eLFH - Pressure Flashcards
Pressure definition
Force per unit area
Pressure formula
Pressure = Force / Area
Definition of one Pascal
Pressure of one Newton acting over an area of one metre squared
Very small therefore more commonly use kPa
Uses of pressure valves in anaesthetic machines
Pressure relief valves
Pressure reducing valves
Gauge pressure
Often pressures measured are relative to atmospheric pressure and therefore gauge pressures
Eg gas cylinder pressures and blood pressure are both gauge pressures
Absolute pressure
Total pressure exerted
Absolute pressure = Gauge pressure + Atmospheric pressure
Commonly used examples of devices that measure pressure
Manometers
Mechanical gauges
Manometer definition
Rely on ability of pressure to displace a column of liquid
Types of manometer
Open ended
Closed ended
Use of mechanical gauges and example
Used to measure pressures significantly higher than atmospheric pressure
Eg Bourdon gauge
Open ended manometer
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
Open ended manometer interpretation when gas pressure and atmospheric pressure are equal
Mercury / fluid level equal on both sides
Open ended manometer interpretation when gas pressure exceeds atmospheric pressure
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)
Open ended manometer interpretation when gas pressure is lower than atmospheric pressure
Mercury / fluid level is higher on the enclosed gas side of the manometer
Closed ended manometer use
When pressures to be measured are less than atmospheric pressures
Measures absolute pressure
Closed ended manometer system
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
Advantage of using water to measure pressure
Readily available and non toxic
Advantage of using mercury to measure pressure
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
Bourdon gauge mechanism
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
How can Bourdon gauges be used to measure temperature
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
Aneroid gauge
Another name for mechanical gauge
Translates as “without fluid” gauge
