Pressure Flashcards
Define force. State and define the SI unit.
Force is that which changes or tends to change the state of rest or motion of an object.
SI unit: Newtons
1 Newton = the force that will give a mass of 1 kilogram an acceleration of 1 meter per second.
N = kg.m.s^-2
What acceleration will the force of gravity acting on an object give. What force will the force of gravity apply to a 1 kg object
a = 9.81 m.s^-2
F = ma
F = 1 kg x 9.81 m.s^-2 F = 9.81 N
What is 1 kilogram weight?
Gravity = 9.81 m.s^-2
Force on 1 kilogram = 9.81 Newtons
As F = ma (Newton first law)
then:
m=F/a
So
m = 1N/9.81 m.s^-2
m = 0.1019 kg weight
m = 102 gram weight
So 1 Newton is the equivalent of 102 gram weight
Define pressure. State and define the SI unit
Pressure is the force applied per unit area.
SI unit: Pascal (Pa)
1 Pascal = a force of 1 Newton acting over an area of 1 square meter
P = f/a
What are the other units of pressure and how are the units interconverted?
1 Bar = Standard Atmosphere = 101.325 kPa = ± 1 bar 14.5 psi 100 kPa 1020 cmH2O 750 Torr 750 mmHg
1 kPA =
- 5 mmHg
- 2 cmH20
List and describe 5 clinical applications of force and pressure.
- Relative difficulty of injecting a liquid from a large syringe and a small syringe.
- Danger during intravenous regional anaesthesia (Bier’s block) as if the cannula is close to the double torniquet pressures might exceed the torniquet pressures leading to local anaesthetic systemic toxicity. –> 100kPa = 750 mmHg
- Bed sores in immobilised patient.
- Pressure limiting/relief/reducing valves
APL valve (Adjustable Pressure Limiting Valve) --> diagram: downward spring with Force F onto diaphragm with area a into breathing system with Pressure P. The downward force can be adjusted by increasing the downward force on the diaphragm hence increasing the pressure within the breathing system.
- Discuss the clinical application of force and pressure - syringes and the danger during bier’s block
- Relative difficulty of injecting a liquid from a large syringe and a small syringe.
- Pressure constant but change double diameter of the plunger –> 4 times force required to create the same pressure. Area is proportional to the square of the radius. Use thub pressure of 25 Newtons.
2 ml syringe
P = f/a
P = 25 N / 5 x 10-5 m^2
P = 500 kPa
20 ml syringe
P = f/a
P = 25 N / 2.5 x 10^-4 m2
P = 100 kPa
Nevertheless this is still 6 x systolic BP (16 kPa or 120 mmHg) –> therefore consider number 2 below
- Danger during intravenous regional anaesthesia (Bier’s block) as if the cannula is close to the double torniquet pressures might exceed the torniquet pressures leading to local anaesthetic systemic toxicity. –> 100kPa = 750 mmHg
Also consider the risk of extravascular infusion with syringe pumps which can infuse at very high pressures.
Discuss the clinical application of pressure and force to bed sores. Show calculations
- Bed sores in immobilised patient.
20kg mass over an area = 10m^-2
F = ma = 9.81 x 20 = 196 N P = f/a = 196N / 10^-2m2 P = 19.6 kPa (SBP = 16 kPa) therefore -> ischaemia
Discuss clinical applications of force and pressure with regards to pressure limiting/relief and reducing valves
Draw diagrams of APL and pressure reducing valve
Describe how the entonox valve works
2 stage valve
- stage 1 –> pressure reducing valve with spring/diaphragm and pressure.
- stage 2 –> diaphragm connected to a rod which regulates flow out of 1st stage. –> when negative pressure is present this rod tilts allowing more entonox out of first stage valve. Only when pressure is subatmospheric then entonox is released.
Describe the valve with regard to the oxygen failure warning device
Low pressures –> reduced upward force on diaphragm –> downward movement of diaphragm –> oxygen then leaks past the small valve to blow the whistle –> same type of valve used to cut of nitrous supply when O2 is low.
What is the relationship between gauge pressure and absolute pressure
Absolute Pressure = Gauge pressure + atmospheric pressure
What is gauge pressure
When the pressure is measured relative to atmospheric pressure this is called the gauge pressure
What is absolute pressure
The zero on this scale is an absolute vacuum. Therefore, Absolute pressure = gauge pressure + atmospheric pressure
What is atmospheric pressure
This is an absolute pressure due to the gravitational force on the air molecules. Thus, the atmospheric pressure depends on the density of air. The density of air depends on the altitude at that point.
What is the principle behind a manometer
The force of gravity acting on liquid molecules gives rise to a pressure. This depends on the height of the liquid above the point of measurement.
A manometer consists of a column of fluid in a tube which can be used to measure pressure.
The height of the fluid column is independent of the cross sectional area of the tube.
The pressure exerted by a column of water 10.2 cm high is 1 kPa.
What is the pressure exerted by a column of water 10.2 cm high
1 kPa
What is the advantage of using a mercury manometer
Mercury is 13.6 denser than water. So the force exerted by its weight is 13.6 x higher. Higher pressures can be measured with a shorter column of mercury compare with a column of water because a kPa supports 7.5 mmHg will support a 10.2 cmH20 column.
What is the effect of a slope in the manometer tube
If a manometer tube is tilted, the liquid rises along until the necessary vertical height is reached. A sloped manometer is used in sensitive pressure measurements
What is the difference between a barometer and a manometer
A mercury barometer is used to measure the atm pressure. The column of mercury is sealed and a vacuum is present above the column of mercury so that the absolute atm pressure is recorded.
The mercury manometer measures the gauge pressure of a specific system
What would happen to the meniscus of a mercury barometer if a few drops of isoflurane were added to the vacuum above it?
If a small amount of any liquid is introduced above the column –> a saturated vapour pressure is introduced at that temperature. This wall cause the meniscus of mercury to fall by a level which corresponds to the saturated vapour pressure of that substance.
Barometric pressure = 760 mmHg
Isoflurane SVP = 250 mmHg
Therefore the meniscus would drop to 510mmHg
What is a Bourdon gauge and how does it work? When are these used and what are their advantages
A bourdon gauge consists of a coiled tube with a pointer which moves on a calibrated scale when subjected to pressure. Increasing pressure causes the tube to uncoil which moves the pointer, via and amplification gauge, to increasing pressures on the scale.
Gas cylinders –> where the column of fluid required would be unacceptably high
The advantage of bourdon gauges is that there is no liquid to spill
What is differential pressure measurement
The measurement of the difference in pressure between two points in the anaesthetic breathing system.
How does surface tension effect the pressure readings in a water and mercury manometer? Why?
Falsely increase readings in water manometer
Falsely decrease readings in mercury manometer
- Mercury does not ‘wet’ the tube. THe cohesive forces between the mercury atoms are stronger than the forces that exist between the mercury atoms and the wall of the cylinder
Opposite for water
