eLFH - Pressure

Question 1

Pressure definition

Answer 1

Force per unit area

Question 2

Pressure formula

Answer 2

Pressure = Force / Area

Question 3

Definition of one Pascal

Answer 3

Pressure of one Newton acting over an area of one metre squared

Very small therefore more commonly use kPa

Question 4

Uses of pressure valves in anaesthetic machines

Answer 4

Pressure relief valves
Pressure reducing valves

Question 5

Gauge pressure

Answer 5

Often pressures measured are relative to atmospheric pressure and therefore gauge pressures

Eg gas cylinder pressures and blood pressure are both gauge pressures

Question 6

Absolute pressure

Answer 6

Total pressure exerted

Absolute pressure = Gauge pressure + Atmospheric pressure

Question 7

Commonly used examples of devices that measure pressure

Answer 7

Manometers

Mechanical gauges

Question 8

Manometer definition

Answer 8

Rely on ability of pressure to displace a column of liquid

Question 9

Types of manometer

Answer 9

Open ended
Closed ended

Question 10

Use of mechanical gauges and example

Answer 10

Used to measure pressures significantly higher than atmospheric pressure

Eg Bourdon gauge

Question 11

Open ended manometer

Answer 11

Vertical column of fluid in a tube
Commonly U shaped with one end open to air (atmospheric pressure) and other end exposed to entrapped gas

Measures gauge pressure

Question 12

Open ended manometer interpretation when gas pressure and atmospheric pressure are equal

Answer 12

Mercury / fluid level equal on both sides

Question 13

Open ended manometer interpretation when gas pressure exceeds atmospheric pressure

Answer 13

Mercury / Fluid level pushed so it is higher on open ended side

Pressure difference is the difference in height of the fluid levels (mmHg / cmH2O)

Question 14

Open ended manometer interpretation when gas pressure is lower than atmospheric pressure

Answer 14

Mercury / fluid level is higher on the enclosed gas side of the manometer

Question 15

Closed ended manometer use

Answer 15

When pressures to be measured are less than atmospheric pressures

Measures absolute pressure

Question 16

Closed ended manometer system

Answer 16

U shaped fluid filled tube attached to enclosed gas, but other end is now sealed rather than open

Space above fluid / mercury in the tube is a vacuum so no atmospheric pressure exists above the mercury / fluid

Question 17

Advantage of using water to measure pressure

Answer 17

Readily available and non toxic

Question 18

Advantage of using mercury to measure pressure

Answer 18

Mercury 13.6 times more dense than water so column of mercury does not need to be as high as a water one would

Particularly useful where higher pressures are being measured

Question 19

Bourdon gauge mechanism

Answer 19

High pressure gas enters coiled tube causing it to uncoil

As tube uncoils, motion transferred to linkage gear train connected to a pointer

Pointer moves over a scale on the dial to indicate pressure value

Question 20

How can Bourdon gauges be used to measure temperature

Answer 20

Bourdon gauge tube attaches to temperature sensing element which contains a gas

As temperature increases, pressure increases as volume is fixed (as per third gas law)

Uncoils tube and moves pointer by same mechanism - scale must be calibrated from temperature rather than pressure

Question 21

Aneroid gauge

Answer 21

Another name for mechanical gauge

Translates as “without fluid” gauge