Flow Flashcards
What is the density of air at sea level?
1.2kg/m3
If the specific gravity of a gas is over 1, what does this mean?
It is more dense than air
(Gas specific gravity = gas density / air density
How do we measure the specific gravity of solids and liquids?
We compare it to water instead of air
What is a Newtonian fluid?
One where the viscosity is only effected by temperature and pressure, not by movement etc.
What happens to a Newtonian vs a non-Newtonian fluid when it is put under shear stress (e.g. stirred)?
Newtonian fluids still flow, whereas non-Newtonian will deform.
What are some examples of Newtonian fluids?
Water and gas
How do we calculate laminar flow?
Q= (pi deltaP r4) / 8nl
N = viscosity
What components of the HP eqn. is turbulent flow directly and inversely proportional to?
Directly proportional to r2 and square root of delta P
And inversely proportional to length and density
What kind of flow occurs along a smooth tube?
Laminar flow with increased velocity at the centre, due to the walls offering greater resistance
Where does turbulent flow occur in a vessel / tube?
At an orifice, which is a tube with a diameter greater than its length
What does a rotameter provide in terms of pressure and orifice?
Constant pressure with a variable orifice
How is a rotameter constructed?
A tapered glass tube (slimmer at the bottom) with a bobbin in it. Gas flow from the bottom via a needle valve.
What happens to flow along the length of a rotameter and how does this alter how the bobbin moves?
Gas flow at the bottom causes pressure to lift the bobbin until gravity matches this and causes it to float. Near the base, the bobbin is close to the side of the tube, so length is greater than diameter (between the bobbin and side of the tube), so flow is laminar. As the bobbin climbs higher is moves further from the wall of the tapered tube, so the diameter of the gap exceeds the length of the bobbin, causing turbulent flow.
(Physics 1 page 7 - diagram)
In a rotameter, where do viscosity and density have more of an effect?
Density has more effect on the turbulent flow at the top and viscocity has more effect on the turbulent flow at the bottom
If we increase the density of the gas in a rotameter, what happens to the rotation of the bobbin?
Less rotation
What are two examples of constant orifice, variable flow meters?
Pneumotachograph and simple pressure gauge
What happens in a pneumotachograph?
Laminar flow through small diameter tubes, resistance causes a pressure drop proportional to flow as in HP eqn. Meter reads this across a small section of tube. Needs calibrated for all different gasses.
When are simple pressure gauges used?
If upstream flow is constant e.g. on a cylinder
What type of meters in terms of orifice and pressure are peak flow meters?
Constant pressure, variable orifice
How does a vane flow meter work?
Exhalation causes vanes to move and uncover increasingly larger orifices. Vanes are held by a spring to make opposing pressure. The size of the orifice revealed is shown be the ratchet (red dial pointer outside).
What other types of flow meter are there other than peak flow and vane meters?
Watersight, bubble, thermistor and ultrasonic
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for oxygen.
Temps in Celsius and pressures in bar.
-183
-219
-118
50
1.4
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for N2O.
Temps in Celsius and pressures in bar.
-88
-91
36.5
72
50.8
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for CO2.
Temps in Celsius and pressures in bar.
-79
-57
30
73
57
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for H.
Temps in Celsius and pressures in bar.
-269
-272
-268
2.3
N/A
Given that O2 has a boiling point of -183 degrees what does this mean for storage?
It needs liquid cold storage for large amounts and gas storage in cylinders
What impact does the critical temperature of N2O have for its storage?
It is above room temperature so is stored as a liquid.
For O2 list the atomic and molecular weights, colour, how it is produced, flammability and form of storage?
16
32
Clear or light blue as a solid/liquid (absorbs red light)
Made by fractional distillation of air
Not flammable but will drive combustion
Stored as a liquid
For N2O list the: molecular weight, vapour pressure, colour, how it is produced, flammability and form of storage?
MW = 44
VP = 44 bar
Colourless and sweet smelling
Heat ammonium nitrate to 240 Celsius (over this = exothermic explosion) and extract N2O from impurities (higher N oxides, ammonia and nitric oxide)
Vapour is not flammable but drives combustion
Stored vertically as a liquid in blue cylinders
For entonox list: mixture, how it is produced, flammability, the psueudocritical temp and form of storage?
50:50 N2O and O2
Poynting effect = bubble O2 through liquid N2O
Not flammable but supports conduction
PCT = -5.5 at 117bar, -7 at 137 bar and 130 at 4.1 bar (pipeline pressure)
Cylinder are stored above PCT to prevent liquefaction of N2O, separation (lamination) and hypoxia mix.
What is a pseudocritical temperature of a gas?
The critical temperatures of a gas combination e.g. entonox
What can we do to prevent hypoxia mixture being delivered from a entonox cylinder?
Store horizontally, use a dip tube and store at an appropriate temperature to avoid liquefaction
What does an entonox dip tube do?
Reaches the bottom of the cylinder to deliver what is at the bottom first, it means that even if N2O has liquefied, the patient is delivered 80% N2O and 20%O2. The mixture then becomes gradually richer in O2 and less rich in analgesia. If they delivered from the top, the O2 would go first and the mixture would get gradually more hypoxic as more O2 left the liquid at the bottom to equilibriate.
What would happen if an entonox cylinder liquified and had no dip tube and was given to the patient vertically?
Liquified N2O would be at the bottom with O2 above. Patient would initially get O2 giving little analgesia and as O2 leaves, more would bubble out, leaving NO at the bottom, which would then be delivered as a hypoxic mix
What is the molecular weight of CO2?
44
How is CO2 made?
Hearing calcium or magnesium carbonate
How is CO2 stored?
5% CO2 and 95% O2 in a black cylinder with grey and white shoulders
Is CO2 flammable?
Non flammable and non explosive
What kind of gas is helium?
Inert (chemically inactive)
What is the molecular weight of helium?
4
What is the density and viscosity of H compared to N?
Less dense but more viscous
What is the specific gravity of H at 0celsius?
0.138
What is the advantage of H in an upper airway obstruction?
If turbulent flow present, a H/O2 mix will have a greater flow than a NO mix
What is a Heliox mix and how is it stored?
79% helium and 21% O2 in a black cylinder with brown and white shoulders
What happens to laminar flow when it reaches a critical velocity?
It turns into turbulent flow.
Is density or viscosity more important in effecting the rate of flow in:
1. Laminar flow
2. Turbulent flow
- Viscosity
- Density
Do we do direct or indirect measurement of gas flow in anaesthetics and how?
Indirect, by looking at the change in pressure across a fixed orifice
