Pressure Flashcards
What is one pascal, expressed in terms of newtons and area?
1 pascal = the pressure of 1N acting on 1m2
If we have a 2ml syringe and a 20ml syringe, with the same amount of force applied, which will have more pressure?
P=F/A
The cross sectional area of a 2ml syringe is a fifth of a 20ml
2ml = F/A
20ml = F/5A
Therefore more pressure is generated for the same amount of force in a 2ml syringe than a 20 (that’s why it’s far harder to plunge a 20ml syringe - need more force to eject the same amount of liquid)
What is the difference between a pressure reducing and a pressure relieving valve?
Pressure relieving = gas pipe with a spring loaded disc above. If pressure gets too high, disc will be pushed up, releasing gas around the seal.
Pressure reducing = segmented pipe. Spring loaded disc in upper chamber, attached to ball valve with ability to block gas flow from the lower chamber. Pressure rises too high, valve will close and only gas that has already escaped bottom chamber, can progress to adjacent lower chamber. Regulates the pressure from a variable pressure cylinder to a standard 400kPa.
(Physics module 1 page 4)
What is gauge pressure?
This is the pressure on a cylinder gauge etc. It will read empty, when there is still remaining gas. The remaining gas is at atmospheric pressure so there will be no flow.
What are examples of things that use gauge pressure?
Clinical measurements e.g. CVP or BP and cylinder gauges etc.
What is absolute pressure?
This is the gauge pressure and atmospheric pressure combined e.g. showing the actual pressure in a cylinder.
(If a question asks for absolute pressure, always add atmospheric on to reading e.g. at sea level add on 101 kPa)
What are examples of absolute pressure metres?
Barometers or altimeters
If a systolic is 120mmHg at sea level, what is the absolute pressure?
120mmHg = 16kPa, so absolute pressure = 117 kPa
Divide mmHg by 7.5 to get absolute
What is barometric pressure at Everest peak?
33kPa
What is the difference between pipeline pressure at sea and on land?
400kPa on land and 500kPa at sea
What is intrapleural pressure at sea level on inspiration?
- 1 kPa from 101kPa = 100 kPa
What is the difference between an open and closed manometer?
Open = one end open to atmospheric pressure, other end attached to pipe etc. to be measured. If the mercury is higher on the atmospheric side than the pipe side, the. The pipe is over atmospheric pressure and vice versa.
Closed. Same set up, but closed end, with a vacuum above the mercury. Good for sub atmospheric pressures as the gas pressure only needs to overcome 0 to create a measurement.
(YouTube and physics Mosul 1, page 4)
Do open manometers show gauge or absolute pressure?
Gauge pressure
Do closed manometers, measure absolute or gauge pressure?
Absolute
(Vacuum is at zero atmospheric pressure)
What is a common example of an open manometer?
sphygmomanometer
