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
If a tube is long compared to it’s diameter, what kind of flow is likely to be present?
Laminar flow.
This is why at low flows, the flow across a bobbin in a rotameter is laminar. The bobbin makes the gas behave as it flowing through a tube, rather than an orifice
What is the Bernouille principle?
Velocity is higher and pressure is lower at the point of constriction in a tube. Due to conservation of energy.
At the point of constriction in a tube, which is higher, velocity or pressure?
Velocity is higher
Pressure is lower
(bernouille principle)
How does low haematocrit affect blood flow?
Low haematocrit results in low viscosity which improves flow (but predisposes to turbulence)
In laminar flow, is resistance dependent on flow?
No. In laminar flow resistance is constant and independent of flow.
Whereas in turbulent flow resistance increases with flow in an exponential manner.
Can flow through an orifice be laminar?
No, flow through an orifice is always turbulent
What is pressure?
Force per unit area
OR
Work per unit volume
What can pressure be expressed as?
Height of fluid in a column
What is the electrical equivalent of pressure?
Potential difference (we substitute pressure for voltage into Ohm’s Law, where current is flow and resistance is the same)
What is the relationship between resistance and radius in laminar flow?
Resistance is inversely proportional to the fourth power of the radius
What is the relationship between resistance and pressure drop in laminar flow?
Flow is proportional to the pressure drop therefore resistance is inversely proportional to it.
What is the shape of Bourdon’s Gauge?
As pressure increases, the cross-section becomes more circular.
What law does Bourdon’s Gauge make use of?
Charles’ Law
How can Bourdon Gauge’s be used for temperature measurement?
They can use Charles’ Law to measure temperature
Can Bourdon Gauges be recalibrated easily?
No- they’re very difficult to recalibrate
What is 1 bar equal to?
1 bar = 1 atmosphere = 1035 cmH20 = 760 mmHg = 760 Torr
Why are readings in mmHg numerically lower than cmH2O?
Due to Hg’s greater density (13 x denser than H2O)
