| REBREATHERS |
DESIGNING A REBREATHER FOR EXPLORING THE ACHERON (ST CLAIR CAVE, JAMAICA)
By Guy Van Rentergem, 2008 ©
In this document we describe the different parts of an oxygen rebreather for exploring and surveying the Acheron with its foul air. The main purpose of the design is to make it possible to visit in the Acheron during three hours (1 hour go, 1 hour return and 1 hour reserve), this with a double safety margin.
This means the rebreather should be reliable for minimal 5 hours doing light too moderate exertion. When in rest, consuming much less oxygen, the dwell time should rise to 15 hours and higher.
Other requirements are:
build, light, cheap and general available components, more or less cave
2. How an oxygen rebreather works
2.1. The very short:
An oxygen rebreather is a closed breathing system filled with 100 % oxygen which recycles the user’s breathing gas. It provides oxygen to, and absorbs carbon dioxide produced by the user. The main parts of a rebreather are: the mouth piece, breathing hoses, counter lungs, scrubber, oxygen supply and regulator.
2.2. The not so very short:
Let's say the rebreather is ready to be used and the counter lung is filled with 100 % oxygen. When we now inhale from the counter lung it will be emptied for a great part. Our body will use the inhaled oxygen it can use and convert it to carbon dioxide. When we breathe out into the mouthpiece there will be now about 4 % CO2 in the oxygen. The exhaled gas passes through the scrubber filled with lime where the CO2is absorbed and then the gas (now mostly oxygen again) is going into and filling the counter lung again where it is ready to be inhaled the next time.
As you can
see, 4 percent of the gas
volume has gone since it has been absorbed by the lime in the scrubber.
why a constant flow of oxygen is added into the loop. So the volume we
absorbing the CO2 is refilled by a constant oxygen flow from
compressed gas cylinder.
If the oxygen flow is too low the counter lung will be empty after a certain time and we can't breathe anymore. This happens when we do heavy work. then we need more oxygen and produce more CO2than added by the constant flow. A manual bypass valve will put more oxygen into the loop.
There are a
lot of complicated
tricks to regulate the oxygen level in a rebreather but for our goal a
flow of oxygen is good enough. This is rather simple in construction at
cost of having some venting when we are resting. A manual bypass will
aid us when we need more oxygen than the normal flow.
Let’s assume the total volume of the breathing loop (this is mouthpiece, hoses, canister and counter lungs) together with our lungs is 10 litres (0.36 cu ft). Now we take one breath from the outside through our nose and exhale into the breathing loop. That makes about 3 litres (0.1 cu ft) of air. The composition of air is 79 % N2and 21 % oxygen. So our 3 litres of air contains 2.4 litres (0.08 cu ft) of nitrogen. 2.4 litres N2 on a total volume of 10 litres means that 24 percent of our breathing loop will be filled with nitrogen gas!!! As you can see any addition of outside air will make the gas in the loop very fast unbreathable. The user of the rebreather will lose consciousness rapidly once there is 84 % percent inert gas (nitrogen and/or CO2)!
There are several causes why the percentage of CO2 in the loop can rise:
breakthrough is not too bad.
This happens during heavy exertion when more carbon dioxide is produced
absorber bed can handle. Once the workload returns to normal the
will stop. The other causes are fatal and must be avoided by careful
Always fill the scrubber with a new load of lime when using the
never know what happened with the lime in the scrubber and old fillings
always be disposed. Only a new load of lime will guarantee us the good
of the scrubber. And the filling of the scrubber must be done very
in a controlled way. The lime must be stacked firm enough but still
gas to be passed but no rattling must be heard when the scrubber is
over air in the rebreather
must be purged before use. Normally purging goes like this: we breathe
the mouthpiece and exhale through the nose (without the nose clips).
This we do
till the counter lung is empty. When ready we must put the nose clips
But to shorten this purge cycle (should be done three times!) the
be pulled vacuum through a manual pump before filling it with oxygen.
3. Parts of a rebreather
The question was if we were going to use full face mask or a mouthpiece. The full face mask seems to be the most favourable but in the hot humid climate where we will use the rebreather the mouthpiece got the preference. The main problem with a face mask is the seal between the face and the mask which is not 100 % air proof. The risk for contaminating the loop with nitrogen from the air is possible. Another problem is fogging of the face mask and excessive sweating, making it all very uncomfortable. We need a good view during the exploration, so everything hampering our sight must be banned.
The mouthpiece of a rebreather has two hoses. One for exhaling and one for inhaling. There are also two valves inside forcing the air in one direction through the breathing loop, enabling the right functioning of the rebreather.
If possible a shutoff valve should be installed. This prevents losing oxygen and polluting the loop with nitrogen when pulling the mouthpiece out of the mouth. But this also makes the construction more difficult. The shut off valve is a must when diving where the risk of flooding the loop with water is prominent. But a plug can do the same job in out of the water situations. The mouthpiece can be extracted from the mouth for a short time enabling the plug to be inserted without too much worries. The loop pressure is higher than the environment preventing nitrogen to rush into the system.
An elastic strap will be attached to the mouthpiece giving some relief to the jaw muscles.
The scrubber is a container with a bed of lime. The air is forced through the lime and will remove the user's exhaled CO2 through an exothermal chemical reaction.
We go for the vertical mounted axial scrubber. The radial scrubber seems to be more efficient but the axial type is a much simpler construction. Also the vertical axial scrubber is less prone to channelling. We can go for this design because the rebreather will be used most of the time in upright position, this in contradiction with a diver which has much more axis of freedom. The diameter of this canister must be > 15 cm. This to assure an optimal surface for the reaction front in the scrubber.
3.3. Breathing hoses
part of the breathing loop
and connect the mouthpiece with the scrubber and counter lungs. They have
flexible enough to assure a certain degree of comfort during use. Also
hoses must have a minimal diameter of 2 cm (3/4 inch) enabling minimal
breathing resistance. A higher diameter is even better but this will
weight of the machine. Remember we carry the rebreather on the back.
diver weight is not really a problem because of the law of Archimedes.
rebreathers do indeed weight more than 100 pounds!
The rule of thumb is that 1 kg of lime is needed for 1 hour rebreather time doing moderate exertion. The scrubber will be filled with 5 kg of lime. This gives us a large enough safety limit.
3.5. counter lung
A rebreather is a closed
when we exhale the air has to go somewhere to be temporary stored. This
in the counter lung.
It is a flexible bag
which expands when we exhale and contract when inhaling.
So the total volume of the loop remains
constant during the breathing cycle.
3.6. Oxygen cylinder with constant flow mechanism
The oxygen is delivered in the rebreather as a compressed gas. See chapter 4 for more details.
3.7. Relief valve
When oxygen consumption is lower than the constant flow, the counter lung is filled to capacity and the system must vent through a relief valve. The pressure for opening the relief valve must be lower than 1.6 bar. At this pressure oxygen becomes toxic!
3.8. Manual demand valve
There is the possibility to bypass the constant flow of the oxygen when the consumption is higher than the delivered amount. This valve should be placed in a convenient place but safe enough to prevent accidental activation or damage. The right side of the rebreather at the lower end seems right.
A lot of heat is generated in a rebreather. A way to dispose the heat is necessary. There are different ways to cool down the air. Some constructions use sealed tubes with low melting point salts like potassium phosphate hydrate. This is rather heavy and only useable till all the crystals have melted. Other designs use ice instead of salts. It has been proved that this method is not feasible for our goal. The ice provokes condensation generating more heat than the ice can take absorb by melting. another way of cooling is a heat exchanger. This with the extra help of water evaporating on the surface seems to be the way to go. This passive method can be given an extra boost by adding an extra fan forcing the evaporation of the water. See also chapter 7.
3.10. Oxygen sensors
We need to know what we breathe. This should be almost 100 % oxygen. But there will be a certain percentage of nitrogen in the gas too. This must be kept as low as possible. Nitrogen can't be absorbed by the scrubber and will accumulate in the loop till the percentage of oxygen will be to low to be breathable. This is also the reason why we purge the system before use. To aid this purging we will pull the system vacuum. Another source of nitrogen will be the gas dissolved in our blood. A certain amount of nitrogen will be freed from our blood since the partial pressure of nitrogen in the loop will be less than that in our blood.
Another gas in the loop will be CO2. Normally this gas should be absorbed by the scrubber. But there is the possibility of a breakthrough of the scrubber or that all the lime in the scrubber is depleted. 10 % CO2 in the loop means a certain death. Problem is that for the moment there is no decent (cheap) way of measuring CO2 in moist air. The only way we can check this is to be on alert for the symptoms of CO2 intoxication. Therefor we will monitor a list of different parameters of the system each five minutes. These parameters could be monitored automatically but it is better that the explorer checks the system himself. This as a self-test. If we don't succeed in this simple task due to concentration problems or fatigue then there are indeed troubles. That's also the reason why we go with two persons, to check each other with this task. The moment there are problems we have to end the exploration and return to base.
3.11. Pressure gauge
gauge is to monitor the
pressure in the oxygen cylinder. The pressure gives an indication of
oxygen we still have and is a good indication of how long we still can
3.12. Water traps
There are different sources of water available in a rebreather:
- The chemical reaction in the absorber produces water, roughly 157 ml of water is formed per hour!
- a lot of saliva is produced because of having the mouthpiece in the mouth. This amount will differ per individual.
- there is also a lot of evaporation of water through our lungs. What we exhale is moist air.
All this water must be kept out of the system because it can cause serious harm. Too much water in the scrubber can influence the efficienty of absorber in a bad way. another problem that can happen is the flooding of certain parts of the loop making breathing harder and hampering the good working of the system.
Throughout the rebreather temperatures goes up and down forcing the air to contain more or less water vapour. This vapour transports water through the loop. Condensation is to be expected at the coldest places of the loop. In the scrubber the temperature can rise to 100 degrees C and can indeed produce steam. What we exhale will always be 36 degrees C. The coldest point in the rebreather will be the cooler, which will have a temperature of probably 30 degrees C. So the cooler must be built as a water trap. Another water trap will be the bottom of the scrubber. This is a big device and can hold a lot of water.
exhale tube saliva will dribble
down and must also be collected or purged. The relief valve will be put
the saliva collects. So when the loop purges this source of water will
4. Human oxygen metabolism
Table of oxygen
metabolism of a
human (male 70 kg)
(Source Mastering Rebreathers, Jeffrey E. Bozanic, p 83)
According to Ake Larsson
(www.teknosofen.com) oxygen consumption as high as 2 L/min is rarely
real dives. In the NIOSH report RI9650 “Performance
Comparison of Rescue Breathing Apparatus” the rebreathers are
In the NIOSH report RI9650 “Performance
Comparison of Rescue Breathing Apparatus” the rebreathers are
VO2 (O2consumption) = 1.35 l/min
Tests are terminated when CO2> 10 % or O2 < 15 %
For the exploration of the Acheron an oxygen consumption of 1.1 l/min will be used. Surveying does not take much energy and can be catalogued as a light activity. Of course exploration can be moderate to heavy exertion. But we have to prevent this and do everything in a slow and relaxed way (the Jamaican way…). A temporary high oxygen demand can be catched by the extra addition through the manual demand valve. But what we have to prevent is the breakthrough of the scrubber bed. A breakthrough happens when the volume of carbon dioxide is bigger than the scrubber can take. Resulting in carbon dioxide passing through the bed, polluting the loop.
5. Oxygen cylinder
We had to find a local source of oxygen in Ja. Thanks to Jan this source was found at IGL (industrial gases ltd) Kingston.
D Size aluminium
Oxygen cylinder containing 425 l of oxygen (15 cu feet)
CGA 540 port valve for
Medical oxygen is shipped at a pressure of maximal 2015 psi (139 bar) for safety reasons with the special CGA870 valve.
The real volume of this bottle (when filled e.g. with water) can be derived by
= P2.V2 pressure 1 x volume 1 = pressure 2 x
volume 2, temperature must be the same
pressure 1 x volume 1 = pressure 2 x
volume 2, temperature must be the same
Since we use a constant mass flow of 1.1 liter or 0.04 cu ft it's possible some of the oxygen will be purged in moments of low activity and at moments of high activity we'll have to bypass the constant mass flow to add more oxygen in the loop. But we are still on the safe side of the requirement of a 3 hours rebreather knowing we have an 5 hours oxygen supply. And that's the way I like it. With normal diving gear we would never get this done with such a safety margin.
Soda lime is composed of water (16-20 %), sodium hydroxide (NaOH) or potassium hydroxide (KOH) and Calcium hydroxide (Ca(OH)2) where calcium hydroxide is the most abundant compound (70-80%). The bulkdensity is about 0.9 kg/l
Commercial Soda lime a specially prepared mixture of calcium and sodium hydroxides. The granules are creamy white in color, hard and processed to minimise dust formation. It is also porous, and irregularly shaped to provide for a larger surface area.
When put in contact with a acidic gas like CO2, a strong, exothermic (heat producing) reaction takes place which gives off water and binds the CO2 by forming a stable Calcium Carbonate. When binding to other acidic gases, other calcium or sodium salts are formed. Sometimes a color indicator is added which aids in indicating the useful life of product. As the soda lime becomes less effective at binding CO2, the color of the product changes to a violet color. This color change should not be your sole method of determining when to replace your absorbant, but it is an indication of use.
Commercial products are e.g.: Sofnolime (Molecular Products ltd), SodaSorb (W.R. Grace & Co.), Divesorb (Drager)
The general description of the reaction is as follows: First the gaseous CO2 reacts with water to form carbonic acid - H2CO3. Then the NaOH reacts with the carbonic acid to produce Na2CO2 and H2O. The Na2CO2 reacts with the Ca(OH)2 which has been disassociated into Calcium and Hydroxide Ions. (Ca++ and OH-) to produce CaCO3 (calcium carbonate, otherwise known as limestone.) The CO2 is now in a relatively stable state. There is a net production of three H2O molecules for every molecule of CO2 which is taken in.
Here is the reaction in
2H2CO3+NaOH+2KOH -> Na2CO3+K2CO3+4H2O (-89.4 kJ/mol)
The caustic alkalis draw the acid gas out of the passing mixture and hold it. The sodium and potassium carbonates then react with the hydrated lime, forming the unsoluble calcium carbonate and regenerating sodium hydroxide and potassium hydroxide
Na2CO3+K2CO3+2CA(OH)2-> 2CaCO3+2NaOH+2KOH (+12.3 kJ/mol)
each mole of absorbed CO2 (44g) will produces three mole of water (54 g)
1 mole of carbon dioxide weights 44 g and has a volume of 22.4 l at atmospheric pressure.
1 l of CO2 weights 1.97 g
1.1 l/min of oxygen is added to the loop which is converted by our metabolism in CO2, giving 1.1 l/min x 60 min/h / 22.4 l/mol = 2.9 mol/h
2.9 mol CO2/h x 3 mol H2O/mol CO2= 8.7 mol H2O/h
8.7 mol H2O/h x 18 g H2O/mol H2O = 156.6 g H20 = 156.6 ml H2O
about 157 ml of water is formed/hour through the chemical reactions of CO2 absorbtion.
6.2. Which mesh size to use?
(granule size of 2.5 to 5 mm or .098" to
.197") is the most popular for recreational dives. The 8-12 (1.0 to 2.5
or .039" to .098") will last about 1.3 to 1.5 times longer then 4-8
given the same dives. If you are doing extreme dives you probably want
8-12, if you are paddling around on reefs, the 4-8 will do just fine.
7. Designing a Cooler
Two sources of heat are available in a rebreather:
As tested by myself (on June 30 2008) air hotter than 50 °C (122 °F) is unbearable. So the air in the rebreather must be cooled to a temperature were we can do the job as comfortable as possible.
Normally, in a rebreather there is no direct source of cooling available. But if the system was used during diving then a substantial amount of cooling would be delivered by the surrounding water. But since we are using the rebreather in the hot moist air of a Jamaican cave no cooling power can be suspected from our environment.
So we have to add a cooler. The first design was using ice as demonstrated in the professional rescue breathing apparatus by Drager, the BG4. In this machine a 1.2 kg (2.65 lbs) block of ice sealed in a plastic container is used. Test conducted by me on 30 June 2008 proved that this is a bad system. One of the weak points is that the cooling ice invokes condensation of the moist air inside the rebreather. Condensation of water vapor releases almost 7 times more heat than the heat needed to melt the same amount of ice. So most of the cooling power is wasted in condensating water. A second drawback is that 1.2 kg (2.65 lbs) of ice can only absorb a limited amount of heat till it is all melted and the water has the same temperature of the inside air of the rebreather. This means that the cooling power is finite in time ( Heat of fusion of ice 335 J/g, Heat of vaporization of water 2260 J/g).
Only reason I see why they use ice in the BG4 is the psychological factor. Something like: it’s ice so it cools the best. And the machine is from Drager and it costs a fortune so it must be good.
On 3 July I did test with another type of cooler. The air was send through a 3 meter (9.8 ft) long copper tube covered with a wet cloth. This setup of passive cooler was impressive. The cooling power was 30 °C (86°F) between both ends of the tube and the system did not need any extra power to cool. Extra ventilation to stimulate the evaporation of the water in the cloth gave an extra boost of 5 to 10 °C.
The construction of a copper spiral was not very practical and after several trails (wasting some meters of good copper tubing) and other design was necessary. The final design is a long thin walled plastic tube which worked remarkably well.
8. Cleaning the rebreather
All parts of the
should be disinfected (except the scrubber). There are different brands
disinfectant like poloxamer-iodine (Wescodyne), Virkon S or RelyOn MDC
but povidone-iodine (10%) (Betadine) can also be used.
9. Purging the rebreather before use
A rebreather has to be purged before use. Otherwise a large volume of inert gas (N2) can cause low oxygen levels leading to hypoxia (lack of oxygen).
The system has to be purged by repetitively inhaling all the gas in the loop through the mouthpiece and exhaling through the nose. Do not inhale from the ambient atmosphere during this procedure. Once the loop is as empty as possible, the loop has to be refilled with oxygen from the supply cylinder. Repeat this three times without removing the mouthpiece. Once this is done the unit ready to use.
Also if you take a breath
for example to speak to someone, you will have filled your lungs with a
quantity of nitrogen. Before resuming use of the rebreather, you must
your lungs of the inert gas by repetitively inhaling from the
exhaling through your nose. If you exhale into the rebreather without
this, you will add inert gas to the system, creating the potential of
THE EXPLORATION OF THE ACHERON