theoretical considerations rb design by older

7/29/2019 Theoretical Considerations RB Design by Older

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THEORElrICAL CC!NSIDERATICiNS IN THEDliBIGN OF CWSED CIRCUIT OXYGENREBREATHING EQUIaI]l'lT

byPaul OlderProject 4/69

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CONT]l'lTSRationale

~ h e o r e t i c a l IdealsStress PhysiologyOxygen SupplyCounterlungCarbon Dioxide Absorption CanisterReducer Flow &By-passValvesRelief Valve & Purge Valve.Resistance to breathingConclusion

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Rubicon Research Repository (http://archive.rubicon-foundation.org)


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TIlE)RETICAL AND PRAOl'ICAL CONSIDERATIONS REGARDINGTHE USE OF OXYGm RSBREATHIl:lG EQUIHolJiNT FORDIVING

RATIONALE FOR TlIlil NEElD eli' HIGH EFFICIJiNCY OXYGlm REBREATHING EQUIPIlllNT.Oxygen rebreathing equipnent has several advantages over compressed

a ir "scuba" or oxygen/inert gas mixture sets. Most important i s that i ti s not possible for a diver to be detected by the presence of bubbles.In view of this the equipnent must be rega:rded as an instrument forolandestins operations. In aooepting this surmise, one must also aoceptthat the p ~ l l i o a l and emotional st ress of the d iver using this squipnentcould be extreme. I t could theoretically be possible for multiple motorunits to be u til iz ed a t once, a si tuat ion which cannot be producedvoluntarily in a laboratory. This would produce over a short period,figures for 0 consumption, 00 production and t idal volume fa:r in excessof e x p e r i m e n t ~ l l y produced sitfiations. e.g. a diver under conditions ofstress m ~ be able t o v en ti la te at his maximum breathing capacity for aperiod of several minutes. Bearing in mind these factors I think thata safety ma:rgin in the use of equipnent should be as high as pos sibl e.I t should be made impossible for equipnent malfunction to threaten thediver.2. THE TlUOCiRETICALLY IDEAL OXYG:EN REBREATHING EQ,UIl\IENT.

Unlese one oan theorise on what is ideal i t is not possibls tounify thought on what improvements are needed in existing closed oirdui to sets . To this end I suggest the follOWing cri tsr ia ars mandatory.(eertain o ther point s a:re more open tcopinion and these are discussedla ter in th is report) .(a) The duration of use of the set must be dictated by one factoronly - the exhaustion cf the 02 supply. This i s a f ini te point in time.

I t represents no d ir eo t t hr ea t to the divers l i fe . The use of pure l imits the depth of dives to approximately 25 feet and a free aecent f ~ o mthis depth would normally be easy to accomplish. The only threat tothe diver from this factor, i s from exposure to possible capture. Inorder to avoid th i s , the second principle i s evolved.(b) An emergency supply must be ca:rriedallowing a minimum of5 to 10 minutes increase duration of the dive. This supply must be

completely independent of the main supply of O2 both with regard to thecylinder i teelr and any reducing valves or constant mass valves associatedwith it.( c)circui t .fourth.

The set must operate on the principle of a completely closedAcoeptance of these three pr inciples imposes aooeptanoe of a

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(d) The carbon dioxide abeorption system must be such that a llexpired CO produced, even under the most extreme conditions, must befu l ly a b s o ~ b e d . I t i s t rue to sa:y that (,)2 rebreathing equipnent is bui l taround i t s CO2 absorption system. This is therefore discussed in greaterdeta i l later.

Use of the above principles as a working basis fo r a set wouldtend to ru le out sets in current use by the Royal Australian Navy e.g.the u:BA 5561. The LAR I I I and Fenzy sets , at present under tes t by thsnavy, would also be unsatisfactory on the same basis. All these wouldbe ruled out on the inadequate CO 2 absorption that these sets offer .In a l l sets the CO 2 elimination is not adequate from the f i r s t minutes ofuse at high t ida l volumes. (Exper imen ts performed in S.U.M. Laboratory,H!IAS P.ENGUm) 3. CONSIDERATIONS OF STRESS PRYSIOLOGY

I t is advisabls to digress from equipnent design and focusattention on the physiology of diving and stress as related to 02rebreathing equipnent. The diver upon entering the water is immediatelya hyperbaric mobile organism and as such i s subject to a ll the l imitat ionsof hyperbaric therapy in a compression chamber. Our knowledge ofhyperbaric 02 far f:t'om complete, but a working knowledge of some of theprinciples involved must be taught to a l l divers . The subje ct of sui tabi l i tytest ing to high pressure breathing is beyond the scope of this art ic le .I t i s doubtful whether s U i ~ a b i l i t y test ing i s even a valid procedure asthe time taken to produce signs of acute 0 , toxici ty vary from man to manand from day to da:y. I t is therefore i m p ~ s s i b l e to use any form ofSUitabil i ty t e s t as a rigid cri ter ia for


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- 3 -The Paul Bert effect is concerned with neuroloeical symptoms andsigns and i s highly relevant. Oxygen at high pressures produceschanges in metabolism a t cellular level and more particularly changes inenzyme activi ty and neuronal and cellular excitabili ty. The end resultbeing unconsciousness, muscular twitching and f inally convulsions. Theseneurological signs are dependant on the part ial pressure of 02 and theduration of exposure. They are not usually encountered under a part ial

pressure of 1400mm Hg. Thus dives with oxygen must be limited to lessthan 33 feet (2 atmospheres absolute ~ 1500 mm Hg).The presence of a raised part ial pressure of CO2 in the blocdgreatly increases the r isk of these complications. It is notedthat ..at . severe exerc ise the part ia l pressure of CO2 in the blood islower than at res t . The excess cellular 00 producedoeing more thanadsquately removed by t he inc reas e in c a r d i a ~ output. The result offailure to absorb a ll expired 00 i s to produce an unsatisfactoryphysiological situation in the bfood. In addition to this ,blood pH andpC02 a re st rong ly interelated. (The Henderson-Hasselbach relationship).Any r ise in 002 causes a r ise in Hydrogen ion concentrat ions andan increase in acidity of the blood. Under conditions of extremeexercise the acidity of the blood is high for metabolic reasons. I fnow the diver is forced to inspire a mixture containing CO2, the acidityof ths blood will be further increased to unacceptable levels.The design of CO 2 towers is t he re fo re the most important singlefaotor in any closed cirCUit diving set .

4. axyum SUPPLY.As was stated, the duration of the ideal set must be dictated by the

supply. In order to e st ab lis h how much 02 is re'luired i t is necessaryto know two :factors!1. How long th e diver may be required to stay in2. What i s the maximum 02 consUljlption per minutecan achieve in the water for this period.

the water.that a man

The most economical swimming speed for a diver would appear to beabout 0 .8 kno ts . (United States Navy Experimental Diving Unit Report,8th March, 1957). At a minute volume of 30 to 40 l i t res the a ctual 02consumption incurred for the work of breathing star ts to increasedisproportionately with the inc reas e in the minute volume. The minutevolume of 40 L/min. would be associated with an overall 02 oonsumption of1650 oc/min. This O2 oonsumption i s equal to a swimming speed of 0.8knots to 1.1 knots aocording to individual variation in the d iv er . Themean is 0.9 knots. In order to last 2 hours underwater a man swimming at0.8 knots could uti l ise looOcc to 1500 cc/min of 02' As the set should bedesigned to have a constant mass valve, this will De the l imi ting factoron maximum duration. In praotice, the constant mass will be around1500 cc/min ., both to limit the duration of the set and to obviate thenecessity of constantly f i l l ing the rebreathing bag whilst swimming.



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- 4 -These refer to flows at 1 atmosphere absolute. At 2 atmospheresabsolute the actftal flow would be half of this but the mass of gaswould remain the same. In other words the number of moleoules of oxygenpassing through the valve would be the same whether the flow 1186 l500ccat 1 atmosphere absolute or 750cc at 2 atmospheres absolute.

To calculate ths maximum amount of that could be u ti li sed in2 hours is very difficult. I t has been Bhilwn that an 02 consumption of2 L/min. is possible over a prolonged period. This means that 240 l i treswill be necessary for 2 hours. Allowing 10% over this gives a figure265 l i t res. I t is therefore suggested that the main oxygen supply to theset should contain at least this . In addition to these requirements anindefinite volume is required for by-passing during dsscents, and for lossincurrsd during ascents. b'or this reason a figure of 300 l i t res will benecessary. I t is very difficult to f i l l cylinders i f the requi red f i l l ingpressure is in excess of 3000 p.e . i . . I t is therefore recommended thatthe oxygen cylinders in oxygen breathing sets should be oapable of holding300 l i t res of oxygen at a pressure of 3000 p.s . i . . This will obviouslydiotate the size of any given cylinder depending on material utilized ini ts construction.

The amount of oxygen carried now places cri ter ia for the 002absorbent Le . the absorbent carried must be able to absorb CO2 for theperiod of time that a swimmer can stay under the water with 300 l i t resof 02' This in turn is controlled by the constant flow valve.Assuming a constant flow of 1.5 l i t res with an 02 capacity in the cylinderof 300 l i tree then the sot must be able to accommodate a diver swimmingalong at 0.8 knots for 200 minutes i .e . 300 divided by 1.5.The above remarks apply to the actual function of the set only,irrespective of time limits imposed upon i t by the navy. However, thef i rs t criteria set out for the set in this report must apply i . e . 02eXhaustion must be the limiting factor on the use of the set . Thisoonoept is v ita l in order to allow for a high safety margin in theequipment even i f i t i s used over and above the usual diving time. The02 oylinder needs to be as small as possible compatible with the f i l l ingpressure mentioned above. (If the f i l l ing pressure were to exoeed 3000p.s . i . , difficulty c o u l ~ be encountered both in getting i t recharged andin testing the cylinderLcA pressure reduoing valva must be incorporated to drop the pressureto around 60 p.s. i . from the 3000 p.s . i . within the cylinder i tself .This will allow for a. flood rate to the reservoir bag of at least 50 L/min Assuming a reservoir bag or counterlung volume of 6 l i t res this flow would

f i l l the bag from empty to ful l in 9 seconds. In order to increase theflow rate above 50 L/min the operating line pressure would have to beabove 60 p. s. i. Certain difficulties are encountered in obtaining a .oonstant f low with an operating pressure of 60 p.B.i. and i t maybenecessary to oompromise and increase the opera ting pressure to above thisfigure. I t has been shown however, with some other sets in current usethat high pressure lines are to be deplored as i t is possible for O-ringsand other connections to brsak away or leak. The ideal O2 rebreathingset therefore would have no high pressure lines that were not welded at thejoints or screw seals, i . e . there would be no O-rings.



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- 5 -An emergency cylinder muet be incorporated to fulf i l twopurposes. First1y in tile event of failure of the reducing valve of themain cylinder there is a reserve supply of 02 to enable the diver notjust to surface, and hence reveal himself, but to enable him to swimsome distance away underwater. Secondly when the diver exhausts hismain 02 supply he s t i l l has enough 02 to swim some distance from wherehe is at the time. The cylinder must not be of sufficient volume to

allow a diver to use this supply as a routine to prolong the dive I t is important however, that the diver should be able to return fromthe si te of his operation on his emergency tank assuming the main tankhas f , i led. I t would be reasonable therefore to make the emergencysupply something in the region of 90 l i t res. This emergency cylinderwould not go through a constant f low valve and the diver would be forcedto open th e valve to f i l l his bag as necessary, "on demand". The mainreason for my objection to the use of another reducing valve andconstant flow valve is that the more sophisticated equipment becomes, thegreater i s the risk of mechanical failure. There i s no need fOI this90 l i t res to be under a high pressure, i f however, dic ta tions of thesize of the set force the loading pressure of this cylinder to be inexcees of 1500 p.s . i . , a pressure reducing valve would become desirable.The extra duration imposed by the addit ion of the emergency 0;> cylinder,muet be taken into account when the duration of actiVity of tfie sodalime canisters are considered.

THE COUNTERLUNG OR REBREATHING BAG.This needs to be placed in a position that m1n1m1ses the effectof hydrostat ic pressure on inspiration or expirat ion. The only partsof a diving eet sub ject to the effects of hydrostatic pressure are thosewhich are compressible, and the most compressible of these is therebreathing bag. The ideal position i s probably at the level of thecarina, but the position of the diver in the water will a ffe ct resistanceimposed by this faotor. The carina seems to be the optimum place forthe bag to be positioned as this represents the mid point in t e lungwhere the gas flow will be deviated to the var ious lobes.The volume of the bag must be at least equal to the maximum Qi ta lcapaoityof any diver using the set . A figure of at least 6 l i t res isprobably the minimum aoceptable. I t is worth remembering however thatthe vi ta l capacity at a depth of 33 feet (corresponding to 2 atmospheresabsolute) would be something less than the v i ta l capacity at one atmosphereabsolute (at the surface). Exact changes in the vital capacity imposedby the increase in pressure have not yet been measured by this laboratory.The material of which the bag is constructed is also of importance.I f the material is too thick then considerable resistance is imposed onbreathing by the work involved in collapsing and re-expanding the bag;i f i t i s too thin there is some risk of the bag being torn or ruptured i fthe pressure should rise suddenly. It has been found that latex rubberis probably the optimum material with whioh to oonstruct this bag.



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- 6 -6. CARBON DIOXIDE AIlSORPl'ION CANISTERS

The efficiency of an 02 se t i s l imited by the efficiency of theCO2 absorption system.

A closed c i rcui t system makes the highest demands on any divingset , on the CO

2absorbent. All expired CO

2must be removed from the

inspired gases. In mixture sets the oonstant blow off removes some ofthe expired CO2 from the circui t .The CO2 absorbent must be able to remove a ll CO2 expired under a lloondit ions. Tlie theoret ioal maximum CO2 output of a diver under exercise(assuming the most disadvantageous respira. tory quot ient of 1) will beapproximately 35OOco per minute fo r sho rt p er iods . I t must also beremembered that the t idal volume under these conditions can also be ashigh as 350000.As stated at th o beginning of th is report , th e dur at ion of theset must be dictated by the 02 supply. In other words the CO2 absorbent

system (With an 02 supply of 300 l i t res plUB a 90 l i t res reserve) mustcontinue at 10rJ%; eff io iency fo r 260 minutes at an 02 consumption of 1.5l i tree per minute or 130 minutes at an 02 consumption of 3 l i t res perminute. I t would not be dif f icul t to design a c an is te r t o perform thisfunction but i t s size would be prohibitive and impractical . The oanistermust therefore be the smallest possible size oompatible with the aboverequirements.

I f the set oarries 390 l i t r es of 02 i t is possible with arespiratory quotien t o f 1 for this to oorrespond to a 002 production ofapprOXimately the same f igure . In other words it i s possible tocal cu la te for any given amount of 02 oarried exaotly how much the maximumamount of CO 2 absorbed would be.In order fo r CO2 to be absorbed i t must oome into con tact w ithabsorbent ohemioal. As th e a otio n i s not instantaneous i t is neoessaryfo r the exhaled gas to remain in contact with ths absorbent for aslong as possible. This means that the whole of one expired breath mustbe aooommodsted inside the absorbent packed oanister . Le . there must bean a ir spaoe between ths granules of say 35OOcc. This faotor above a l lothers imposss a minimum size on the oanister . I f the canister a ir spaoewas able to accommodate say only 2500co of a ir when paoked, thefollowing sequenoe of events would occur. Let us assume that the diverexpires 30000c a i r . The f i r s t 13000s or so would be dead spaoe a i r .This would not oontain any CO2 , The remaining 285000 would oontain CO2at a par t ia l pressure of around 45mm Hg. As the oanister could on ly hold2500 oos of gas, 35000 (2859 minus 2500) of gas oontaining CO would oometo res t outside the oanister and out of contaot wi th a b s o r b e n ~ . Some ofthe CO2 (an unknown quantity) would be absorbed in passage through theoanister . This means that the div er would inhale gas containing CO:;>,This would cause a rise in t idal volume whioh af ter a few breathe wouldfur ther impair absorption e ff io iency o f the oanister .



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- 7 -In order to provide an air space of 35OOcc, i t is necessary to use12.5 Ibs of soda lime of mesh size 4-8 (with an average of 6). Witha smaller granule a greater weight is required to provide this a ir space.

Wi th a larger granule less weight is necessary.The concept of air space is vi ta l to an understanding of the sizerequirements. Reducing the size to an equivalent of 10 Ibs of soda limereducee the air space. Under condition of exercise the CO2 absorptionwith this quantity would never be l O ~ efficient even for a few minutes.In other words there would always be a l i t t le CO2 in the inspired gas The cri terian for efficiency of soda l ime canisters must becomplete removal of a ll expired CO? Anything less is liable to re su ltin a rapid CO2 bUild up, under conditions of severe exercise.Having provided enough soda lime to give sufficient air space theduration of the canisters becooJes academic as it far exceeds anyrequirement. In our own experiments they were not exhausted after 4t hours.I t has been eho\vn by Adriani that the shape of CO2 canisters isimportant i n rel at ion to getting an even f low of gas through a ll parts ofthe tower. This wae done with anaesthesia in mind, when conditions arebasal and CO? production is around 200cc a minute. Care must be takenin e x t r a p o l a ~ i n g this to can is te rs designed for diving.Under anaesthesia peak flow rates rarely exceed 30 L!min., withdiving PFR can be as high 180 l it res per minute. Further work needs tobe done on this problem. Bearing in mind the flow rates and CO2production i t is not unreasonable to assume that the shape of can is te rsis even more crit ioal than Adriani suggests.He has shown that the overall shape should be cylindrical and thatthe length to diameter ratio should not exceed 2 to 1. This is ofimportance with regard to both good distribution of gas, the preventingof channel ling and the prob18m of resistance.The problem of res is tance to breathing in the canister is discussedelsewhere, i t can be overcome by using a twin canister r ig with thecanister in parallel. This would reduce resistance and flow rates throughthe towers. A system like this has been tested and found to be highlyefficient. The eize of these canisters is at the moment too large tobe practicable in diVing eqUipment.

7. THE REDUCDTG VALVE AHD CONSTANT FWVI VALVES.The redUCing valve must be simple in design and small in size.I t must reduce the high pressure in the cylinder to the operating pressureof the set Which, as We have said before should : ' ~ e a l 1 y be arct:lld60 p.s . i . . The orifice .hould allow a oonstant mass of gas a t d if fe ringambient pressures from one to three atmospheres.



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- 8 -The problem of whether to supply constant flow valves or demandvalves is a difficult one. I t is probably more convenient for the diverto have a demand system, bu t certainly in the interest of safety a constantflow valve is more desirable. With a dsmand system i t is possible for thediver to breathe his own expired gas until the bag is fi l led with an everincreasing concentration of nitrogen and a decreasing concentration ofA constant flow of 1.5 I i tres per minute ooming into the bag will mean tliati t will be difficult for the diver to get to hypoxio levels of 02 i f he hasonly partially denitrogenated his lungs or the oounterlung. The constantflow valve will not replae the need for denitrogenation of the lungI t is merely a safety device in case the denitrogenation is not completelyadequate. This point must s t i l l be made to a ll divers and the physiologyof this problem explained in detail .

8. VALVllB.Uni-directional valves are essen ti al t o prevent rebreathing ofexhaled gas. These must be designed so that no decrease in oross sectionalarea oocurs and the minimum disturbance of gas flow is incurred. Owing

to the variOUS position in which the set will be used these valves cannotbe gravity operated. Spring loaded valves are not satisfactory owingto the high resistance at high flow rates. Flap valves have the advantagethat they can be made light in weight and al ter the direotion of the flowof the gas to a lesser extent than the spring loaded valve. The positionof these velves in a ful ly clossd cirCUit , is probably not important exoeptthat they must be capable of being examined for wear and be easy toreplace. I t is necessary to incorporate two valves into the circuit andnot just one. I t has been found that teflon provides the ideal materialof which to construct the flaps of these valves. Cine of the problems ofrubber is that i t tends to stick up on the valve seating and also tendsto vib rate when a high flow of gas passes across i t . These problems wouldappear to be irradicated by the use of Tef lon.9. ItELI.i:lF' VALVES AIm PURGE VALVES.

A rel ief valve is required for the expulsion of gas during ascent.' ~ t h o u t i t , gas must escape from the mouthpiece or mask - with a resultantloss of a watertight f i t t ing. The volume of gas expelled may be 10 l i t resor more during ascent from 33 feet , ( i .e . total lung volume plus breathingbag volume). Without this source of escape, the expanding gas may wellpredispose to pulmonary barotrauma.

A mouth to atmosphere purge valve is required to deni trogJnatethe lungs and the set, and is of value in closing the circuit and usingthe equipment as a buoyancy vest, in emergency.



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- 9 -10. RESISTAlICE TO BREATHING.

This problem must be considered from both physiological andpractical points of view. Various f igu re s a re quoted as being acceptableto divers, a total set res is tance of 10 om H20/L. f low/sec was regarded bythe U.S. Navy as acceptable in 1957. (E.D.D, reports). They believethat the canister should no t contribute more than one third of the totalresistance. (This figure would appear to be empirical and I can seenc reason for agreeing with this. I t is the total set resistance whichmatters.) In 1965 the U.S. Navy changed the total resistunce figure tc12.5 ems/L flow/second. For an 02 consumption of 2.5 L/min, the t idalvolume (breathing pure (2) will be apprOXimately 60 L/min or one L/see.In other words swimming at a rate corresponding to an 02 consumption of2.5 l i tres a minute, an acceptable resistance figure to the U.S. Navywould be 10 to 12.5 em H20 at one atmosphere absolute.

I do not bel ieve these figures are subjectively acceptable.I f these figures are halved to 5-6 an H20/L flow/sec there is virtuallyno sUbjective sensation of resistance to breathing.

P t : ~ i o l o g i s t s have shown that although 10-12 em H20/L flow/sec .is an acceptable figure from the point of view of the ventllation perfusionrat io, subjectively figures in excess cf 6 to 8 an Hproduce difficultywi th respiration. Werk done en bicyCle ergometers shewn that whilethe sub ject is most happy with the res is tance to respiration on expiration,actual PhySiologieal performance is better with the resistance oninspiration. (Hesearch werk performed at Royal Prince Alfred HospitalSydney). This can be expla ined in the terms of the Starling resister andth e Waterfall theory. Actual impairment of ventilation/perfuSiOn ratiois apparently minimal with resistance en expiration. This problemtherefore would appear mere theoretical than practical and I wouilld havothought the most important fact r te be considered was the subjectivesensation of an;\, pGIson us ing the set .

With a minute volume of 60 L/min and a peak flow rate of threetimes this (PFR II V), a resistance of 3 om H20 t o inspi ra tion and6 em H ~ O to expiration does not produce any not1ceable effort in breathing.I suggest therefore the fol lowing figures be adopted as a working maximumfor total resistance in 0 sets at one atmosphere absolute. Resistancete expiration, 6 om H20/LYsec, resistance to inspiration, 3 em H20/L/sec.Wet carbon dioxide absorbent creates an increasing resistance bya factor which is difficult to calculate. At the end of a 2 hour dive

i t is probable that the total resistance would have risen unless the volumeef absorbent is so large as to create a problem with the size ef the Bet.In our own experiments, resistance to expiration at the commencement of atest was 5 em H20 total resistance, after 3 hours i t W2.S 6 an H20total resistance. This could enly have cemc from a change in res is tancein the canister i tself as other factors remained unaltered.

u con esearc epos tory ttp: arc ve.ru con- oun at on.org


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At this stage i t is perhaps pertinent to consider the factorsinvolved in creating a breathing resistance. Gases flowing througha tube system with a laminar flow, follow Pouiselles law

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From these figures i t is possible to calculate whether the flow inthe canisters and the circuit tubing i s turbulent. How important thisis depends on how the resistance to breathing increases in any given set,as the pressure rises from one to two atmospheres absolute. Obviouslyi f the pressure fails to r ise by more than I centimeter of water thereis no point in computing the Reynolds number for that set. If howeverthe resistance rises dramatically or doubles, i t would suggest the resistanceto flow in the set is being dictated by the density o f the gas. In thiscase a thorough breakdown of the resistance in the individual parts ofthe set will become imperative. Turbulent flow is encouraged bydistortion of the pathway of the flUid, such as junctions in the circuitory.I t is therefore important that all direction changes of the gases f lowbe achieved by gradual curves rather than right angled joints.

Suddon changes in diameter are almost un,,,voidablo, thus inducingchanges in velocity within the set. These changes wil l e ffe ct gasvelocity according to Bournellis theorum. This states that the crosssectional areas multiplied by the velocity is a constant for a fixedvolumo of fluid in a given circuit . I t is therefore possible to computewhether tho change is sufficiont to cause turbulence. The only otherfactor involved in changing r es is tance in the circuit is the slightdifferenoe in cone it itut ion of th0 g < ~ o a during inspiration and expiration.During expiration the gas would contain approximately 40mm Hg partialpressure of carbon dioxide which would alter the v is cosi ty r ela tive t oinspired mixture which contains only I mm llg carbon dioxide. ThisChange i s so slight that i t may be ignored.

In summary then the circui tory must be of large diameter tubingof noar constant diameter. This must be of such a size so that thetendency for the flow becomes turbulent is minimised. Uni-directionalvalves must no t constrict this diameter, The overall length of thetubing must be kept down to a minimum poesible compatible with the easeof use.11. CONCLUSIoN

A description on the ideal form of oxygen breathing equipment i sgiven. Many of the requiranents are not met by existing equipment.Although operation diving sets w ill never reach the ideal, they couldbe designed to be much closer to i t than is seen in current equipment



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-12-OOl'lTRlBUTORI

Dr. Paul Older M.B., B.S., F.F.A.R.C.S., F.F.A.R.A.C.S.Consultant Anaesthetist to the R.A.N.Research adviser to R.A.N .S.U .M

Approved by

( Carl Edmonds.)Surgeon Lieutenant Commllllder RANO:rficer-in-ChargeSchool of Underwater Medicine

SCHOOL OF UNDERWATER MElCINEH.M.A.S. I E l . ~ G U m


theoretical considerations rb design by older

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