2. Content
Measurement of Liquid and gas.
Gas laws
Fick’s principle
Adiabatic Changes
Dalton law of Partial pressure
Avogadro’s hypothesis
Universal gas constant
Critical and pseudo-critical temperature
3. Objectives
To know about Measurement of Liquid and gas.
To know about the Gas laws
To know about the Fick’s principle
To know about Adiabatic Changes
To know about Dalton law of Partial pressure
To know about Avogadro’s hypothesis
To know about Universal gas constant
To know about Critical and pseudo-critical
temperature
4. Measurement of Gas flow
Gas flow Meters
• Variable orifice (fixed pressure change) flowmeters
e.g. Rotameter, peak flow meter
• Variable pressure change (fixed orifice) flowmeters
e.g. Bourdon gauge, pneumotaco-graph
9. Measurement of gas volume
• Benedict Roth Spirometer
• Vitalograph
• Dry gas meter
• Compact wright respirometer
• Electronic volume monitor
10. Benedict Roth Spirometer
• The Benedict Roth spirometer is widely used for both physiological and clinical
studies.
• A light bell moves with the patient's breathing.
• This movement may be recorded by a pen on a rotating drum
• The motion of the bell being transferred to the pen through a connecting wire
which passes over two pulleys.
• A water seal prevents the leakage of gas from the bell and this seal is small in
order to reduce the volume of gas which dissolves in the water.
13. • More portable than the Benedict Roth spirometer.
• In the Vitalograph bellows are used to measure gas volume. The top
plate of the bellows is pivoted and its motion is transferred to a
scriber which records volume changes on a chart. The chart is motor
driven and automatically starts to move as soon as the bellows move
from their empty position. This allows expired volume-time graphs to
be plotted.
Vitalograph
17. Wright Spirometer
• The compact Wright respirometer is more convenient for measuring tidal
volumes in anaesthesia
• It has a set of gears but in this case the volume measurement is achieved by
monitoring the continuous rotation of a vane as it is moved by the flow of gas.
• The effect of the slits is to cause a circular motion of the airflow which rotates
the vane.
• The vane does not rotate when the flow is reversed.
• The Wright respirometer is calibrated for use for tidal volume measurement
and for tidal ventilation.
• Its calibration is inaccurate if it is used to measure a continuous flow.
18.
19. Measurement of Liquid flow
• The measurement of IVF infusion rate is the
most common type of liquid flow
measurement in anaesthesia, as the drops
passing through the drip chamber of a fluid or
blood administration set provide a quick
visual indication of flow.
26. • Gas is a substance which exists above its critical temperature.
• Atom or molecules
• Lattice formation
• When heated melts
• Vander walls force exists between liquid molecules
• Change into vaopur or gas when gets heated.
27. Boyle’s Law
• At a constant temperature, the absolute pressure of a given mass of gas is
inversely proportional to the volume.
• Mathematically: pressure(P)
1
𝑉(𝑣𝑜𝑙𝑢𝑚𝑒)
PV= k(where k is constant)
P1V1=P2V2=Constant…………(i)
• describe the effects of altitude on gases in closed cavities within the body.
28. Q)
• “E” type cylinder showing pressure of 1900psig. If we use oxygen
@3L/min, oxygen will last for ……
a) b)
c) d)
29. Cont..
As we know
• The volume of an E type of cylinder is approximately 5 Litres (V1)
• The service pressure at which the cylinder is filled is 1900 psig(P1).
P1V1= P2V2
1900 X 5= 14.7 X V2 (where P2 is atmospheric pressure
ie.1atm =14.7 psi)
V2=1900 x 5/14.7=646 litres (approx.-660 liters)
• So if we use 3 litres/min of oxygen, the E type full cylinder will deliver for
660/3= 220 mins)
30. 2)Inhalation and Exhalation:
During the natural breathing process, when a person inhales, the
diaphragm and intercostal muscles contract, expanding the
volume of the thoracic cavity. This increase in thoracic volume
leads to a decrease in intrathoracic pressure (Boyle's Law). Air
flows from a region of higher pressure (outside the body) to a
region of lower pressure (inside the lungs). Exhalation is the
reverse process, with a decrease in thoracic volume leading to
an increase in intrathoracic pressure, pushing air out of the lungs.
31. 3) Positive Pressure Ventilation:
Ventilators used in mechanical ventilation work on the principle of
positive pressure ventilation. In this mode, the ventilator delivers
a positive pressure to the airway, which increases the pressure
inside the lungs (Boyle's Law). This increase in pressure causes
the lungs to expand and air to flow into the lungs (inhalation).
When the ventilator releases the positive pressure, the lungs
recoil, and air is pushed out (exhalation).
32. 4)Adjusting Ventilator Settings:
Ventilator settings, including inspiratory pressure (PIP - peak
inspiratory pressure) and expiratory pressure (PEEP - positive
end-expiratory pressure), to control the volume and rate of air
delivered to the patient's lungs. Understanding Boyle's Law helps
in setting appropriate pressures to achieve adequate tidal volume
and oxygenation.
33. 5)Lung Protective Strategies:
In critical care settings, understanding Boyle's Law is essential
for developing lung-protective ventilation strategies. Excessive
pressure or volume can damage the delicate lung tissues,
leading to barotrauma and volutrauma.
34. Charle’s Law
• At constant pressure, volume of a gas is directly
proportional to the temperature.
Volume(V) Temperature(T)
V / T=Constant
V1/T1=V2/T2=Constant……..(ii)
36. As gases are inspired, we can see from the relationship described in
Charles’s law that warming from 20 degrees C (273 degrees K) to 37
degrees C (310K) will cause an increase in the volume of inspired
gases. For example, an adult tidal breath of 500 ml of air at room
temperature will increase to a volume of 530 ml, when it reaches the
site of gas exchange as it warms up to body temperature.
37. • Charles’s law can be also be used to calculate the amount of nitrous oxide
remaining in a gas cylinder.
• A nitrous oxide cylinder contain a mixture of gas and liquid at 20 degrees
C room temperature (as its critical temperature is 36.5 degrees C).
• As nitrous oxide gets removed, the liquid nitrous will boil, and the nitrous
oxide gas will then expand, so some (e.g., Bourdon) pressure gauges will
indicate a constant pressure until all the liquid nitrous oxide has boiled and
there is relatively little nitrous oxide left.
• To calculate the amount of nitrous oxide left, you need to weigh the
cylinder. Using Avogadro’s law (1 gram molecular weight of gas will
occupy 22.41 L at STP), and knowing the molecular weight of nitrous
oxide is 44, we can calculate the amount of nitrous oxide available to us.
38. • If the empty weight of an ‘E’ cylinder is 5.9 kg and the current weight
is 8.8 kg, we will have approximately 2900 g of liquid nitrous oxide
and therefore (2900 x 22.41)/44 = 1477 liters of nitrous oxide at 273
degrees K. We can then apply Charles’s law; as room temperature is
293 K (273+20), to work out that there are (1477/273)x293 = 1585
liters of nitrous oxide remaining in the cylinder.
39. Third Gas Law
At constant volume, the absolute pressure of the given
mass of gas is directly proportional to the temperature.
pressure(P) T (temperature)
Application:
i. Medical gases are stored in cylinders having a constant
volume and high pressures (138 Barr in a full oxygen /
air cylinder). If these are stored at high temperatures,
pressures will rise causing explosions.
ii. Molybdenum steel can withstand pressures till 210
bars. Weakening of metal in damaged cylinders are at a
greater risk of explosion due to rise in temperature.
41. Fick’s Law
• Rate of diffusion of a substance across a unit area is
directly proportional to the concentration gradient or the
partial pressure gradient across the membrane.
44. Cont..
Clinical Relevance
a) Anesthetic vapors diffuse into breathing circuits and later acting as
Vaporizers at the time of discontinuation of anesthetic gases.
b) N2O gas diffuses into cuff of endotracheal tube.
c) Diffusion of N2O into air filled cavities as in pneumoecephalous,
pneoumoperitoneum etc.
d) Diffusion of CO2 and O2 in lungs.
45. Adiabatic Changes And Joule
Thompson Effect
• Energy is required to be added to or taken from a gas as
the change occurs.
• When a gas is subjected to sudden compression, the heat
energy is produced rapidly and the reverse occurs when
there is sudden expansion. There is no exchange of energy
with the surroundings. This is an adiabatic change.
• In joule thompson effect, when a gas is allowed to escape
through a narrow opening, there is a sudden temperature
drop.
46. Cont..
• Clinical Relevance:
-When a valve of an oxygen cylinder is opened suddenly,
oxygen will rush into high pressure hose or stem of oxygen regulator
and on reaching the end of hose, adiabatic process might occur.
-That suggests that local pressure is much higher than that of
filling pressure of the cylinder (about 150 bar) for a very brief time. The
process is adiabetic because compression occurs without any heat
from outside.
47. Cont..
APPLICATION
• Used in manufacture of oxygen when air is cooled by
external cooling and is made to suddenly expand, it loses
further temperature as energy is spent in order to hold the
molecules together i.e. the Vander Waal forces.
• This sudden loss of temperature is due to JOULE
THOMSON’S EFFECT.
• When this is repeated many times the temperature reduces
to less than -183oC and through fractional distillation, liquid
oxygen collected in the lower part is separated from
nitrogen with a boiling point of -197oC which collects at the
top of the container.
48. Dalton’s Law Of Partial
Pressures:
In a mixture of gases,
• The total pressure exerted by the mixture is equal to the
sum of the partial pressures of the individual gases,
provided the gases do not mix with each other.
P = p1+p2+p3….
49. Eg:The mixture remaining in the cylinder is 50% nitrous
oxide, 50% oxygen and so each gas occupies half the
cylinder volume.
• According to Dalton's law, the pressure exerted by the
nitrous oxide in the cylinder is the same as it would
exert if it alone occupied the container.
• But if it were to do this, the available space for the
nitrous oxide would have increased from half the
cylinder to a full cylinder.
In other words, it would have doubled its volume.
50. Avogadro’s Hypothesis And
Ideal Gas Equation:
• Equal volume of gases contain equal number of
molecules at standard temperature and pressure (273K
and 760mm Hg).
• The law can also be defined as one gram molecular
weight (one mole) of a gas contains 6.023x1023
(avogadro’s number) molecules = occupies 22.4L at
STP.
• PV = n RT is the ideal gas equation.
Where R is the universal gas constant = 1.987
J/degree/mole in SI units.
52. • This equation may be used in anaesthetics when
calculating the contents of an oxygen cylinder.
• constant room temp
• fixed internal volume,
• R is a constant
Only variables now are P and n so that P ∝ n
• Therefore pressure gauge acts as a content gauge for
gases – measure of amount of O2 left in a cylinder.
The universal gas equation
53. Avogadro’s Hypothesis And
Ideal Gas Equation:
Clinical Relevance
• Nitrous oxide cylinder contains 3.4kg N20
• Mol wt. of N2O = 44 (1mole)
• 1 mole occupies 22.4 L at standard temp and
pressure
• Therefore 3400 g occupies 22.4 x 3400/44 = 1730L
54. Critical Temperature &
Pseudo Critical Temp
• Temperature beyond which a gas cannot be
compressed to the liquid state.
• The pressure of the gas at the critical temperature is
called the critical pressure and the volume occupied by
the gas is called the critical volume
• POYNTING EFFECT: When two gases one of high and
the other of low critical temperatures are mixed in a
container, the critical temperature of the gas with the
higher value is lowered (pseudo critical temp) and the
mixture will remain gaseous above the pseudo critical
temperature.
55. Eg:
• Entonox is a 50:50 mixture of O2 & N2O.
• The critical temperature of oxygen is -118oC and of N2O
is 37oC.
• when these gases are mixed in a same cylinder, then the
critical temperature of the mixture will be -6oC due to
POYNTING EFFECT and the mixture will remain as gas
at room temperature.
Critical Temperature & Pseudo
Critical Temp
57. Clinical application
• In cold climates if the temperature is less than -6oC, then N2O will
separate into its liquid form and will remain in the bottom of the cylinder
and the patient will get only O2 initially and hence will not produce any
analgesia.
• Later patient gets only N2O which can result in hypoxia. Hence in such
situation cylinder should be thoroughly shaken
58.
59. References
1. Fundamentals Of Anaesthesia
2. Understanding Anesthesia Equipment By
Dorsch And Dorsch
3.Basic Physics And Measurement In
Anaesthesia,4th edition G.D.Parbrook
