6. General Aspects of Pressure Flashcards
Define pressure.
Pressure is the force applied per unit area. Its SI unit is the pascal (N/m2).
What is force?
Force is a vector quantity
that can cause an object with
mass to accelerate.
Newton’s second law defined
force as the mass of an object
multiplied by its acceleration.
Its SI unit is the newton.
One newton will accelerate a
1 kg mass at
1 m/s2 in a vacuum.
Gravity gives any object
an acceleration of 9.81 m/s2
making one newton
equivalent to a 102 g weight.
This is a small pressure when applied
to a squared metre area,
so pressure is normally
expressed in kilopascals (kPa).
What other units of pressure are there?
1 bar is equivalent to:
> 1 atmosphere
> 14.5 lb/in2 (psi)
> 30 inches of Hg
> 101 kPa
> 760 mmHg (torr)
> 1020 cm H2O
Are you more likely to dislodge a blockage in a cannula when using a 2 or 20 mL syringe?
A 2 mL syringe,
as pressure is force over area.
The 2 mL syringe has a smaller
cross-sectional area so
the force applied by the thumb is spread
over a smaller area
generating a higher pressure.
For this reason care must be taken
when injecting fluids with a
small syringe as the high pressure
generated could cause tissue damage.
What is the difference between partial and total pressures?
Dalton’s law states that in a gas mixture
the pressure exerted by each
individual gas is equal to the pressure
that gas would exert
if it occupied the container alone.
This is the partial pressure
(the term ‘tension’
refers to the pressure
exerted by a gas
dissolved in liquid).
Total pressure is the sum of all
the partial pressures in the mixture.
Is atmospheric pressure constant?
No.
Atmospheric pressure is created
by the force of gravity
acting on the molecules that
make up the atmosphere
and therefore atmospheric pressure
will depend on the
height of the atmosphere
and
its density.
This means that atmospheric pressure
will fall with altitude
and rising temperature.
As a consequence,
the partial pressure of the gases
that make up the air will also fall,
leading to low partial pressures of oxygen at altitude.
What is gauge pressure?
Gauge pressure refers to
pressure measurements
above or below atmospheric pressure.
An empty cylinder pressure
will have a gauge pressure of zero.
What is absolute pressure?
Absolute pressure refers to
pressure measurements incorporating
atmospheric pressure –
it is gauge pressure plus atmospheric pressure.
An empty cylinder will have an absolute pressure of 1 bar.
Is blood pressure an absolute or gauge pressure?
Blood pressure is a gauge pressure.
How do manometers work
> A manometer consists of a
fluid-filled column,
which is open to the atmosphere
and therefore reads gauge pressure.
> Gravity acts on the fluid
to produce a pressure,
which is dependent on
the density
+
the height of the fluid
but independent
of the cross-sectional area of the column
(pressure = height × density × gravitational force).
> They are used to measure low pressures.
> Inaccuracies can be caused
by surface tension.
This leads to an over-reading
in water manometers
and
an under-reading
in mercury manometers
(in practice this has no clinical significance as a 6-mm-wide column of water will only over-read by 0.04 kPa).
> They can be made more sensitive by
using fluids with a low density
(e.g. a pressure of 1 kPa will
support a column of mercury 7.5 mm high
or a column of water 10.2 cm high).
What are barometers?
> Barometers are closed
to the atmosphere
and therefore measure absolute pressure.
> A mercury barometer has a
Torricellian vacuum above it,
which contains mercury vapour
at its saturated vapour pressure (SVP).
> Barometers like the
Fortin’s barometer
and Goethe’s device
can be used to measure
sub-atmospheric pressures.
Here, the height of the
measuring column falls
rather than rising
and this principle was
used to predict bad weather;
low atmospheric pressures would
cause a fall in the height of the
fluid in a ‘thunder tube’,
predicting an oncoming storm.
How do aneroid gauges work?
> Aneroid gauges
(from the Greek meaning ‘no water’)
such as the Bourdon gauge are used
to measure high pressures
where manometers would be impractical
(e.g. to measure the pressure of a
137 bar oxygen cylinder you would
need a mercury column 104 m high
or a water column 1394 m high!).
> A Bourdon gauge consists
of a coiled metal tube linked
to a cog and a pointer.
The cross-sectional area of the
tube is elliptical and
when exposed to increases
in pressure it changes to a
circular cross-sectional shape,
which causes the tube to uncoil,
moving the pointer across a scale.
