# 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
• 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
• 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 platinum 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

• Accurate
• Fast

• 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 2019-07-18