit:
      • One faces into the gas flow
      • The other faces away from the gas flow
    • The pressure difference between tubes is proportional to flow

Pros

  • Accurate
  • Continual measurement
  • Allow calculation of volumes

Cons

  • Increased resistance
  • Increased dead space
  • Require laminar flow
    Inaccurate when:
    • Flows are higher than what the system is designed for
    • Alteration in gas density
      • Change in gas mixture
    • Alteration in gas temperature

Hot Wire Flowmeter

Components:

  • Two fine platium wires in the gas circuit
    • One heated to 180°C at OL.min-1
    • One at 0°C
  • Ammeter

Method:

  • As gas flows, the wire cools
  • Rate of heat dissipation is proportional to gas flow
  • The amount of current required to return the wire to 180 is measured, and is proportional to flow

Pros

  • Accurate
  • Fast

Cons

  • Fragile

References

  1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
Last updated 2017-10-04

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