Oxygen Rebreather Design and Fabrication

Going on the loop, Acadia NP

To Swim is Human,

…To Dive is Sublime,

……To Suck Thine Own Bubbles is Extreme

Dr. Robert R. lannello, Director, CEDU

There is something romantic about making a machine that to allow you to travel past the limits of terrestrial physiology. From the early sci-fi of Jules Verne to the Apollo missions, life support systems have been an icon of adventure for hundreds of years. Here I will document my own prototype oxygen rebreather, as well as some figures for those seeking to make their own dive equipment, including detailed drawings of the prototype rebreather I made.

Theory Of Operation

Click through slideshow for annotations

The type of rebreather I built is the oldest and simplest type- an oxygen rebreather. As its name implies, the gas that is breathed and added is 100% oxygen. If a human breathes in a breath of 100% oxygen, they will breathe out a mixture of CO2 and oxygen. If the toxic CO2 is removed, you can re-breathe the remaining gas- hence the name.

In a rebreather, CO2 is removed by a chemical scrubber, which is the white stuff in the horizontal tube in the above photo. In order for the CO2 to be removed, it needs to be pushed through this tube of scrubber. In a rebreather, the lungs of the diver are used to move gas by breathing. Obviously, a diver would not be able of compressing all the gas in their lungs into a rigid container, so there are soft counterlungs in the gas path. The gas moves back and forth from the lungs of the user and the counterlungs, through the scrubber. The simplest oxygen rebreathers do not even move gas in a loop- they are of a type called a “pendulum rebreather”, where the bulk of the gas is in the counterlung or in the lungs of the user. This rebreather is slightly more sophisticated- the gas moves in a loop instead.

This is achieved by a one-way valve in the mouthpeice. It only allows exhalation to one side of the scrubber, and inhalation from the opposite side of the scrubber. In a pendulum rebreather, it is easy to have dead volume on the ends of the pendulum- short breaths can create a buildup of CO2 which is bad. This is less likely to happen with a loop.

Of course, with oxygen is consumed by the body, and CO2 is absorbed by the scrubber chemicals, so over time the volume of gas in the loop decreases until the diver can no longer breathe. The silver item with two hoses coming out of it is the manual add valve (MAV). This allows injection of oxygen into the system.

Experienced divers (or even beginner divers, or anyone familiar with boyles law) will realize that on ascent, gas expands and that there needs to be some kind of gas dump. In order to minimize leakage potential, the gas dump for this unit is the users nose, which vents from the lungs to the water.

Design Philosophy

Finished CAD model

The driving philosophy behind this rebreather was to make something that was simple, but not too simple. For example, instead of using zip ties, clear PVC tubing and hose barbs, I went to the trouble of machining fittings for proper scuba hoses- in my opinion these are a lot safer, even though they are more expensive and require more complicated machining. On the other hand, there is no overpressure dump valve, because I did not want to botch something that could accidentally allow water into the system, which would be unpleasant.

Engineering and Physiological Constraints

rough size comparison of the ubiquitous AL80 to a bernz-o-matic disposable cylinder

The kernel of this rebreather design and sizing is the duration that it can be used to support life. In this case, the limiting reagent is oxygen. If you use more oxygen, you will be up and out of the water faster. The amount of oxygen also determines the amount of scrubber which is appropriate. For this prototype, I am using disposable welding gas cylinders. They are quite low pressure, roughly 450PSI*. This is calculated from knowing that there are roughly 30L in the cylinder, and the volume of the cylinder is about 1L. Therefore it should be 30*15 PSI. This corroborates the information found here.

*N.B. feeding this directly into scuba inflator hoses violates the hoses 300 PSI rating- a regulator would help with that. However, the volume of gas is so small that after only a small amount of O2 is dispensed, we are back under the pressure rating. I can’t say anything here aside from that I turn the O2 on very slowly, and that these hoses will probably have a very low cycles.

An M4 medical cylinder of about the same size filled to 2200 PSI might carry 113L of gas, and a scuba AL6 cylinder would carry ~160L gas. These are all very similarly sized cylinders, but the higher pressure allows for a HUGE improvement in capacity.

One question to ask is “how long will 30L of O2 allow someone to live?”. The answer (as always) is that it depends. The rule of thumb for O2 consumption that I used (pulled from here) is 3.5mL/(kg*min) at one MET where the weight is you body mass. That means as a 90 kilo person I consume about 19 L of oxygen per hour at one MET. One MET (metabolic equivalent) is if I am sitting in a chair- a walk might be 3 MET and something like running could be higher- therefore, the duration that the oxygen will last is heavily dependent on body size and personal physiology. However for a very relaxed dive, it seems like 30L could last 15 minutes or so (testing on the couch seems to confirm this rough estimate). Unlike OC scuba, there is no volumetric penalty for breathing at depth, since the gas is recycled- although more gas is required to fill the lungs to the minimum volume at depth.

The next question to ask is “How big should the scrubber be?”. One of these disposable oxygen cylinders contains about 32.5 L of oxygen, which is 44 grams. Using some math (and chemistry) we can see that 44 grams of O2 means that there are 60.5g of CO2 produced. I used this paper to derive what the ratio mass:mass of sofnolime to CO2 should be. The paper gives the time until complete scrubber breakthrough, where ppCO2 is about .1 Atm, which suggests a 3.3:1 ratio of sofnolime to CO2. I backed off of that to where the curve started to really take off around .05 Atm, and calculated using a 3.7:1 mass:mass ratio. In my calculations I also added a factor of safety of 2 to account for any variations in scrubber quality or condition. This gives me a scrubber length (for the designed diameter) of about 4″, but embarrassingly I decided to make the canister much longer to match it with the width of the O2 bottle+regulator I had, assuming they would also be mounted horizontally. On the bright side, I could pack the scrubber once, do a dive, swap tanks, and then get in another dive. You can see my math here.

Old chest mounted design- the valve was to be manually operated (no MAV) to add gas to the loop

On this iteration, I decided that the end caps of the design would route the hoses out horizontally. Ultimately, this made the unit very wide, and I think it would have been wiser to have the hoses routed straight up. However, this did allow for more configurations- front mount or back mount with a larger bend radius, and therefore less force on the divers jaw.

Components

The next few sections describe details of various components of the rebreather.

DSV

A dive surface valve (DSV), allows the loop to be closed off by closing the opening to the mouthpeice when at the surface. This is useful for when you are between dives and do not want to let ambient air into the system. The DSV I chose is labeled NOT FOR REBREATHER USE- it is the Argonaut kraken DSV. It’s definitely another part that one could make, but the complicated shape makes it more appropriate to buy since it will be easier to maintain/replace.

Counterlungs

Testing the old unit. CL attachment is on viewers left, and it goes into the bag at the bottom. This causes all the air to go to the top of the lung due to water pressure, and requires the lung to be very full to work.

The previous design I worked on had a huge counterlung problem: the lung tended to form a bubble at the top, far away from the hole which led back into the loop. This caused the fabric of the lung to close up against the inlet, like a flapper valve. Gas would get trapped in an un-breathable portion of the lung, which led to excess buoyancy and to gas getting trapped outside of the loop- this is bad for breathing, and scary for the diver.

For this next design, I opted for two counterlungs- one on either side, which should make it easier to breathe at any diver attitude. I used hydrapak 4L “seeker” bags. As best as I can tell, these are RF welded urethane, and they are tough and O2 compatible (or at least they are not contraindicated).

The new design better accommodates changes in diver attitude. Rolling to one side should cause at least some gas to be forced into the other lung. Pitching up and down should be accommodated by the piccolo tubes fitted to the lungs. The volume of the lungs was determined by breathing into empty 4L hydrapak seeker bags. The metric here is vital capacity- the most you can breathe out after the deepest breath (about 6L for me). My tidal volume is much much lower than that, and that is the minimum the bags need to be inflated to. However, excess capacity is not bad, in that it can help prevent immediate lung overinflation on ascent.

These 4L bags were fitted to pipe unions, allowing them to be removed and rinsed. The sheet metal casing of the unit forms a boundary for the lungs as well, preventing them from floating up and torquing the scrubber, or pinching themselves off. Two colors were used to indicate inhale and exhale, as one piccolo tube is longer than the other for hose routing reasons.

MAV

The last system was the MAV. Originally it was to have a CMF orifice as well, but that was not installed due to concerns about pre-dive leaks/overfilling of the lungs prior to the dive. The MAV is one of the more fraught parts of the system in terms of cost and rework, and thought. Originally I was planning on machining the whole valve myself, but that is annoying- parts corrode quickly in seawater, and I don’t want to have to re-do hours of lathe work every time it leaks. I want to build a rebreather, not a valve.

After some research, I found Duncan Price’s website which turned me on to clippard cartridge valves. These things are awesome! Drill a hole, drop it in, and you are good to go, aside from the rest of the plumbing.

Ultimately, that plumbing consists of two BC hoses with the 3/8″ end screwed into the MAV, and a single -4 SAE port plug to plug up the end where the valve goes in. Because the hoses are very different lengths, I opted to make both connections on the MAV side the same, because there’s little danger of assembling the unit with the wrong length of hoses.

The hoses place the MAV mid chest for ambidextrous use, which is very important, since the MAV provides life-giving oxygen.

Mistakes Were Made

I think it is important to address a few safety errors in the design of the unit. The two major lapses in the design are the use of aluminum in the MAV and scuba hoses which are not rated for an appropriately high pressure.

The hose issue is obvious in retrospect, and could easily be resolved with a regulator by finding a higher pressure rated hose and actually, I have no idea what the output pressure of the regulator I have is- I dont have an gauge that can read it- I have assumed it was tank pressure, but it may be reduced. The real trouble is with material compatibility and high pressure oxygen.

I recently got a copy of Vance Harlows Oxygen Hackers Companion (highly recommended- worth the money and you won’t find a pdf). It explains in great detail that material compatibility with high pressure oxygen is critical for preventing rapid unplanned and uncontrolled oxidization (oxygen fires). While unlikely, a sparking metal shaving, accelerated by the high pressure oxygen from the cylinder, could spark and cause the MAV to catch fire. Brass would be a much better material than 6061 (NB- 6061 does contain a small amount of iron and could possibly create sparks) brass is considered to be “non-sparking”.

On the bright side, the lack of ferrous metal shavings and oils by careful cleaning seems to have prevented any kind of unpleasant exothermic reaction. The only removable threaded connection is that of the oxygen cylinder to the brass regulator. The regulator is very soft and is unlikely to create steel slivers from the steel oxygen bottle. The other two connections are scuba BC inflator hose QDs which do not have tight metal-to-metal fits.

Drawings

Ah yes…engineering drawings

Here are the drawings of the entire unit, including part numbers for off the shelf parts. N.B. that all the mistakes under the “Mistakes were made” heading are documented here- not corrected. Keep that in mind when you look at them.

Resources/Inspiration

A little joke, courtesy of Merriam-Webster online dictionary

CEDU diver website, by Robert R. lannello. Down to earth and tongue-in-cheek (or perhaps very serious?), this website has a huge amount of information for and shows what you can do with a garage and a little knowledge.

Right up there with Dr. Bob is Alex Pierce. His youtube series covers many many dive subjects, including a DIY O2 rebreather that he and his friends built!

Jef Mangelshots excellent collection of links on the starck electrolung, one of the first closed circuit mixed-gas rebreathers was very inspirational and incredibly informative.

The rebreathersite is of course, about rebreathers. It is probably the greatest resource for research, complete with detailed photos of commercial and DIY units, including non-diving rebreathers. It also has information on the theory and operation of rebreathers.

DIY rebreathers has a lot of information, but is mostly a good collection of links to random information.

Rubicon foundation (possibly down) hosts some amazing images/text from the development of early rebreathers, including early lambertsen reabreathers (LARU).

The airspeed press has great books on technical subjects for divers, especially divers playing around with voodoo gasses. I have read the oxygen hackers companion and I am excited to dig into the rest of the books published there soon!

Richard Kinch’s website, which is mostly not about scuba, but provides some good tips on how to make a dash 3 SAE ORB fitting without a reamer.

Written by Hans Haas, an influential oxygen rebreather diver, is an honest-to-goodness book. “MANTA: Under the Red Sea with Spear and Camera” is an interesting look back at history, and a lens into early diving. Some parts of the book have not aged well, but the photos and the sense of adventure it conveys are worth a read.

Suppliers

These websites also are chock-full of useful info and are staffed by smart people. The difference compared to the above is that they will also sell you stuff! Good stuff!

Mcmaster-carr.com (need I say more?) Screws, aluminum extrusion, aluminum sheet…almost anything you could need.

vintage scuba supply and vintage double hose are good places to look for hose sets. Much nicer than anything I found at home depot.

dive gear express– sofnolime, hoses, fittings galore. Be sure to check out their BCD parts section too!

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