Fluids and pressure Flashcards
How do a rotameter and pneumotachograph differ?
Rotameters are variable orifice flowmeters whereas pneumotachographs are variable pressure, constant orifice flowmeters
What is the pressure drop across the bobbin in the rotameter?
Constant pressure drop
Why is the flow turbulent at high flows and laminar at low flows in the rotameter?
This is because the annulus of the bobbin is narrow, compared to its length against the flowmeter wall.
Therefore it tends to act as a tube, and flow tends to be laminar. If the annulus was wide and the length was short, it would behave as an orifice and therefore flow would be turbulent.
What are rotameters lined with?
Gold - antistatic
What does Reynold’s number mean?
If the number is >2000 then flow is likely turbulent
if it is <2000 it’s likely laminar.
What is the critical velocity for flow?
Critical velocity is the gas velocity at which laminar flow changes into turbulent flow.
What happens to flow/Reynold’s number if you breathe in helium?
Helium decreases the density of inspired gas so reynold’s number is reduced and therefore turbulent flow is more likely.
Is Reynold’s number dependent on tube thickness?
No
What does the lumen of the Fleish pneumotachograph do?
The lumen expands into a large number of smaller tubes
What does the pneumotachograph have across it’s resistor?
It has a sensitive differential pressure transducer across the resistor. The pressure gradient across the transducer allows flow to be calculated.
In the pneumotachograph, will output be affected by gas viscosity?
Yes.
Flow is affected by viscosity, therefore the pressure difference across the transducer will also be affected.
What does water vapour do to the pneumotachograph?
Water vapour can block differential pressure transducers, or smaller tubes in a Fleisch pneumotachograph.
Therefore it can cause erroneous readings.
What Law is the pneumotachograph working on the principle of?
Poiseuille’s law
Which kind of flow does the Hagen-Pouiseulle equation relate to?
ONLY laminar flow
How does temperature affect Reynold’s number?
Increasing gas temperature will reduce it’s density - that reduces Reynold’s number and flow is more likely to be laminar
