Fractional Distillation

Fractional distillation is the most common and least expensive method for producing O₂. The process involves several related steps. First, atmospheric air is filtered to remove pollutants, water, and carbon dioxide (CO₂). The purified air is liquefied by compression and cooled by rapid expansion (Joule-Thompson effect).

The resulting mixture of liquid O₂ and nitrogen (N, N₂) is heated slowly in a distillation tower. N₂, with its boiling point of 195.8° C (320.5° F), escapes first, followed by the trace gases of argon, krypton, and xenon. The remaining liquid O₂ is transferred to specially insulated cryogenic (low-temperature) storage cylinders. An alternative procedure is to convert O₂ directly to gas for storage in high-pressure metal cylinders. These methods produce O₂ that is approximately 99.5% pure. The remaining 0.5% is mostly N₂ and trace argon. U.S. Food and Drug Administration (FDA) standards require an O₂ purity of at least 99.0%.³

Physical Separation

Two methods are used to separate O₂ from air.⁴ The first method entails use of molecular “sieves” composed of inorganic sodium aluminum silicate pellets. These pellets absorb N₂, “trace” gases, and water vapor from the air, providing a concentrated mixture of more than 90% O₂ for patient use. The second method entails use of a vacuum to pull ambient air through a semipermeable plastic membrane. The membrane allows O₂ and water vapor to pass through at a faster rate than N₂ from ambient air. This system can produce an O₂ mixture of approximately 40%. These devices, called oxygen concentrators, are used primarily for supplying low-flow O₂ in the home care setting. For this reason, details about the principles of operation and appropriate use are discussed in Chapter 51.

Markings and Identification

Medical gas cylinders are marked with metal stamping on the shoulders that supplies specific information (Figure 37-3).^1,9 Although the exact location and order of these markings vary, the practitioner should be able to identify several key items of information.

The letters DOT or ICC (Interstate Commerce Commission) are followed by the cylinder classification (3A or 3AA) and the normal filling pressure in pounds per square inch (psi). Below this information usually is the letter size of the cylinder (E, G, and so on) followed by the cylinder serial number. A third line provides a mark of ownership, often followed by the manufacturer’s stamp or a mark identifying the inspecting authority. An abbreviation indicating the method of cylinder manufacturer is usually on the opposite side of the cylinder. Also in this area is information about the original safety test and dates of all subsequent tests.

Safety tests are conducted on each cylinder every 5 or 10 years, as specified in DOT regulations.1,9 During these tests, cylinders are pressurized to five thirds of their service pressure. While the cylinder is under pressure, technicians measure cylinder leakage, expansion, and wall stress. The notation EE followed by a number indicates the elastic expansion of the cylinder in cubic centimeters under the test conditions. An asterisk (*) next to the test date indicates DOT approval for 10-year testing. A plus sign (+) means the cylinder is approved for filling to 10% greater than its service pressure. An approved cylinder with a service pressure of 2015 psi can be filled to approximately 2200 psi. After hydrostatic testing, cylinders are subjected to internal inspection and cleaning.

In addition to these permanent marks, all cylinders are color-coded and labeled for identification of their contents.1,10 Table 37-2 lists the color codes for medical gases as adopted by the Bureau of Standards of the U.S. Department of Commerce.¹¹ For comparison, the color codes adopted by the Canadian Standards Association also are included. Color codes are not standardized internationally. For this reason, cylinder color should be used only as a guide. As with any drug agent, the cylinder contents always must be identified through careful inspection of the label. To be absolutely sure about the O₂ concentration provided by a cylinder, the user must analyze the gas before administering it (see Chapter 18).¹²

Cylinder Sizes and Contents

Letter designations are used for different sizes of cylinders (Figure 37-4). Sizes E through AA are referred to as “small cylinders” and are used most often for transporting patients and anesthetic gases. These small cylinders are easily identified because of their unique valves and connecting mechanisms. Small cylinders have a post valve and yoke connector. Large cylinders (F through H and K) have a threaded valve outlet (Figure 37-5) (discussed later).

Cylinder Safety Relief Valves

In a closed cylinder, any increase in gas temperature increases gas pressure. Should the temperature increase too much (as in a fire), the high gas pressure could rupture and explode the cylinder. To prevent this type of accident, all cylinders have high-pressure relief valves. These relief valves are of three basic designs: frangible disk, fusible plug, and spring-loaded. The frangible metal disk ruptures at a specific pressure. The fusible plug melts at a specific temperature. The spring-loaded valve opens and vents gas at a set high pressure. In each case, the activated valve vents gas from the cylinder and prevents pressure from becoming too high.

Most small cylinders have a fusible plug relief valve. Most large cylinders have a spring-loaded relief valve. These safety relief valves are always located in the cylinder valve stems.

Filling (Charging) Cylinders

How a cylinder is filled depends on whether its contents will be gaseous or liquid. Some gases stored in liquid form can remain at room temperature, but others must be maintained in a cryogenic (low-temperature) state. Cryogenic storage is discussed later.

Measuring Cylinder Contents

Because of the previously described differences in the physical state of matter of compressed and liquid gases, different methods are needed to measure the contents of the cylinder.

Liquid Gas Cylinders

In a liquid gas cylinder or container, the measured pressure is the vapor pressure above the liquid. This pressure bears no relationship to the amount of liquid remaining in the cylinder. As long as some liquid remains (and the temperature remains constant), the vapor pressure and the gauge pressure remain constant. When all the liquid is gone and the cylinder contains only gas, the pressure decreases in proportion to a reduction in volume. Monitoring the gauge pressure of liquid gas cylinders is useful only after all the liquid vaporizes. Weighing a liquid-filled cylinder is the only accurate method for determining the contents.

Figure 37-6 compares the behavior of compressed gas and liquid gas cylinders during use. The vapor pressure of liquid gas cylinders varies with the temperature of the contents. The pressure in an N₂O cylinder at 21.1° C (70° F) is 745 psig; at 15.6° C (60° F), the pressure decreases to 660 psig. As the temperature increases toward the critical point, more liquid vaporizes, and the cylinder pressure increases. If a cylinder of N₂O warms to 36.4° C (97.5° F) (its critical temperature), all the contents convert to gas. Only at this temperature and higher does the cylinder gauge pressure accurately reflect cylinder contents.

Estimating Duration of Cylinder Gas Flow

When a cylinder of therapeutic gas is used, it often is necessary to predict how long the contents will last at a given flow. The duration of flow of a cylinder can be estimated if the following are known: (1) the gas flow, (2) the cylinder size, and (3) the cylinder pressure at the start of therapy. For a given flow, the more gas a cylinder holds, the longer it lasts. Conversely, the higher the flow, the shorter the emptying time. The duration of flow of a cylinder is directly proportional to the contents and inversely proportional to flow, as expressed in the following formula:

< ?xml:namespace prefix = "mml" />Duration of flow=ContentsFlow

The units commonly used in the United States for measurement of these quantities are not the same. Cylinder contents are generally specified in cubic feet or gallons, whereas gas flow normally is measured in liters. Table 37-3 provides the factors needed to convert these units.

Rather than memorizing various cylinder contents and constantly converting metric and English units, the user can quickly calculate duration of flow by using cylinder factors. Cylinder factors are derived for each common gas and cylinder size with the following formula:

Cylinder factor(L/psig)=Cubic feet(full cylinder)×28.3Pressure(full cylinder)in psig

In the numerator of the previous equation, the English-metric conversion constant (28.3) is used to convert cubic feet to liters. Dividing the resulting volume by the pressure in a full cylinder yields the cylinder factor. The derived factor represents the volume of gas leaving a given cylinder for every 1-psig decrease in pressure. Table 37-4 provides cylinder factors for the therapeutic medical gases and common cylinder sizes.

When the factor for a given gas and cylinder is known, calculating the duration of flow is a simple matter of applying the following equation:

Duration of flow(min)=Pressure(psig)×Cylinder factorFlow(L/min)

A wide margin of safety must be allowed in estimation of cylinder duration of flow. This principle is especially important if the RT cannot be present during use and must return with a full cylinder. Some clinicians return 30 to 40 minutes before the calculated time; others compute duration of flow to a level of 300 to 500 psig rather than 0 psig (empty). Assuming the calculations are correct and there is no change in flow, both methods ensure an uninterrupted supply. The accompanying Rule of Thumb presents a shortcut for estimating cylinder duration of flow.

Mini Clini

Computing Cylinder Duration of Flow

 Problem

The RT needs to determine how long a G cylinder of O₂ with a gauge pressure of 800 psi set to deliver 8 L/min will last until empty.

Solution

Step 1: Determine the cylinder factor for an O₂ G cylinder (see Table 37-4), in this case 2.41.

Step 2: Apply the duration of flow equation:

Duration of flow(min)=Pressure(psig)×Cylinder factorFlow(L/min)

Duration of flow(min)=800×2.418=241 minutes(approximately 4 hours)

Estimating Duration of Liquid Oxygen Cylinder Gas Flow

The only accurate method for determining the volume of gas in a liquid-filled cylinder is by weight. Because 1 L of liquid O₂ weighs 2.5 lb and produces 860 L of O₂ in its gaseous state, the amount of gas in a liquid O₂ cylinder can be calculated with the following formula:

Amount of gas in cylinder=Liquid O2 weight(lb)×8602.5 lb/L

After the amount of O₂ remaining in the cylinder is determined, the duration of the gas in minutes can be calculated with the following formula:

Duration of gas(min)=Amount of gas in cylinder(L)Flow(L/min)

As with gaseous O₂ cylinders, a wide margin of safety is needed for estimation of cylinder duration. This margin of safety varies with the size of the portable O₂ unit or large storage container.

Mini Clini

Computing the Duration of a Liquid Oxygen Container

 Problem

The RT needs to estimate how long Mrs. Jones’ portable liquid O₂ container will last if it contains 3 lb of liquid O₂ that supplies an O₂ delivery device running at 2 L/min.

Solution

Step 1: Determine the amount of O₂ in the cylinder.

Amount of gas in cylinder=Liquid O2 weight(lb)×8602.5 lb/L=3×8602.5=1032 L

Step 2: Calculate the duration of the gas in the container.

Duration of gas=Amount of gas in the cylinder(L)Flow(L/min)=1032 L2=516 minutes60(min/hr)=8 hours 36 minutes

Gas Cylinder Safety

The following guidelines for cylinder safety are from the recommendations of the National Fire Protection Agency (NFPA)² and the Compressed Gas Association (CGA).¹ For ease of use, these safety guidelines are divided into cylinder storage, transport, and use.

Gas Supply Systems

There are three types of centrally located gas supply systems: an alternating supply system or cylinder manifold system, a cylinder supply system with reserve supply, and a bulk gas system with a reserve.² The alternating supply or cylinder manifold system consists of large (normally H or K size) cylinders of compressed O₂ banked together in series (Figure 37-7). This alternating supply system has two sides: a primary bank and a reserve bank. When the pressure in the primary bank decreases to a set level, a control valve automatically switches over to the reserve bank. When this occurs, the primary bank is taken off-line, and the empty cylinders are replaced with full ones. The replenished primary bank becomes the reserve bank. Some large alternating supply systems are permanently fixed and are refilled on site by a supply truck. These cylinder manifold systems have pressure-reducing valves for regulation of delivered pressure and normally have low-pressure alarms. These alarms sound when reserve switchover occurs, and they warn of impending depletion or malfunction. Cylinder manifolds or alternating supply systems are used to supply O₂ from a central location in small facilities or to supply specialty gases, such as N₂O, to operating rooms (Figure 37-8).

A cylinder supply system with a reserve consists of a primary supply, a secondary supply, and a reserve supply. When the primary gas supply is depleted by the demand, this supply system automatically switches to the secondary supply. Master signal panels indicate that the changeover has occurred. This supply system operates in a manner similar to the alternating system except that this system has a reserve supply if primary and secondary supplies become depleted. Liquid containers may be used as the primary and secondary gas sources, but the reserve supply usually is high-pressure gas cylinders. Gas cylinders are used as the reserve supply because low-pressure liquid containers lose approximately 3% of the supply per day.²

For economy, safety, and convenience, most large health care facilities use a liquid bulk O₂ system. A small volume of liquid O₂ provides a very large amount of gaseous O₂ and minimizes space requirements. However, along with this advantage comes a major problem. O₂ has a critical temperature well below room temperature (−118.6° C [−181.4° F]).¹ Liquid O₂ must continually be stored below this temperature, or it reverts to its gaseous state.

To stay in liquid form, O₂ is stored in large stand tanks (Figure 37-9) at relatively low pressure (<250 psig). These stand tanks are similar to giant thermos bottles, consisting of inner and outer steel shells separated by an insulated vacuum chamber (Figure 37-10). Because it eliminates heat conduction, the vacuum keeps the liquid O₂ below its critical temperature without refrigeration. When it flows through vaporizer coils exposed to ambient temperature, the liquid O₂ quickly converts back to a gas. With the O₂ in its gaseous form, the pressure is decreased to the standard working pressure of 50 psi by a pressure-reducing valve. A safety vent allows vaporized liquid O₂ to escape if warming causes cylinder pressure to increase above a set limit.

Smaller liquid cylinders are used for home O₂ supply. These cylinders come in several sizes and hold between cubic ft and cubic ft of liquid O₂. Small liquid O₂ cylinders are refilled on site by means of transfer of liquid O₂ from a large cylinder. Chapter 51 describes the use of these small liquid O₂ cylinders in the home.

Bulk Oxygen Safety Precautions

The NFPA sets standards for the design, construction, placement, and use of bulk O₂ systems.² A key provision in these standards is the requirement for a reserve or backup gas supply to equal the average daily gas usage of the hospital. To meet this requirement, most large facilities have a second, smaller liquid stand tank. Smaller facilities may use a cylinder gas manifold as the backup.

Failure of bulk O₂ supply systems has been reported with resultant major problems.13–15 Failure of a bulk O₂ supply can be life-threatening to any patient receiving O₂ or gas-powered ventilatory support. For this reason, the respiratory care staff must be prepared. Adherence to an established protocol is a quick way to identify and prioritize all affected patients. When affected patients are identified, staff members move appropriate backup equipment to the bedside (e.g., portable cylinders, bag-valve-mask resuscitators). Trained personnel bypass the failed system and provide needed patient support, while engineers determine the cause of the failure and correct it.

American Standard Safety System

Adopted in the United States and Canada, the ASSS provides standards for threaded high-pressure connections between large compressed gas cylinders (sizes F through H/K) and their attachments.¹⁶ Specifications exist for more than 60 gases and gas mixtures. Figure 37-13 shows a typical ASSS connection between a threaded cylinder outlet and a pressure-reducing valve nipple. Use of the ASSS standards makes misconnections difficult because the size (bore) of the cylinder outlet and its threading differ based on the type of gas in the cylinder.

Because there are only 26 connections for the 62 listed gases and mixtures, each gas may not have a unique connection. Some gases have identical connections. Catalogs of cylinder equipment show the connection specifications for each type of cylinder and gas. A typical description for a large cylinder of O₂ is as follows: CGA-540 0.903-14NGO-RH-Ext. The connection for the threaded outlet of this cylinder is listed by the CGA as connection number 540. The outlet has a thread diameter (bore) of 0.903 inch; there are 14 threads per inch; and the threads are right-handed (RH) and external (Ext). It generally is necessary to use only one or two outlet connections because most of the gases that are used by RTs are grouped within a few connector sizes. However, practitioners should be familiar with the classification scheme in general because expanding instrumentation and scope of services may bring RTs in contact with other gases and gas systems.

Pin-Index Safety System

Pin indexing is part of the ASSS but applies only to the valve outlets of small cylinders, up to and including size E. These cylinders have a yoke type of connection. Figure 37-14 illustrates the general structure of the pin-indexed yoke connection. The upper yoke fits over the lower valve stem. Two pins, projecting from the inner surface of the yoke connector, mate with two pinholes bored into the valve stem. Proper pin position aligns the small receiving nipple of the yoke with the recessed cylinder valve outlet. Tightening the hand screw on the yoke firmly seats the receiving nipple into the valve outlet. A nylon washer or bushing typically is used to ensure a leak-free connection.

Similar to the ASSS, the PISS helps prevent accidental misconnections between pieces of equipment. The exact positions of pins and pinholes vary for each gas. Unless the pins and holes align perfectly, the yoke nipple cannot seat in the recessed valve outlet. Six holes and pin positions constitute the total system. Because overlapping holes cannot be used, there are 10 possible pin combinations. Figure 37-15 is a diagram of the location of all six possible holes and their index numbers. Table 37-5 lists the gases included in the PISS system, including their index positions.

Diameter-Index Safety System

The ASSS and the PISS provide standards for high-pressure connections between cylinders and equipment; the DISS was established to prevent accidental interchange of low-pressure (<200 psig) medical gas connectors.¹⁷ RTs typically find DISS connections (1) at the outlets of pressure-reducing valves attached to cylinders; (2) at the station outlets of central piping systems; and (3) at the inlets of blenders, flowmeters, ventilators, and other pneumatic equipment.

As shown in Figure 37-16, the DISS connection consists of an externally threaded body and a mated nipple with a nut. As the two parts are joined, the shoulders of the nipple and the bores of the body mate, with the union held together by a hand-tightened hex nut. Indexing is achieved by varying the dimensions of the borings and shoulders. There are 11 indexed DISS connections and 1 connection for O₂, for a total of 12.¹⁷ The standard threaded O₂ connector (0.5625 inch in diameter and 18 threads per inch) preceded adoption of this safety system. Nonetheless, it has been assigned a DISS number of 1240.

Although O₂ and air are generally used from a central outlet, it may be necessary to administer other gases that have different DISS connections. To avoid stocking a large variety of pressure regulators, flowmeters, and connectors for special gas use, adapters can be used to convert various DISS connections so that they can be used for different purposes. Using adapters to bypass a safety system carries the increased risk of misconnection. For this reason, RTs should exercise extreme caution when adapting equipment connections. Misconnections have occurred, with negative patient consequences.13,18

Quick-Connect Systems

Station outlets at the patient’s bedside allow quick access to a bulk supply of O₂ and air or a vacuum source. Station outlets have DISS connections or quick-connect systems that are gas-specific or vacuum-specific. Various manufacturers have designed specially shaped connectors for each gas (Figure 37-17). Because each connector has a distinct shape, it does not fit into an outlet for another gas, and each manufacturer has its own unique design. For this reason, connectors from different manufacturers are not interchangeable. As long as a facility is standardized for a single quick-connect system, this incompatibility is seldom a problem.

A variety of safety systems help prevent inadvertent misconnections between medical delivery systems and equipment. Figure 37-18 summarizes the use of and relationships between the ASSS, PISS, and DISS systems as applied to cylinder gases. Proficiency in the proper use of these systems is a basic skill of RTs.

High-Pressure Reducing Valves

There are two basic types of high-pressure reducing valves: single-stage and multiple-stage. Reducing valves are available as preset or adjustable. Although all of these valves function on the same principle, the design, features, and use are different. This section differentiates preset reducing valves and adjustable reducing valves and discusses multiple-stage reducing valves.

Preset Reducing Valve

Figure 37-19 shows the basic design of a high-pressure preset reducing valve. High-pressure gas (2200 psig for O₂) enters through the valve (A), with the inlet pressure displayed on the pressure gauge (B). The body of the valve is divided into a high-pressure chamber (C) and an ambient-pressure chamber (D) by a flexible diaphragm (E). Attached to the diaphragm in the ambient-pressure chamber is a spring (F), which is fixed to the other side of the chamber. Also attached to the diaphragm, but in the high-pressure chamber, is a valve stem (G) that sits on the high-pressure inlet (H). Gas flows through the valve inlet (H) into the high-pressure chamber and on to the gas outlet (I). The pressure chamber is supplied with a safety vent (L) preset to 200 psig to release pressure in the event of malfunction.

The spring tension is calibrated to give when the pressure on the diaphragm exceeds 50 psig. When this happens, the valve stem is pushed forward and closes the high-pressure inlet, preventing further entry of gas into the reducing valve. However, as long as gas is allowed to escape from the pressure chamber through the outlet (I), the inlet valve remains open and allows gas flow. The regulator maintains a balance between outlet flow and inlet pressure. Automatic adjustment of the diaphragm-spring combination keeps the pressure in the high-pressure chamber at a near-constant 50 psig—hence the name preset. Preset reducing valves are normally used in conjunction with high-pressure gas cylinders to decrease the pressure to the standard 50 psig used with most respiratory care equipment.

Adjustable Reducing Valve

Although most respiratory care equipment works at the standard 50 psig, some devices need variable pressures. To provide variable outlet pressures from a high-pressure gas source, an adjustable reducing valve is needed. Figure 37-20 shows the basic design of a high-pressure adjustable reducing valve. As with the preset design, the inlet valve (H) remains open until the gas pressure exceeds the spring tension, displacing the diaphragm and blocking further gas entry. However, while the preset reducing valve provides a fixed pressure, the adjustable reducing valve allows a change in outlet pressure. Outlet pressure can be changed with a threaded hand control (K) attached to the end of the diaphragm spring. Changing the tension on the valve spring varies pressure over a wide range, usually between 0 psig and 100 psig.

The adjustable reducing valve commonly is used in combination with a Bourdon-type flow gauge (discussed later). The combination of a flowmeter with a reducing valve is called a regulator.

Low-Pressure Gas Flowmeters

As with drugs, giving a medical gas to a patient requires knowledge of the dosage being delivered. Physicians often prescribe O₂ dosage as a flow, in liters per minute. In addition, certain gas-mixing equipment requires accurate knowledge of input flows, sometimes involving two or more gases. Flowmeters allow the rate of gas flow to a patient to be set and controlled. When the gas source is a high-pressure gas cylinder, a regulator (reducing valve plus flowmeter) is required. However, when the source is a bulk central supply system, the pressure has already been reduced to 50 psig by the time it reaches the outlet stations; this eliminates the need for pressure reduction and requires only a flowmeter.

Three categories of flowmeters are used in respiratory care: the flow restrictor, the Bourdon gauge, and the Thorpe tube. The Thorpe tube has two different designs: pressure compensated or not pressure compensated (uncompensated). Although uncompensated Thorpe tubes are rare, they may still be used at some institutions. For this reason, the principles underlying each of the four types of flow metering devices are compared and contrasted.

Flow Restrictor

The flow restrictor is the simplest and least expensive flowmeter device. As shown in Figure 37-21, a flow restrictor consists solely of a fixed orifice calibrated to deliver a specific flow at a constant pressure (50 psig). The operation of the flow restrictor is based on the principle of flow resistance, as described in Chapter 6. Specifically, the flow of gas through a tube can be quantified with the following equation:

R=P1−P2V

Rearranging the equation to solve for flow (V) yields the following:

V=P1−P2R

where V is the volumetric flow per unit time, P₁ is the pressure at the upstream point (point 1), P₂ is the pressure at the downstream point (point 2), and R is the total resistance to gas flow.

By design, a flow restrictor requires a source of constant pressure (usually 50 psig). As long as the source pressure remains fixed, P₁ − P₂ should stay constant. With a fixed-size orifice, the flow resistance (R) also remains constant. The rate of gas flow through a flow restrictor can be increased by increasing P₁ (upstream pressure) or by selecting a larger orifice size. Both fixed and adjustable orifice flow restrictors are used clinically. Commercially produced flow restrictors are calibrated at 50 psig. Table 37-6 summarizes the advantages and disadvantages of flow restrictors.

TABLE 37-6

Advantages and Disadvantages of Flow Restrictors

Advantages	Disadvantages
Low-cost, simple, reliable (no moving parts)	Different versions required for different flows
Cannot be set to incorrect flow	Accuracy varies with changes in source and downstream pressures
Can be used in any position (gravity-independent)	Cannot be used with high-resistance equipment

Bourdon Gauge

A Bourdon gauge (Figure 37-22) is a flowmeter device that is always used in combination with an adjustable pressure-reducing valve. Similar to the flow restrictor, the Bourdon gauge uses a fixed orifice. In contrast to the flow restrictor, the Bourdon gauge operates under variable pressures, as adjusted with the pressure-reducing valve. The Bourdon gauge is a fixed-orifice, variable-pressure flowmeter, so increasing the upstream pressure increases gas flow out of the device unless downstream pressure also increases.

As shown in Figure 37-23, a Bourdon gauge has a calibrated fixed orifice (A), which creates outflow resistance. The gauge itself is attached to the flow stream with a connector (B) located proximal to the orifice. Inside the gauge is a curved, hollow, closed tube (C) that responds to pressure changes by changing shape. The force of gas pressure tends to straighten the tube, causing its distal end to move. This motion is transmitted to a gear assembly and indicator needle (D). A numbered scale is calibrated to read the needle movement in units of flow (liters per minute).

As with the flow restrictor with fixed orifice, the output flow of the Bourdon gauge is proportional to the driving pressure. However, the Bourdon gauge provides a continuous range of flow, which the user adjusts by altering the driving pressure. Although the gauge actually measures pressure changes, it displays the corresponding flow.

As with a flow restrictor, gravity does not affect a Bourdon gauge. The Bourdon gauge is the best choice when a flowmeter cannot be maintained in an upright position. This situation is common when a patient is being transported with a portable O₂ source. In these instances, keeping the E cylinder upright is seldom easy, and movement of both the O₂ supply and the patient is common. Combined with its continuous range of flows, this feature makes the Bourdon gauge the metering device of choice for patient transport.

The main disadvantage of the Bourdon gauge is its inaccuracy when pressure distal to the orifice (downstream pressures) changes. Specifically, if downstream pressure increases (as when high-resistance equipment is used), the pressure difference across the orifice and actual output flow decrease. However, the Bourdon gauge flow reading depends on upstream pressure, which stays constant. In this situation, the gauge reading is falsely higher than the actual delivered flow. Because it measures upstream pressure, the gauge registers flow even when the outlet is completely blocked (Figure 37-24). A user who needs accurate flow when using a device that creates high resistance should not select a Bourdon gauge. A compensated Thorpe tube should be used instead.

Integrated O₂ cylinders (Figure 37-25), including the Grab ’n Go System (Praxair, Inc, Danbury, Connecticut), have combined the O₂ cylinder with a pressure regulator and an adjustable flow restrictor to meter O₂ flow. These portable O₂ systems eliminate the need for separate O₂ tanks, Bourdon gauge regulators, and O₂ keys or wrenchs (needed to turn on standard E-cylinders). These integrated systems virtually eliminate problems and delays associated with incorrectly mounted regulators. The practitioner simply selects the flow on the flow-adjusting knob and connects the O₂ tubing to the system connection and the patient.

Thorpe Tube

The Thorpe tube flowmeter (Figure 37-26) is always attached to a 50-psig source, either a preset pressure-reducing valve or a bedside station outlet. Compared with the flow restrictor and the Bourdon gauge, the Thorpe tube functions as a variable-orifice, constant-pressure flowmeter, so increasing the size of the orifice increases the gas flow. Figure 37-27 shows how a Thorpe tube works. The key component in this device is a tapered transparent tube that contains a float. The diameter of the tube increases from bottom to top. Gas flow suspends the float against the force of gravity. To read the flow, one simply compares the float position with an adjacent calibrated scale, normally calibrated in liters per minute.

Although the Bourdon gauge measures pressure, the Thorpe tube is used to measure true flow. Flow measurement involves the complex interaction of gravity and fluid dynamics. When gas begins to flow into a Thorpe tube, the initial pressure difference lifts the float. As the needle valve is opened, the float rises in the widening tube, the space available for flow around it increases, and resistance to flow decreases. The float ultimately stabilizes when the pressure difference across the float (an upward force) equals the opposing downward force of gravity.

As the needle valve of the flowmeter is opened, the increase in flow initially disrupts this balance, causing an increase in the pressure difference across the float. With the upward pressure difference greater than the downward force of gravity, the float rises. However, as the float rises, the available “orifice” increases in diameter. Flow resistance around the float decreases, and the pressure difference again equilibrates with gravity. The float position stabilizes at a higher level, proportionate to the greater flow around it.

Thorpe tubes come in two basic designs: pressure compensated and pressure uncompensated. The term pressure compensation refers to a design that prevents changes in downstream resistance, or back pressure, from affecting meter accuracy. All manufacturers now supply only pressure-compensated Thorpe tubes for administration of medical gas. However, some ventilators and anesthesia machines still use uncompensated Thorpe tubes. For this reason, clinicians using these devices must understand the effect of back pressure on the accuracy of these devices. Downstream resistance increases when the user connects a flowmeter to certain types of equipment. Almost all therapy gas equipment produces some flow restriction. Devices such as jet nebulizers produce very high downstream resistance. Depending on their design, Thorpe tube flowmeters respond to resistance in one of two ways.

The uncompensated Thorpe tube flowmeter is calibrated in liters per minute but at atmospheric pressure (without restriction). Gas from a 50-psig source flows into the meter at a rate controlled by a needle valve located before the flow tube (Figure 37-28, A). When the user attaches flow-restricting equipment to the meter, downstream resistance increases, increasing pressure in the flow tube. As long as this pressure does not exceed 50 psig, gas continues to flow through the tube. However, the added downstream resistance increases the pressure in the flow tube above atmospheric pressure. At higher pressures, a greater amount of gas flows through a given restriction than at atmospheric pressure. With the float at a given height, more gas flows through the tube than is indicated on the scale. Under these conditions, an uncompensated Thorpe tube falsely shows a flow lower than that actually delivered to the patient.⁴

In contrast, the scale of the compensated Thorpe tube flowmeter is calibrated at 50 psig instead of at atmospheric pressure. Its flow control needle valve is placed after (distal to) the flow tube (see Figure 37-28, B). The entire meter operates at constant 50-psig pressure. Knowing that the compensated Thorpe tube operates at 50 psig helps identify it. When a compensated Thorpe tube is connected to a 50-psig gas source with the needle valve closed, the float “jumps” and then returns to zero as the Thorpe tube is pressurized. Because the entire meter operates at constant pressure, an increase in downstream resistance increases pressure distal to the needle valve only. As long as the downstream pressure does not exceed 50 psig (in which case flow ceases), the position of the float accurately reflects actual outlet flow. For this reason, the pressure-compensated Thorpe tube is the preferred instrument in most clinical situations.

The only factor limiting the use of a pressure-compensated Thorpe tube is gravity. Because it is accurate only in an upright position, a Thorpe tube is not the ideal choice for patient transport. In these cases, the gravity-independent Bourdon gauge is a satisfactory alternative. Figure 37-29 summarizes the effects of downstream resistance, or back pressure, on the Bourdon gauge and pressure-compensated and uncompensated Thorpe tube flow metering devices.

Mini Clini

Selection of Devices to Regulate Gas Pressure or Control Flow

 Problem

Three staff RTs are given three separate requests to set up O₂. (1) Mark has an order to transport Ms. Patel to radiology with O₂. (2) Carmen needs to set up a pneumatically powered ventilator with O₂ in the ambulatory clinic (where there are no O₂ outlets). (3) Monica has to set up O₂ therapy with a jet nebulizer for a patient in the intensive care unit (ICU). What equipment should each RT select?

Solutions

1. Because he has to transport a patient using O₂, Mark should select an E cylinder with an adjustable regulator that includes a Bourdon gauge (unaffected by gravity) or an integrated O₂ cylinder that includes an adjustable flow restrictor.

2. Because pneumatically powered ventilators require 50 psig and no central outlets are available, Carmen needs a preset (50 psig) reducing valve and a large G/H O₂ cylinder.

3. Because all modern ICUs have central wall outlets for O₂, Monica need only select a flowmeter with the appropriate quick connect. A compensated Thorpe tube is required for metering flow through high-resistance equipment such as jet nebulizers.

Summary Checklist

• All therapy gases must contain at least 20% O₂; all such gases support combustion.

• Medical gases are stored either in portable high-pressure cylinders or in large centralized bulk reservoirs.

• For positive identification of the contents of a medical gas cylinder, the label must be carefully read.

• The pressure in a gas-filled cylinder indicates its contents; the pressure in a liquid-filled cylinder does not.

• To compute duration of flow (minutes) of a medical gas cylinder, multiply the cylinder pressure (pounds per square inch) by the cylinder factor, and divide the result by the set flow (liters per minute).

• Gas supply systems provide gas at 50 psig to outlets throughout a facility through a network of pipes. Such a system must include both zone valves for repairs or fire and alarms to warn of failure.

• Failure of a bulk gas supply system can threaten the lives of patients receiving O₂ therapy or being supported with pneumatically powered devices. A protocol must exist to deal with this emergency.

• Indexed safety systems help prevent misconnections between equipment. The ASSS provides high-pressure connections with large cylinders; the PISS does the same for small cylinders; and DISS connections are for low-pressure outlets, typically 50 psig.

• A reducing valve is used for reduction of gas pressure. A flowmeter is used for control of gas flow. A regulator is used for control of both pressure and flow.

• A flow restrictor is used to provide fixed low flows of O₂. A Bourdon gauge is used to meter flow during patient transport. A compensated Thorpe tube is used when accurate flows are needed with high-resistance equipment.