Principles of Flow Measurement

Franklin L. Scamman, MD

Physics Series

November 26, 2002

Principles of Flow

• Q = Pressure/Resistance

• Laminar

• Turbulent

Vertical Flow Tube

How a Vertical Flow Tube Works

Tandem Flow Tubes

Rotameter

Positive Displacement- Narcomed

Rotameter- Ohmeda SE

Pneumotach

Flow Measurement Principle

P1

P2Counterpressure sensor

flowin

flowout

Electronic flow measurement principle- laminar flow restrictionscreate a pressure difference P1 - flow is calculated from P1

Used with Oxygen, Side gas, Bypass, and Agent flows

Laminar flow restriction

P1Ambientpressure

P2

P1 = pressure difference

P2 = correction (off-set) to ambient pressure

Clinical Spirometry• Pitot tubes measure the

pressure difference created by the gas flow

• Knowing the amount of pressure difference allows the gas module to calculate flow

• Flow is mathematically integrated over time to calculate volume

Pitot Tube Principles

Agent AdjustmentBypass flow measurement

Agent flowmeasurement

AladinTM cassetteTemperature sensor

Cassette ID Magnets

Agent proportional.valve

Inflowclosevalve

Outflowclosevalve

One wayvalve

liquid flowprevention

Restrictor

Agent Adjustment

1

2

3

3

Cardiac Thermodilution

Mathematics of Thermodilution

• Quantity = Volume X Concentration

• Volume = Flow X Time

• Quantity = Flow X Time X Concentration

• Flow = Quantity / (Time X Concentration)

Plot of Washout Curve

High and Low Cardiac Output