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The flow meter assembly ( Fig. 9-6 ) precisely controls and measures gas flow to the common gas outlet. With traditional glass flow meter assemblies, the flow control valve regulates the amount of flow that enters a tapered, transparent flow tube known as a Thorpe tube. A mobile indicator float inside the flow tube indicates the amount of flow passing through the associated flow control valve. The quantity of flow is indicated on a scale associated with the flow tube.[8] [9]
Figure 9-5
The oxygen failure protection device (OFPD) responds
proportionally to changes in oxygen supply pressure. The OFPD consists of a seat-nozzle
assembly connected to a spring-loaded piston. (Modified with permission
from Narkomed 2A Anesthesia System: Technical Service Manual, 6th ed. Telford,
PA, North American Dräger, June 1985.)
Figure 9-6
Oxygen flow meter assembly. The oxygen flow meter assembly
is composed of the flow control valve assembly plus the flow meter subassembly.
(Adapted from Bowie E, Huffman LM: The Anesthesia Machine: Essentials for
Understanding. Madison, WI, Ohmeda, a Division of BOC Health Care, 1985.)
Opening the flow control valve allows gas to travel through the space between the float and the flow tube. This space is known as the annular space ( Fig. 9-7 ). The indicator float hovers freely in an equilibrium position where the upward force resulting from gas flow equals the downward force on the float resulting from gravity at a given flow rate. The float moves to a new equilibrium position in the tube when flow is changed. These flow meters are commonly referred to as constant-pressure flow meters because the pressure decrease across the float remains constant for all positions in the tube.[8] [32] [33]
Flow tubes are tapered, with the smallest diameter at the bottom of the tube and the largest diameter at the top. The term variable orifice designates this type of unit because the annular space between the float and the inner wall of the flow tube varies with the position of the float. Flow through the constriction created by the float can be laminar or turbulent, depending on the flow rate ( Fig. 9-8 ). The characteristics of a gas that influence its flow rate through a given constriction are viscosity (i.e., laminar flow) and density (i.e., turbulent flow). Because the annular space is tubular at low flow rates, laminar flow is present, and viscosity determines the gas flow rate. The annular space simulates an orifice at high flow rates, and turbulent gas flow then depends predominantly on the density of the gas.[8] [33]
The assembly of the flow control valve (see Fig. 9-6 ) is composed of a flow control knob, a needle valve, a valve seat, and a pair of valve stops.[8] The assembly can receive its pneumatic input directly from the pipeline source (50 psig) or from a second-stage pressure regulator. The location of the needle valve in the valve seat changes to establish different orifices when the flow control valve is adjusted. Gas flow increases when the flow control valve
Figure 9-7
The clearance between the head of the float and the flow
tube is known as the annular space. It can be considered an equivalent to a circular
channel of the same cross-sectional area. (Adapted from Macintosh R, Mushin
WW, Epstein HG: Physics for the Anaesthetist, 3rd ed. Oxford, UK, Blackwell Scientific
Publications, 1963.)
Contemporary assemblies of flow control valves have numerous safety features. The oxygen flow control knob is physically distinguishable from other gas knobs. It is distinctively fluted, projects beyond the control knobs of the other gases, and has a larger diameter than the flow control knobs of other gases. All knobs are color coded for the appropriate gas, and the chemical formula or name of the gas is permanently marked on each. Flow control knobs are recessed or protected with a shield or barrier to minimize inadvertent change from a preset position. If a single gas has two flow tubes, the tubes are arranged in series and are controlled by a single flow control valve.[7]
The flow meter subassembly (see Fig. 9-6 ) consists of the flow tube, the indicator float with float stops, and the indicator scale.[8]
Contemporary flow tubes are made of glass. Most have a single taper in which the inner diameter of the flow tube increases uniformly from bottom to top. Manufacturers provide double flow tubes for oxygen and nitrous oxide to provide better visual discrimination at low flow rates. A fine flow tube indicates flow from approximately 200 mL/min to 1 L/min, and a coarse flow tube indicates flow from approximately 1 L/min to between 10 and 12 L/min. The two tubes are connected in series and supplied by a single flow control valve. The total gas flow is that shown on the higher flow meter.
Contemporary anesthesia machines use several different types of bobbins or floats, including plumb-bob floats[10] [11] [12] [15] [16] ; rotating, skirted floats[13] [14] ; and ball floats.[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] Flow is read at the top of plumb-bob and skirted floats and at the center of the ball on the ball-type floats.[8] Flow tubes are equipped
Figure 9-8
Flow tube constriction. The lower portion of the illustration
represents the lower portion of two flow tubes. The clearance between the head of
the float and the flow tube is narrow. The equivalent channel is tubular because
its diameter is less than its length. Viscosity is dominant in determining gas flow
rate through this tubular constriction. The upper portion of the illustration represents
the upper portion of a flow tube. The equivalent channel is orificial because its
length is less than its width. Density is dominant in determining gas flow rate
through this orificial constriction. (Adapted from Macintosh R, Mushin WW,
Epstein HG: Physics for the Anaesthetist, 3rd ed. Oxford, UK, Blackwell Scientific
Publications, 1963.)
The flow meter scale can be marked directly on the flow tube or located to the right of the tube.[7] Gradations corresponding to equal increments in flow rate are closer together at the top of the scale because the annular space increases more rapidly than does the internal diameter from bottom to top of the tube. Rib guides are used in some flow tubes with ball-type indicators to minimize this compression effect. They are tapered glass ridges that run the length of the tube. There are usually three rib guides that are equally spaced around the inner
The flow meter subassembly for each gas on the Datex-Ohmeda Modulus I, Modulus II, Modulus II Plus, CD, and Aestiva is housed in an independent, color-coded, pin-specific module. The flow tubes are adjacent to a gas-specific, color-coded backing. The flow scale and the chemical formula or name of the gas are permanently etched on the backing to the right of the flow tube.[10] [11] [12] [15] [16] Flow meter scales are individually hand-calibrated using the specific float to provide a high degree of accuracy. The tube, float, and scale make an inseparable unit. The entire set must be replaced if any component is damaged.[10] [11] [12] [15] [16]
North American Dräger does not use a modular system for the flow meter subassembly. The flow scale, the chemical symbol, and the gas-specific color codes are etched directly onto the flow tube.[19] [24] [25] [26] [27] The scale in use is obvious when two flow tubes for the same gas are used.
Flow meter leaks are a substantial hazard because the flow meters are located downstream from all machine safety devices except the oxygen analyzer. [1] Leaks can occur at the O-ring junctions between the glass flow tubes and the metal manifold or in cracked- or broken-glass flow tubes, the most fragile pneumatic component of the anesthesia machine. Even though gross damage to conventional glass flow tubes is usually apparent, subtle cracks and chips may be overlooked, resulting in errors of delivered flows.[34] The elimination of conventional glass flow tubes from some newer anesthesia machines (i.e., Datex-Ohmeda S/5 Anesthesia Delivery Unit [ADU] and the Dräger Fabius) may help to minimize potential sources of leaks[20] [21] [35] [36] (see "Electronic Flow Meters").
In 1963, Eger and colleagues[37] demonstrated that, in the presence of a flow meter leak, a hypoxic mixture is less likely to occur if the oxygen flow meter is located downstream from all other flow meters.[33] Figure 9-9 is
Figure 9-9
The flow meter sequence is a potential cause of hypoxia.
A and B, In the event
of a flow meter leak, a potentially dangerous arrangement exists when nitrous oxide
is located in the downstream position. C and D,
The safest configuration exists when oxygen is located in the downstream position.
(Adapted from Eger El II, Hylton RR, Irwin RH, et al: Anesthetic flow meter
sequence—A cause for hypoxia. Anesthesiology 24:396, 1963.)
A leak in the oxygen flow tube can produce a hypoxic mixture even when oxygen is located in the downstream position ( Fig. 9-10 ).[1] [34] Oxygen escapes through the leak and nitrous oxide flows toward the common outlet, particularly at high ratios of nitrous oxide to oxygen flow.
A flow error can occur even when flow meters are assembled properly with appropriate components. Dirt or static electricity can cause a float to stick, and the actual flow may be higher or lower than that indicated. Sticking is more common with low flow because the annular space is smaller. A damaged float can cause inaccurate readings because the precise relationship between the float and the flow tube is altered. Backpressure from the breathing circuit can cause a float to drop so that it reads less than the actual flow. If flow meters are not aligned properly in the vertical position, readings can be inaccurate because tilting distorts the annular space.[8] [36] [38]
Before the standardization of flow meter scales and the widespread use of oxygen analyzers, at least two deaths resulted from confusion created by ambiguous scales.[34] [38] [39] The operator read the float position beside an adjacent but erroneous scale in both cases. This error
Figure 9-10
An oxygen leak from the flow tube can produce a hypoxic
mixture, regardless of the arrangement of the flow tubes.
Some newer anesthesia machines such as the Datex-Ohmeda S/5 ADU and the North American Dräger Fabius GS have conventional control knobs and flow control valves, but have digital flow meters. The output from the flow control valve is represented graphically or numerically, or both, in liters per minute on the workstation's integrated user interface. These systems depend on electrical power to provide a precise display of gas flow. However, even when electrical power is totally interrupted, because the flow control valves are not electronic, oxygen should continue to flow.[20] [35] [36] Because these machines do not have individual flow tubes that physically quantitate the flow of each gas, a small, conventional, pneumatic fresh gas or total flow indicator is provided that indicates the total fresh gas flow from all flow control valves. This miniature flow tube informs the user of the approximate quantity of gas that is leaving the anesthesia workstation's common gas outlet.[20] [35] [36] [37] [38] [39] [40]
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