Vapourisers: Physical Principles and Classification

Vapourisers: Physical Principles and Classification1Vapour1 ml of liquid anaesthetic on vapourisation produces 200 ml of its vapour.Vapour output of ml of liquid agent = 22,400(molar volume) x DensityMolecularweight(MW) Millilitres of liquid used/hour set percentage fresh gas flow (FGF) in L/min 3

(for sevoflurane use 3.3),2MAC (minimum alveolar concentration)MAC value for an inhalational agent is one that causes a lack of response to painful stimulation in 50% of patientshalothane (Fluothane):0.75enflurane (Ethrane):1.68isoflurane (Forane):1.15methoxyflurane: 0.16sevoflurane (Sevorane, Ultane): 1.7desflurane (Suprane): about 6

These MAC numbers referred to volumes % of end-tidal alveolar gas at 760 mm Hg (1 atm)3Saturated vapour pressure (Psv)In a closed container vapour starts forming over its surface and its pressure increases. At equilibrium it reaches a maximum at a particular temperatureand is called saturated vapour pressure (Psv).Psv of isoflurane is 239 mm Hg.

4Boiling point (BP)A liquid's boiling point is the temperature at which its vapor pressure is equal to the atmospheric pressure. The BP will be lower with lower atmospheric pressure. Anaesthetic agents with lower BPs are more affected by variations in barometric pressure (Pb) than agents with higher BPs.

5Daltons Law of Partial Pressures The total pressure exerted by a mixture of gases is the sum of their individual partial pressuresPtotal = Pa + Pb + Pc + etcGASPartial Pressure (mmHg)InhaledAlveolarExhaledNitrogen594.70570569Oxygen160.00103116CO20.304028H2O vapor5.004747TOTAL760760760PARTIAL PRESSURE of AIR6 Latent heat of vapourisationThe amount of heat energy utilised in the formation of vapour from a liquid without a change in temperature is defined as latent heat of vapourisationMore volatile liquid vapourises faster and cools faster.7Specific Heat

substance's specific heat is the quantity of heat required to raise the temperature of 1 g of the substance by 1C.Specific heat is important when considering the amount of heat that must be supplied to a liquid anesthetic to maintain a stable temperature when heat is lost during vaporization.Copper and water are used as reservoirs of heat8Thermal conductivityThis is a measure of the speed with which heat flows through a substance. The higher the thermal conductivity, the better the substance conducts heat.Thermostabilization is achieved by constructing a vaporizer of a metal with high thermal conductivity (copper, bronze) to minimize temperature changes when the vaporizer is in use. In a vaporizer containing a wick, it is important that the wick be in contact with a metal part so that heat lost as a result of vaporization can be quickly replaced9CRITICAL TEMPERATURE

Vaporizer Design

11ANESTHETIC VAPORIZERSA vaporizer is a device that changes a liquid anesthetic agent into its vapor and adds a controlled amount of that vapor to the fresh gas flow or the breathing system.Ideal vaporizerIt should be simple, safe, satisfactory and more practical.It should have low resistance to gas flow.It should be temperature compensated for uniform vaporization. It should permit a relatively constant concentration at different flow rates of carrier gases and thus it should have flow stability. It should permit precise, accurate, controllable and predictable delivered concentration of the vapor to the patient. Performance of vaporizer should not be affected by changes in fresh gas flow, volume of liquid, ambient temperature and pressure, decrease in temperature due to vaporization and pressure fluctuation due to mode of respiration. It should be light weight with small liquid requirements.Construction should be corrosion and solvent resistant.It should have good quality control.12 Vaporizer design

Concentration calibration Variable bypass vaporizersElectronic vaporizersVaporization methodsFlow overInjection Bubble through type Temperature compensation Mechanical thermocompensation Supplied heatComputerized thermocompensation13Location of vaporizerOutside the breathing system Inside the breathing system Specific anesthetic agent Agent specific Multiple agent

GRAY AND NUNN CLASSIFICATION Plenum vaporizers These are named after plenum system which means the system in which fresh air is forced into a chamber. These vaporizers are used with fresh gas flow from anaesthetic machines. 14Examples are : Boyle vaporizers, copper kettle, fluotek Mark 2, Fluotek Mark 3, Halox vaporizer, Pentec (for methoxyflurane), Enfluratec (for enflurane). Vaporizers are meant for unidirectional gas flow and have a relatively high resistance to gas flow.They are not suitable for use as draw over vaporizers or in circle system. Inhalers or draw over vaporizers Usually have a very low resistance to gas flow. Examples are EMO vaporizer, Emotril, Tecota, Oxford miniature inhaler.15Flow over vapouriserObjective is to create Psv in the space over the liquid anaesthetic agent to deliver known amount of vapour.

The Psv of isoflurane is 239 mm Hg, concentration of isoflurane created is 239/760 (31% V/v in the VC)

Simple flow over vapouriserFlagg canBoyles bottle16Measured flow vapouriserMeasured FGF is led into VC with the help of a separate flow meter. It carries vapourand this is mixed with a separate measured fresh flow with the aid of another flow meter to give required concentration, e.g. Copper Kettle

FOREGGER COPPER KETTLE 195217Measured flow vapouriser1% isofluraneinflow rate of oxygen into the VC - 100 ml/min (isoflurane Psv at 20C = 239 mm Hg, Pb = 760 mm Hg).VC will form 239/760 = 31.4% vapour.

Split ratio for vapourisers with split ratio control knob at the VC inlet

=100 Psv 1 PbPsv .Set vol.% on the dial; if it is 1% then =1, Pbambient pressure, Psv-saturated vapour pressure of the concerned agent. The total out flow from VC is 146 ml FGF of 4500 ml/min will decrease the concentration to very nearly 1% vol./vol 46 ml of the vapour diluted in total 4646 ml gives very nearly 1%

18

19INJECTION VAPORIZERSA known amount of liquid agent or pure vapor is injected into the gas stream to provide the desired concentrationA calibrated throttle valve is opened or closed by the anaesthetist. The more it is closed (more resistance to the FGF), the higher the pressure transmitted by the FGF into the VC.This pressure tends to force liquid to atomize at the injector nozzle. The number of molecules of liquid injected is proportional to the resistance to gas flow at the throttle valveThe liquid is not vapourising within the VC, thermal compensation is not necessaryEg. TEC 6 desflurane vaporizer (it is more ideally classified as a gas-vapor blender)

Vapouriser inside or out of circuitvapouriser is placed in FGF line and not in the circle system.Boyles bottle or Goldman bottle with low resistance could be inserted within the inspiratory limb of the circle system - vapouriser inside circuit (VIC)newer generation plenum vapourisers high resistance characteristics cannot be used either in the draw over circuit or in the closed circuit as VIC21Variable Bypass Vaporizers

Two channels are created making the FGF entering the vapouriser divide into two streams

One stream flows into the VC and gets fully saturated with anaesthetic vapourThe other portion passes through the bypass pathwayThe diluted vapour concentration in the combined gas flow is the output concentration. The ratio of the two streams is the splitting/split ratio

(a) Splitting of fresh gas flow by the control valve at the inlet, bimetallic strip, bellows assembly (b) advanced vapouriser incorporating W-wicks, baffles, etc.22Split ratio1% for isoflurane the split ratio is 44.5:1, i.e. if FGF into the vapouriser is 5000 ml;110 ml will flow into the vapourising chamber, whichwill pick up 50.5 ml of isoflurane vapour (31% of 110ml+50.5ml=160.5ml) and 4890 ml bypasses and The resulting output concentration is 1% (50.5 ml vapour in total outflow of 5050.5).23Split ratioThe split ratio is varied by a control regulator. This controller in older vapouriser was at the inlet of the VC and in newer it is at the outlet.

Split ratio for vapourisers with split ratio control knob at the VC inlet

=100 Psv 1 PbPsv .Set vol.% on the dial; if it is 1% then =1, Pbambient pressure, Psv-saturated vapour pressure of the concerned agent.Split ratio for vapourisers with split ratio control knob at the VC outlet

=100 Psv 1 Pb 24Gas flow splitting ratios (20oC)Desired anesthetic percentagehalothane (Fluothane)enflurane (Ethrane)isoflurane (Forane)sevoflurane (Sevorane, Ultane)methoxyflurane1%46:129:144:125:11.7: 12%22:114:121:112:10.36:13%14:19:114:17:1maximum possible = 2.7% at 20oC.25 WicksAt the higher rate of flow the time for the fresh gas to saturate with vapour is inadequate. This causes the output concentration to fall.

To increase the surface area of vapourisation to increase the efficiency of vapourisation. This is achieved by providing wicks.

Simple plates or channels (baffles) are also incorporated in VC

Another method is to bubble the FGF through the agent. As the agent vapourises it cools. The Psv falls. This decreases the output oncentration.(supply heat)26RESISTANCEPLENUM ( Latin = fullness)Vaporizers with high resistance which depend on compressed gas driven under pressure are called plenum vaporizers. Eg. Boyle bottle, copper kettle, TEC vaporizers Plenum vapourisersBoyles bottlevariable bypass, can operate as bubble through or flowover without wicks,nonagentspecific.has low resistance incircuithas no interlock system (allow only one vapouriser at a time)not temperature compensated

28DRAW OVERCarrier gas is drawn through the vaporizer either by the patients own respiratory efforts, or by a self-inflating bag or manual bellows

Drawover systems operate at less than, or at ambient pressure

Flow through the system is intermittent, varying with different phases of inspiration, and ceasing in expiration. RESISTANCEDRAW OVERHave a low internal resistance to gas flowSo that they may be used within the breathing circuit, the gas flow being driven through them by the patients breathing.

They may be used in a non rebreathing DRAW-OVER APPARTUS, or as IN-CIRCUIT VAPORIZERS in a CIRCLE ABSORBER SYSTEM.

Eg. Goldman bottle, EMO

Measured flow device - Copper KettleMeasuredflow, nontemperature compensated,bubblethrough, out of the circuit and agent nonspecific.

The FGF enters the device and bubbles through a sintered disc or porex unit and is mixed with the vapour free FGF.

Flow through the VC and the flow bypassing the chamber are controlled by separate flow meters

To get the required anaesthetic concentration the SVP of the anaesthetic vapour at the indicated temperature is used as input into the calculation and

the rates of the two flows, bypass flow and VC flow are suitably altered.

This calculation is aided by a circular slide rule provided with the vapouriser

31Vernitrol Flow Calculator (Whiz wheel)

32Copper Kettle

33TEMPERATURE COMPENSATION34TEMPERATURE COMPENSATION

TEMPERATURE COMPENSATIONTo maintain a constant output from the vaporizer, mechanisms to compensate for the fluctuations in temperature are to be employed

Alteration in the splitting ratio (automatic compensation Eg. Bimetallic strip in tec vaporizers, Ether filled bellows in penlon vaporizers, EMO

Computer control electronic vaporizers

Supplied heat tec 6 (electrically heated) Automatic temperature compensating unit (TCU)A metallic bellows assembly with volatile liquid inside such as ether or Freon.

As the liquid anaesthetic cools the bellows starts collapsing since the content inside gets smaller in volume. This opens the inlet port wider allowing more flow into the chamber.

A metal rod also can be used since the metal contracts with the drop in temperature.

Another method uses another physical property of a metal. Dissimilar metals have different coefficients of expansion when they are heated. Two different kinds of metal strips are fused together to form a bimetallic strip. When cooled one metal shrinks more than the other making the bimetallic strip to bend in one direction and in opposite direction as it warms. It is fixed in such a way that as the temperature falls it alters the splitting ratio by its deflection away from the port in the VC inlet37THERMOCOMPENSATIONMost variable bypass vaporizers compensate for changes in vapor pressure by altering the splitting ratio.Done by using a thermosensitive element (BIMETALLIC STRIP) incorporated in the vaporizing chamber or bypass chamber.So, the splitting of gas is controlled by 2 valves: (1) the dial we set (splitting valve)(2) the temperature compensating valve. THERMOCOMPENSATIONIn a bimetallic strip, two metals with very different coefficients of thermal expansion are fixed together.

THERMOCOPENSATION When the temp. of the vaporizing chamber drops, the bimetallic strip bends and moves away. This reduces the resistance to flow and thus more flow occurs into the vaporizing chamber

SUPPLIED HEATAn electric heater can be used to supply heat to a vaporizer and maintain it at a constan temperature Eg. Tec 6

Ohmeda 'Tec series

Tec 3- a bimetallic strip increases flow through the bypass chamber when temperature increases.

Tec 2 - a bimetallic strip decreases flow through the vaporizing chamber when temperature increases.

Ohio 100 - a bellows and thimble valve increases flow through the bypass when temperature increases.

Drger 19 - an annular valve constructed of dissimilar metals increases flow through the bypass when temperature increases.Fluotec, - halothane, Pentec - methoxyfluraneIsotec/Fortec - isoflurane

42Drger 19.1

An annular valve constructed of dissimilar metals increases flow through the bypass when temperature increases

43Classification

Datex-Ohmeda Tec 4,Tec 5, SevoTec, and Aladin (AS/3 ADU); Drger Vapor 19.n, Vapor 2000Copper Kettle, VernitrolDatex-Ohmeda Tec 6(Desflurane)Splitting ratio(carrier gas flow)Variable-bypass (vaporizer determines carriergas split)Measured-flow (operator determines carrier gassplit)Dual-circuit (carrier gas is not split)Method of vaporizationFlow-over (including the Aladin for desflurane,which does not require added heat like the Tec 6)Bubble-throughGas/vapor blender (heat produces vapor, whichis injected into fresh gas flow)Temperature compensationAutomatic temperature compensation mechanismManual (i.e., by changes in carrier gas flow)Electrically heated to a constant temperature(39C; thermostatically controlled)CalibrationCalibrated, agent-specificNone; multiple-agentCalibrated, agent-specificPositionOut of circuitOut of circuitOut of circuitCapacityTec 4: 125 mLTec 5: 300 mLVapor 19.n: 200 mLAladin: 250 mL200-600 mL (no longer manufactured)390 mL44FACTORS AFFECTING VAPORIZER OUTPUTFLOW THROUGH THE VAPORIZING CHAMBER: Varying the proportion of gas passing through the vaporizing chamber and bypass chamber

2. SURFACE AREA OF THE LIQUID GAS INTERFACE: Greater the surface area, more will be the vaporization. Bubble through > flow over

3.TEMPERATURE: as temperature increase, output increase

4.TIME: Output concentration tend to fall over time

5. GAS FLOW RATE: (A) At high flowrates, the gas leaving vaporization chamber is less saturated (B) Alters the total flow that passes through the vaporization chamber

6. CARRIER GAS COMPOSITION:(a) Changes in viscosity & density may affect the proportion of the total flow passing through the vaporization chamber(b) N2O dissolves in the flow, thus altering the effective volume passing through the vaporization chamber

7. BOILING POINT: Higher the boiling point, less will be the vapor output

8. AMBIENT PRESSURE: - SVP is solely a function of temp. so if ambient pressure is reduced, the constant SVP becomes a greater proportion of the total pressure-> output increases. - Agents with low boiling points are more susceptible to the influence of ambient pressure

FACTORS AFFECTING VAPORIZER OUTPUTEFFECT OF LOW ATMOSPHERIC PRESSURE CONCENTRATION CALIBRATED High resistance pathway through the vaporizing chamber offers less resistance, under hypobaric conditions and so a slight increase in vapor output occurs. Deliver higher conc. if measured in vol. % but deliver same PP so clinically affect unchanged %1 = %cal x Pcal = 2% x 760 = 2% x 2 = 4% in 380mmHg = 2% 760mmHg P1 380CONCENTRATION CALIBRATEDATM PRESSUREINCREASES -> Density of gasCHANGES -> More resistance to flow of gas through the vaporizing chamber -> Decreased vapor O/P (Partial Pressure & Volume Percent) Effect on partial pressure is less dramaticEFFECT OF HIGH ATMOSPHERIC PRESSUREEFFECT OF INTERMITTENT BACK PRESSUREWhen assisted or controlled ventilation is used, the positive pressure generated during inspiration is transmitted from the breathing system back to the machine & some way may be transmitted to the vaporizers.

Also seen with the use of oxygen flushPUMPING EFFECTThe increase in vaporizer output concentration due to back pressure

PUMPING EFFECTWhen the bag is squeezed, pressure is transmitted back into both, the by pass channel and also to the vaporizing chamber. The fresh gas tries to move forward and gets compressed both in the by pass channel and the vaporizing chamber. However, the vaporizing chamber volume is much larger than the by pass channel volume, an thus, more fresh gas gets compressed into it than into the by pass channel.

PUMPING EFFECTThis extra fresh gas that enters the vaporizing chamber collects anaesthetic vapor

PUMPING EFFECTWhen the positive pressure is suddenly released (expiration) the previously compressed gases now suddenly expands in all direction.

Some of the rapidly expanding gas (containing vapor) enter the inlet of the vaporizer and cross over into the by pass channelPUMPING EFFECTNormally, a vaporizer by pass channel does not have vapor. So his vapor due the pumping effect is additional. When this by pass vapor flows across to the exit, it meets the vapor from the vaporizing chamber. The addition of the by pass vapor to the vapor from the vaporizing chamber raises the final concentration of anaesthetic delivered. i.e. the pumping effect increases the delivered concentration of anaesthetic agent.

PUMPING EFFECTSEEN ESPECIALLY WHEN-> CARRIER GAS IS LOW-> AGENT IN VAPORIZING CHAMBER IS LOW-> DIAL SETTING IS LOW-> PRESSURE FLUCTUATIONS ARE HIGH & FREQUENT.

MODIFICATIONS TO MINIMIZE PUMPING EFFECTKeep the vaporizing chamber small OrIncreasing the size of the bypass chamber. MODIFICATIONS TO MINIMIZE PUMPING EFFECTAdd long spiral or large diameter tube to lead to the vaporizer chamber. The extra gas forced into this tube & subsequently returned to the bypass does not reach the vaporizing chamber

Increase resistance to gas flow through the vaporizer

MODIFICATIONS TO MINIMIZE PUMPING EFFECT

Check valve to prevent backward flow of gas

MODIFICATIONS TO MINIMIZE PUMPING EFFECT

Exclude wicks from the area where the inlet tube joins the vaporizing chamber.

Outlet tube may be made longer so that further back before picking up anesthetic vapor.

Connections of oxygen flush valve line to the common gas outlet be designed to minimize pressure fluctuations that may produce a pumping effect

Limit pressure transmitted to vaporizer to 20%MODIFICATIONS TO MINIMIZE PUMPING EFFECTPRESSURIZING EFFECT The pressurizing effect on the contrary is seen with higher flowsis due to compression of fresh gas in the chamberthe amount of vapour added to it remains same since the Psv is not affected by ambient pressureDuring expiration when the pressure is released, the gas expands and its total volume is increased; however the amount of vapour remains same and hence dilution in output concentration.PRESSURIZING EFFECT

1988 ASTM MACHINE STANDARDS FOR VAPORIZERSThe effects of variations in ambient temperature and pressure, tilting, back pressure, and input flow rate and gas mixture composition on vaporizer performance must be stated in the accompanying documents.

The average delivered concentration from the vaporizer shall not deviated from the set value by more than 20% or 5% of the maximum setting whichever is greater without back pressure. The average delivered concentration from the vaporizer shall not deviate from the set value by more that + 30% or 20% or by more than + 7.5% or 5% of the max. setting whichever is greater, with pressure fluctuations at the common gas outlet of 2 kPa with a total gas flow of 2 L/minute or 5 kPa with a total gas flow of 8 L/minute

A system that prevents gas from passing through the vaporizing chamber or reservoir of one vaporizer and the through that of another must be provided

The output of the vaporizer shall be less than 0.05% in the OFF or zero position if the zero position is also the OFF position. All vaporizer control knobs must open counterclockwise

Either the max. and min. filling levels or the actual usable volume and capacity shall be displayed

The vaporizer must be designed so that it cannot be overfilled when in the normal operating position.

Vaporizer unsuitable for use in the breathing system must have noninterchangeable proprietary or 23 mm fittings. Conical fittings of 15 mm and 22 mm cannot be used. When 23 mm fittings are used the inlet must be male and the outlet female. The direction of gas flow must be marked. 10.Vaporizers suitable for use in the breathing system must have standard 22-mm fittings or screwthreaded, weight-bearing fittings with the inlet female and the outlet male. The direction of gas flow must be indicated by arrows and the vaporizer marked for use in the breathing system.AGENT SPECIFIC FILLING SYSTEMS

A vaporizer designed for a single agent be fitted with a permanently attached agent specific device to prevent accidental filling with a wrong agent. They prevent accidental filling with the wrong agent.Reduce air pollution.Vaporization chamber TYPESKEYED FILLING SYSTEMSCREW CAPPED FILLING SYSTEMPIN SAFETY SYSTEMAGENT SPECIFIC FILLING SYSTEMSBOTTLE COLLARAttached at the neck of the bottle.2 projections, one thicker than the other are there. This mates with the corresponding indentations on the bottle adaptor

AGENT SPECIFIC FILLING SYSTEMS

AGENT SPECIFIC FILLING SYSTEMS

BOTTLE ADAPTORAT ONE END ISThe bottle connector with a screw thread to match the thread on the bottleSkirt that extends beyond the screw threadsSlots that match the projections on the bottle collar

AGENT SPECIFIC FILLING SYSTEMS

AT THE OTHER END IS The male adaptor that fits into the vaporizer filler receptacle.A short length of plastic tubing with 2 inner tubes connects the ends.The tube allows the bottle to be held higher or lower than the vaporizer.

AGENT SPECIFIC FILLING SYSTEMS

MALE ADAPTERHas a groove on one side to prevent the probe from being placed in the incorrect vaporizer2 holes on the other sideLARGER for agent to leave/enterSMALLER for airAGENT SPECIFIC FILLING SYSTEMS

FILLER RECEPTACLE(Filler socket/vaporizer filler unit/fill & drain system)Must permit the insertion of the intended bottleMust have a means of tightening the male adapter, to from a tight seal.Must have a means of sealing (plug) the adaptor when bottle adaptor not inserted. AGENT SPECIFIC FILLING SYSTEMS

FILLINGCap of bottle removed

Adapter screwed to the collar

AGENT SPECIFIC FILLING SYSTEMS

FILLINGVaporizer turned offPlug removedBottle with adapter inserted such that the grooves match; (tube bent such that bottle is below inlet)

AGENT SPECIFIC FILLING SYSTEMS

FILLINGRetaining screw tight Fill valve (vent) openedBottle held highAir from the vaporizer displaced by the liquid moves through the other tube and enters the air space inside the bottle Gentle up and down motion may help

AGENT SPECIFIC FILLING SYSTEMSQuick-Fil System

Vaporizer ComponentThe vaporizer filler has a screw-on cap. The filler neck has three grooves that can accept only a special filler device, which comes attached to the bottle

Bottle ComponentThe bottle has a permanently attached, agent-specific filling device that has three ridges that fit into slots in the filler. A valve prevents liquid from draining when the bottle is inverted78

AGENT SPECIFIC FILLING SYSTEMS - KEYED FILLING SYSTEMFillingThe bottle is pushed into the vaporizer component as far as it will go and held firmly in place. This will open a valve and allow liquid agent to flow into the vaporizer.DRAININGThe drain attachment is fitted to the bottom of the blockThe bottle is then inserted into the drain attachmentThe drain plug is unscrewed by using the tool attached to the filler cap. Fluid will flow from the vaporizer. After draining, the drain plug is tightened, and the filler cap is replaced and tightened

80Desflurane Filling Systems

This bottle has a crimped-on adaptorThe bottle adaptor has a spring-loaded valve that opens when the bottle is pushed into the filling port on the vaporizerTo fill the Tec 6 vaporizer, the bottle is fitted to the filler port and pushed up against the springit is rotated upward and the bottle is held in this position while fillingWhen bottle is empty, the bottle is rotated downward and removed from the vaporizer

81Difficulty in fillingMisalignment of adaptor in filler receptacleAdaptor not sealing at the bottle end Leak in the bottle adaptorAir bubbles.Lost bottle adaptorFailure of keyed systemLiquid leaksAGENT SPECIFIC FILLING SYSTEMS PROBLEMS10.VAPORIZER TIPPINGIf filler receptacle on vaporizer extends beyond the base it cannot be set upright on a flat surface this can cause tipping prevented by setting the vaporizer receptacle at the edge of the surface or a ring can be fitted at the base that extends below the projection of the filler reeptaclePOOR DRAINAGEBROKEN INNER TUBEAGENT SPECIFIC FILLING SYSTEMS PROBLEMSLOCATIONBetween the flowmenters & common gas outletBetween the common gas outlet & breathing systemIn-system vaporizersINTERLOCK DEVICESApplied to control dial of vaporizer so that only one vaporizer can be turned on at a timeEnsure that-Only 1 vap. is turned on at a time Gas only enters that which is ON at that timeTrace vapor output is minimized when OFF Vap.s are locked onto gas circuit hence correctly seated INTERLOCK DEVICESSelector valveSelectatec systemBack bar devicesOhio switchDrager lock

ORDER OF VAPORIZERSLess potent -upstreamMore potent-downstreamIf equipotentLow VP-upstreamHigh VP-downstreamAlsoIf explosive -downstreamTrilene-downstreamEasy to clean-downstream

ORDER OF VAPORIZERSUP STREAM DOWNSTREAMSEVOFLURANE ENFLURANE ISOFLURANE HALOTHANE DESFLURANESVP- 157175238243669

ADCBUP STREAM DOWN STREAMPick out correct Sequence of non-interlocked vapourisers?