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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