The SI unit for pressure is the pascal, Pa.
Newtons are the product of mass (kg) multiplied with acceleration (meters per second
per second squared). This is expressed as kg⋅m⋅s-2.
By going back to the base units as above you can derive that a pascal is therefore:
Joules are relevant to respiratory physiology and lung compliance.
Question: Given that the joule is force x distance (which later becomes relevant to
respiratory physiology and lung compliance), can you describe it in terms of base units?
Answer: Force is the newton and is expressed as kg⋅m⋅s-2. The SI unit for distance is the
metre, m. As joule is force x distance in SI units it must therefore be kg⋅m⋅s-2 x m.
This is simplified to kgm2/s2.
Flow
Laminar Flow =
Turbulent Flow=
p= density
Relation between Voltage and flow
At low flows, how will the rotameter behave?
Answer: At low flows, the bobbin is at the narrower end of the tube. This means that
the gas flows across an obstruction of greater length than diameter, i.e. the gas will
behave as if flowing through a tube and flow will be laminar.
Question: At high flows, how will the rotameter behave?
Answer: At high flows, the bobbin rises higher, into the wider portion of the tube. This
means that the obstruction caused by the bobbin has greater diameter and reduced
length, i.e. the gas behaves as if flowing through an orifice and flow is turbulent.
Question: Why are rotameters gas specific?
Answer: Because gases have different densities and viscosities, which in turn affect flow
under different conditions.
Question: What factors might affect the density or viscosity of a gas and have an effect
on a rotameter's accuracy?
Answer: The factors that might affect rotameter accuracy include:
•
Warmer gas has a lower density and viscosity, which may cause over-reading
•
Reduced atmospheric pressure reduces the density of a gas, which may
cause over-reading (though viscosity is unaffected by pressure)
No equation defines turbulent flow.
However, Reynold's number (Re) can give an indication of when flow may be turbulent.
Re is a dimensionless number (i.e. has no units) that can be calculated to help you to
predict whether fluid flow is likely to be laminar or turbulent:
•
Re <2000 flow is likely to be laminar
•
Re >2000 flow is likely to be turbulent
•
ρ = fluid density
•
v = fluid velocity
•
d = tube diameter
where:
• η = fluid viscosity
So, the critical velocity = Reynold's number = 2000. Transition from laminar to turbulent
flow depends on the gases present.
The Venturi effect:
Based on the Bernoulli principle, where a drop in pressure (analogous to potential
energy) is accompanied by a rise in flow (analogous to kinetic energy). Venturi masks
use this principle to increase flow by entraining a second fluid into the flow path of the
driving fluid.
Logarithm Approximate Value
Actual Value
log 1
0
0
log 3
0.5
0.48
log 5
0.7
0.70
log 7
0.9
0.85
log 10
1.0
1.0
.
Graham’s Law of Effusion
Thomas Graham found that the rate of diffusion was
inversely proportional to the square root of the molecular
mass of the gas. So the larger the molecule the slower it
diffuses. Effusion relates to the both the direction and the
rate of change of diffusion. In our consideration this could
be from alveolus to the plasma.
So Rate of effusion is proportional to 1/Molecular mass
This explains the second gas effect when using nitrous and
a volatile in Oxygen
Rate of effusion of A/Rate of effusion of B = Square root of
molecular mass of B/Square root of molecular mass of A
So if B is more massive than A, A will effuse out of the
alveolus quicker than B, leaving behind more of B and so
raising its concentration. Since, for example, halothane is
more massive than nitrous oxide, Graham’s law will
indicate that the nitrous will diffuse quicker and so raise
the concentration of the halothane in the alveolus.
Henry’s Law
Henry’s law states that for a gas-liquid interface the amount of the gas that dissolves in the liquid is
proportional to its partial pressure. So Henry’s law helps to predict how much gas will be dissolved in the
liquid. The actual amount also depends on the solubility of the gas as well as its partial pressure.
Dalton’s Law of partial pressure
John Dalton observed that the total pressure of a gas mixture was the
sum of the pressures of each of the gases if they were to exist on their
own.
Therefore P mixture = P1+P2+P3 + …
This means that to calculate the total pressure in a cylinder for a
mixture of gases just add up all the partial pressures.
So if a cylinder of gas mixture at 400 kPa contains 21% oxygen and
79% helium, then if the oxygen existed on its own it would exert a
partial pressure of
21% of 400 kPa = 88 kPa
The helium therefore exerts a partial pressure of 400 – 88 = 312 kPa
Capacitor
T=R.C
Current is proportional to rate of change of
voltage
Current is directly proportional to frequency
CAPACITANCE (C): Is the ability of an object to
store electrical charge. It is equal to the charge
per unit voltage. It is measured in farads
(Farad)
Inductor
T= L/R
Voltage is directly proportional to rate of
change of current
Voltage is directly proportional to frequency
Opposite of capacitor(henry)
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Fick’s law states that the rate of transfer across a membrane is proportional to the concentration
gradient across the membrane and can be expressed as:
Rate of diffusion across the cell membrane, Q= KpAC /T
where C = concentration difference, A = area of membrane, Kp = Membrane pemeability
T = membrane thickness
Electrical Impedance
Electrical impedance is directly proportional to frequency for an inductor (Z= 2 x Pi x f x
L).
Z: is the impedance in ohm
L: is the value of the inductor in Henrys (H)
Electrical impedance is independent of frequency for a resistance (Z= R).
Electrical impedance is inversely proportional to frequency for a capacitor
Z= 1/(2. Pi .f. C).
The relative humidity is calculated from S.V.P. at dew point divided by S.V.P. at ambient
temperature.
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The electromagnetic spectrum consists of radiation in ascending order of frequency as
follows: radio waves, infrared, visible light, ultaviolet, x rays and gamma rays. Thus the
wavelength of ultraviolet light is shorter than infrared as it has a higher frequency
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Power = VI= I2R
Electric Energy = Power x Time