
The annular space. The clearance between the
head of the float and the flow tube is known as the
annular space.

upper portion
› orificial
› Density is
dominant

lower portion:
› tubular
› Viscosity is
dominant

The oxygen flow control knob is
physically distinguishable:
› distinctively fluted
› projects beyond the other knobs
› larger in diameter

All knobs:
› color-coded
› chemical formula or name marked on each
› protected with a shield or barrier

FLOW TUBES:
› tapered glass tubes
› double flow tubes
 better visual discrimination at low flow rates
200ml to 1 L/min and 1 to 10 to 12 L/min
 connected in series
 total gas flow is that shown on the higher flow meter
•
INDICATOR FLOATS AND FLOAT STOPS:



floats: plumb-bob, rotating skirted, ball
float stops: visible
SCALE:
› marked directly on the flow tube or to the right of the tube
› Rib guides are used in some flow tubes

SAFETY FEATURES:
› individually hand-calibrated
› The tube, float, and scale make an inseparable unit ( The
entire set must be replaced if any component is
damaged)

An oxygen leak from the flow tube can produce a
hypoxic mixture regardless of the arrangement of the flow
tubes
Sticking of the indicator float
 damaged float can cause inaccurate
readings
 Backpressure from the breathing circuit
 not aligned properly in the vertical
 ambiguous scales


The output represented in L/minute:
 graphically
 numerically
 or both
Even when electrical power is totally
interrupted, oxygen should continue to
flow (flow control valves are non
electronic)
 Small conventional pneumatic “fresh
gas” or “total flow” indicator

prevent the creation and delivery of a
hypoxic mixture
 minimum oxygen concentration at the
common gas outlet is between 23% and
25%


Limitations:





Wrong Supply Gas
Defective Pneumatics or Mechanics
Leaks Downstream
Inert Gas Administration
Dilution of Inspired Oxygen by Volatile Anesthetics

Ohmeda Link-25 Proportion-Limiting Control
system

North American Dräger Oxygen Ratio Monitor Controller

Allows direct communication between the oxygen
high-pressure circuit and the low-pressure circuit

Actuation of the valve delivers 100% oxygen at 35 to
75 L/min to the breathing circuit

Provide a high-pressure oxygen source suitable for jet
ventilation in ??

Hazards:
› Barotrauma
(if the anesthesia machine does not incorporate fresh gas decoupling or an appropriately
adjusted inspiratory pressure limiter)
› Patient awareness


“old” copper kettle–like technology
“new” computerized control technology
Physics:
 Vapor Pressure
(boiling point: desflurane 22.8°C, isoflurane 48.5°C, halothane 50.2°C, enflurane 56.5°C, and sevoflurane
58.5°C)

Latent Heat of Vaporization
(number of calories required to change 1 g of liquid into vapor without a change in
temperature)

Specific Heat
( the number of calories required to increase the temperature of 1 g of a
substance by 1°C)

Thermal Conductivity
(the speed at which heat flows through a substance)

Flow Rate
Output is less than the dial setting at very low and very high flow rates (<250 mL/min and
15 L/min)

Backpressure
(pumping effect, is more pronounced at low flow rates, low dial settings, and low levels of
liquid anesthetic)

Temperature
(Automatic temperature-compensating mechanisms)

Carrier Gas Composition
(sudden transient decrease in vaporizer output)
Simplified schematic of the Ohmeda Tec–type vaporizer
Simplified schematic of the North American Dräger Vapor 19.n and 20.n
vaporizers