nitrox and trimix rebreather rigs

7/15/2019 Nitrox and Trimix Rebreather Rigs

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4/8/11 7:ebreathers - Nitrox and Trimix Rebreather rigs

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Consumption

Inh gas temp

What's a rebreather? What types of rebreathers are on the marktoday?

A rebreather is an apparatus that recirculates your exaled breath, it remov(scrubs) out the CO2 produced by metabolism and controls the O2 additio(to replenish its consumption). Different types of rebreathers adistinguished by the method of oxygen addition. Rebreathers are classifias:

CCRO2 Closed Circuit Oxygen RebreathersCCR Closed Circuit Mixed gas Rebreathers (electronic control)CCCR Chemical Closed Circuit Rebreathers (Chemical control)SCCR SemiClosed Circuit Rebreathers (part of your exaled breath is venteto the environment

Rebreathers : Types of Rebreathers

Rebreathers : advantages / disvantages

> CCR O2 - ARO

In this kind of apparatus the oxygen addition control is linked with tcounterlung volume decrease in volume, as there is no inert gas in the looan automatic or manual addition system povides the oxygen delivery. order to respect the way the addition system works, we must inspirate frothe loop and exale the breath outside to eliminate the air present in tbreathing loop. If this is not done, the nitrogen in the system continues increase with each breath. This results in a great risk of Hypoxia whileads to unconsciouness and death. Thus, this apparatus requires purging regular intervals.

The loop can be a "pendulum" loop or a closed-circle loop:The pendulum loop has only one hose where the gas goes fore and back, this way it passes the scrub twice, but there is a little amount of gunscrubbed , so is important to get full breath to avoid the CO2 rise.The closed-circle loop doesn’t have this problem, because there are twhoses: one for exhalation and the other for inspiration. To work correctthere are two oneway valves whose design and maintenance are a delicamatter.These rebreathers are ideal for militay operations. Because they are usextensively by the military, & because they are depth-limited (due of thazard of CNS oxygen toxicity), They are generally are not availale civilian divers.

Mixed gas apparatus

Because of the limitations of CCRO2 rebreathers, other systems that woube able to dive deeper were developed. Unfortunately, to dive deeper wneed more complex & heavy systems. To resolve the depth problem toxygen in the system must be diluted with an inert gas. This requires aO2 & diluent addition system. The problem has been solved in variuos wayand each one generates different types of rebreathers systems.

> CCR Closed Circuit RebreathersClosed circuit mixed gas rebreathers usually employ an electronic controll

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CCRO2- Pendulum

CCRO2-Circle loop

system that mixes oxygen with a diluent (Titrox, Trimix or Heliox). Tprinciple of how a CCR operates is simple, but its design and usage are noAs with any rebreather we find counterlungs and scrubber. Additonalthere is a diluent tank, an oxygen tank, and electronics (the brain of tapparatus that controls the composition of breathing gas). The key is tPpO2 set control point: there are three sensors that generate a voltaproportional to PpO2 in the loop, the electronics of the apparatus compathe avrerage sensors output, and if lower than the set PpO2 they opensolenoid to admit a pulse of oxygen into the loop. Electronics compare althe difference of each sensor's output to detect failures. Depending on thmake & model of rebreather the electronics may depend on

microprocessor which controls various things, like the battery's level.Diluent is fed by an automatic addition valve, to mantain the volume of tloop as the diver descends. As the diver ascends the overpressure relvalve opens to vent the breathing gas into sorrounding water. There aalso at least two displays to show the status of the rebreather and PpOinto the breathing loop: one is the primary display, like the "idiot lights" the car that tells if everything is ok; the secondary one shows the partpressure readings of each sensor. More modern CCR’s have two identicdisplays. The primary is the one you switch on first, the secondary operatif there is a failure of the primary. Both show all available information.CCR’s have great depth capabilities and extended bottom times because thsupply only the oxygen necessary for metabolic consumption. Moreovethey mantain a set PpO2, therefore the oxygen percentage changes widepth. This results in great benefits for decompression.

> CCCR Chemical Closed Circuit RebreathersThis kind of Oxygen control was developed in Russia and is used both fpure oxygen and nitrox closed circuit rebreathers. The apparatus is vesimple. It is like a CCRO2 rebreather, but with two scrubbers. One is fillwith a scrubber agent, the other with a special Superoxyde that producan amount of oxygen proportional to the absorbed CO2. From physiological point of view it's correct, because the CO2 that metabolisproduces is proportinal to the O2 consumption. The choice of Superoxyde also appropriate because the Russian’s dive in a low temperatuenvironment. The superoxide chemical reaction supplys heat useful for tscrubber agent efficacy. There is an oxygen or nitrox tank to fill the looThis 02 control system seems to be a good idea for its simplicity, bSuperoxyde is a harmful chemical. The exotermic reaction is caustic, whiwhen exposed to water can kill you. It is also highly flamable, and if watsaturated (as in a system flood) can melt down the canister and everythinsurrounding it. It also contains Asbestos, so if it doesn't kill you now, it wkill you later. This is the same chmical that caused the fire on the Mspace station.

> SCCR Semiclosed Circuit RebreathersIn this kind of apparatus oxygen feed is linked to the replacement breathing mixture: a little part of the breating gas is wasted to tsorrounding environment and replaced with premixed gas that supplies tnecessary oxygen.

Active addition rebreathers-Constant Mass Flow-PremixedThis is the more common kind of SCCR, in this apparatus a metered valinjects a constant mass flow from a premixed tank. The flow doesn't depe

on depth or the percentage of O2 of the mix. The surplus gas in the loopwasted by an overpressure valve, like the "Wings"one. This way the oxygdelivery rate is constant with the depth, as the metabolic oxygconsumption is not dependent on the depth but rather the work rate: Thrate varues from 0.3 lt/min at rest to 2.5-3 lt/min at maximum work ratTherefore the percentage of oxygen in the loop is dependant on the worate. Our body must have a minimum PpO2 of 0.16 bar not to go hypoxand a maximum PpO2 1.6 bar not to risk oxygen SNC toxcity,.The max operating depth calculations are dependent upon the tank's O2 mpercentage, as the oxygen percentage can't be greater of the tank's onThe minimum PpO2 in the loop would occur at the surface with max worate, so we must calculate the minimum constant flow to feed enoug





CCR- Closed Circuit Rebreather

oxygen for this situation (if we want to use a single mix from bottom surface). A leaner mix will require an higher flow.

Since the percentage of inspired oxygen varies in the loop depending on thwork rate , we have to calculate our decompression obligations in relatioto the max PIN2 in the loop. This occurs at max work rate, when there less oxygen and the Nitrogen percentage is highest. Doing so we will do very conservative decompression, but we loose theNitrox decompressiadvantage. There are special computers available that can calculate odecompression by monitoring PpO2. Since the actual PIN2 present into th

loop is calculated, our decompression obligations are optimized.

Thus, with SCCR our mixture doesn’t vary with depth, only workload. If wwant to go deeper we must choose a leaner mix, which means we must sa greater flow, in order not to go hypoxic at surface.

Metered orifice-sonic nozzle

In active addition SCCR a metered orifice supplies a constant mass flow mix, into the loop. When the gas flows through an orifice, if the upstreapressure is more than twice the downstream pressure, the velocity at torifice becomes sonic and the mass flow becomes constant. Furthincreases of upstream pressure are dissipated in shock waves at tdownstream orifice.The mass flow rate is proportional to the size of the orifice and tupstream pressure, so in order to get a desired flow we need different sizof orifice, or a needle adjustable one.As the flow is proportional to upstream pressure, the design of the pressureducer that supplies the upstream pressure is also important : If tpressure reducer is a normal first stage, the intermediate pressure grows the depth increases and so does the flow. This causes a little waste of mibut not so great if compared to the consumption for BCD or dry sinflation. Furthermore the oxygen percentage in the loop will be a lithigher with advantagous deco obligations. If the pressure reducer is a"absolute pressure reducer", it will supply a constant intermediate pressurregardless of the depth, and so the mass flow will be nearly constant as tdepth increases. As the speed of sound depends on the density of the g(hence by the pressure), with an absolute pressure reducer the flow slighincreases with depth. Furthermore, in order to get an upstream pressuthat is twice the ambient (downstream) pressure at maximum depth, tintermediate pressure must be set to a higher value. This means the size the orifice will be smaller and its reliability is more critical.

SCCR calculations for constant mass flowLet's now calculate the percentage of oxygen into the loop.

Being:

fO2 - oxygen percentage into the loop

FO2 - oxygen percentage into the tank

VO2 - metabolic O2 consumption lt

- mix constant flow lt/min

the formula to compute what's in the loop is:





CCCR- Chemical Closed Circuit Rebreather

SCCR- CMF Semiclosed Circuit Rebreather/ConstantMass Flow

Example#1:

FO2 0.4 (40%-Nitrox 40)

= 9. lt/minVO2= 2.5 lt/min

So :

is about 17% of oxygen , so at surface the minimum PIO2 may be 0.bar: too close to hypoxic limit.. To be more consrevative we must choosehigher flow (or a mix with an higher oxygen percentage)

Example#2

FO20.4 (40%- Nitrox40)

=10.4 lt/minVO2= 2.5 lt/min

So :

The percentage of the mix is the same as air and the flow ensures enougoxygen at surface

Example#3

If 1 lt/min is the average O2 consumption we compute the average oxygpercentage in the loop:FO=20.4

=10.4VO2= 1 lt/min

So:

Example #4

We are going to compute oxygen percentage at minimum work rateFO2=0.4

=10.4VO2=0.3

So:

Self mixing constant mass flow rebreathers

With this kind of rebreather the flow is supplied by two tanks. One contai





SCCR- Semiclosed Circiut Constant Mass flow Selfmixing

SCCR- Passive Addiiction-Inhalation phase

pure oxygen and the second a gas mixture (Nitrox, Trimix or Heliox). Theis a pressure reducer and a metered nozzle on each tank, so the flow & thmix is composed of the combination of both the flows. A feedback systemtied to the exhaust valve, provides for adjustment of the flows according the depth (to ensure the correct PpO2). There are two metered nozzles fthe by-pass too, in order to supply the correct percentage of the mixture.Of course self-mixing rebreathers are more complicated than premixerebreathers, and are not as efficent as Closed Circuit. But they do have tadvantage of not depending on electronics, and they have the ability adjust the mixture by the depth.

Passive addition rebreathers

In passive addition rebreathers, a small portion of the gas mixture in tloop is vented to the surrounding atmosphere. This amount is then replacefrom the system by a demand valve. In this way consumed oxygen is rsupplied from the mixture in the tank. The modality with which the portiof mixture is vented characterizes the various apparatus types. One type system is constant flow. A constant flow system has the followinadvantages: 1. A fixed amount of gas is replaced at each ventilation; 2.stable percentage of 02 is injected into the loop, since only a fixed amouof gas is injected; 3. The relationship between metabolic consumption of Oand ventilated volume, RMV/VO2, are comprised between 0.035 to 0.05

with the metabolic varied oxygen consumption between 0.3 lt/min to 2.5lt/min. 4. If something goes wrong, the bag volume will diminish untilcomplete inspiration is impossible, thus signalling the need to switch to obailout system.

The way in which the passive addition occurs defines the system. Additiosystems in use are: Manual, and Automatic which in turn can be eithFixed or Depth Compensated. Manual system: In this system, aftergiven number of breaths a small amount of gas is expired to the outsidand then is replaced from "fresh" mixture. The number of respiratory cyclincreases with the depth, and consequently so does the pressure the oxygemass in the circuit. Automatic units: The automatic units let out opercentage of the ventilated volume every respiratory cycle. This percentagcan be fixed, depth compensated, or partially depth compensated.

Fixed percentage are the simpler of the two systems. A fixed percentage the ventilated volume is expelled from a system. The system is composed two lung bags (inhalation which is larger & exhalation which is usually on30% to 50% the size of the inhalation bag). At each inspiration the largbag swells compressing that smallest one, which vents to the outsidthrough the overpressure valve. The relationship of volume of the two bais equal to the percentage of venting. With this system, the autonomy estimated like opened circuit, but multiplied from the relationship "venting". Therefore, if the unloaded percentage is 1/12, the autonomy wbe 12 times greater. With increasing depth, the composition of the mixtuin circle approaches the mixture in the bottle. As depth increaseconsumption increases, therefore it is attempted to correlate the relationshbetween the two bags with the depth.Depth Compensated. The ideal would be to obtain an expelled volum

inversely proportional to depth. This would waste one constant gas mass, nmatter what depth. This would be similar to the efficiency of a CCR withohaving to depend on the electronics. This has been a great technologicchallenge for the constructors. To date, no systems on the market haachieved true depth compensation. The currently marketed units attempt correlate depth to bag volume in some way. Therefore they are actually onpartially depth compensated. This is confirmed by the declarations manufacturing companies, when they assert that an increase in depcauses the mixture in the bag to approach that of the bottle.

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SCCR- Passive Addiiction-Exalation phase

nitrox and trimix rebreather rigs

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