Components ready for assembly.

In February 2011 I was invited to the rEvo factory in Bruges Belgium to do a two day course to learn how rEvo rebreathers are built and tested.

I already knew quite a bit about the manufacturing and testing process as I’d previously had a friend do exactly the same trip and I’ve spent quite some time with Paul Raymaekers (the designer and manufacturer).

The ANSTI breathing machine.

The week before I was due to visit the factory Paul was in England at ANSTI who were going to undertake some independent tests on the rEvoIII. This was  for the notified body for the 100 metre CE certificate. The results were very encouraging and the rEvoIII  met the standards and the data was submitted.

When I started diving the rEvo I liked it and the more that I dived it the more that I liked it. This continues to this day and I really and truly believe this to be the safest rebreather on the market.

Paul Raymaekers did not just invent this rebreather, it evolved over a number of years to the machine that it is today. If something can be changed to make the unit safer or easier to use then it is, without compromise.

The breathing cycle.

The testing of the rebreather is not done by just jumping into the water with it strapped on your back. It has been done with a huge investment in testing equipment and a considerable amount of time.

rEvo rebreathers occupies a small part of a big industrial factory. Raytech is the business that Paul started and it specialises in precision laser cutting and chemical treatment of metals, mainly Stainless steel and Titainum. I was shown some samples of stents (metallic tubes that are inserted into veins and arteries). They looked about 1.5 millimeters and when put under a microscope you could see they were not a solid tube but a seive type tube and the seive portion has been cut by the ultra fine lasers used at Raytech.

I used to be impressed with the accuracy of the rEvo case until I saw this.

The rEvo rebreather workshop is annexed off from the factory into a separate unit and everything is in there including the testing laboratory. Each part of the rEvo is manufactured and assembled with the same care and attention to detail that you would expect from a company that can make tubes for surgical procedures.

The parts department.

When the parts are assembled they are tested along the way and when the whole unit is complete it is passed off ready for testing. The person who assembles the rebreather has nothing to do with the testing. This is undertaken by somebody who was independent to the assembly process. There are 122 tests that are undertaken and documented.

The investment that has gone into the testing of rEvo rebreathers includes a testing laboratory with a full ANSTI breathing machine. The ANSTI machine is a machine that can simulate breathing under different diving conditions. It is able to go to 100 metres and at different water temperatures and breathing rates. In the time that I was in Bruges the ANSTI machine was working full time doing at least two dives a day. The testing that was being done whilst I was there was to test different types of scrubber material at different temperatures to measure when a CO2 breakthrough occurs.

The mechanical lung.

The ANSTI machine is set at a breathing rate and the machine has a piston chamber to simuate breathing in and out – this is called the tidal volume – the speed that this works is known as the breaths per minute. Then there is the depth and the tempreture to set and even the pre-breathe is simulated. Carbon dioxide is flowing into the breathing loop and this again is variable to the breathing rate and depth. Then there are sensors on the inhale side of the mouthpiece that measure what the diver is breathing after it has been through the rebreather.

Paul is collecting data and is always willing to share his findings. When I learned to dive a rebreather I remember being told that the pre-breathe was to get the scrubber warm and activated and only then does it begin to work and remove the Carbon Dioxide. Some rebreather manufacturers have built their machines around this thinking and their software will not allow progress through the pre dive sequence until the scrubber temperature has risen sufficently. However when you have the benefit of a whole testing laboratory you find out that this is a myth and that the scrubber material works from the first breath. When Paul learned that this was the case he published his findings on the internet and  it caused a lot of controversy.

Radial scrubber parts.

Another myth was that radial scrubbers were better than Axial. The radial scrubber is easier to breathe through but no more efficient than the axial. There is an option to have radial scrubbers on the rEvo and this mearly allows a greater quantity of absorbent material to be used, giving a longer duration (4 kilo insted of 2.6 kilo). It allows cave divers the ability to dive for in excess of 6 and a half hours at a time.

All other rebreather manufacturers have chosen to have a single scrubber, the rEvo has two. It seems that rEvo is different to all other rebreathers because of this and you have to ask why? Well the two scrubbers give you a very low profile, that’s a bonus. The other benefit is that they link both counterlungs to the full width of the scrubber. If you imagine that most scrubbers are fed from a narrow tube, the rEvo is not. It is fully open on the whole width to the counterlung. That’s a bonus as the flow is even throughout the scrubber and in addition it helps the work of breathing.

The pressure chamber.

What is the biggest fear of the rebreather diver? Well you probably guessed it, it’s CO2. Carbon dioxide to the rebreather diver is the worst enemy as it creeps up on you and without knowing you’ve got Hypercapnia. The two main catalysts to hypercapnia are over use of the scrubber or channeling.

Exceeding the manufacturers recommendations on scrubber usage is foolhardy. I have now seen the manufacturer doing the tests to see where the safe time limit is on scrubber usage. If you comply with this then you will have no worries. Scrubber channelling however is another problem. It can come through inproperly packing the scrubber, through settling or humidity.

If you imagine that there is a channel in a scrubber cannister from a rebreather with only one scrubber. Lets say that it’s giving you a 5% breakthrough. That breakthrough is a major problem because you only have one scrubber cannister. The next breath will contain 5% CO2. Now if you have two scrubber cannisters and you are unfortunate enough to have both scrubbers with a 5% breakthrough then the problem is nowhere near as bad. I’ll explain.

If we measured the gas after the breakthrough from the first scrubber it will be 5% CO2 however the gas then mixes as it flows into the second scrubber and you still have a 5% breakthrough but only 5% of the total gas from the first scrubber breaks through, therefore you have a 5% of 5% CO2 breakthrough into the inhale lung.

Test diving the cells

When it comes to cells rEvo don’t just buy them, store them and ship them. They buy them, then test them to ensure that the millivolts are between 9 and 13, then they test dive them for 5 dives in the ANSTI machine and only upon passing these tests are they fit for use in a rEvo. They are then re-sealed into their original packaging.

When I slip under water on every dive with a rEvo I know that I’m not testing this rebreather for a low budget manufacturer. I know that it’s been tested and continues to be tested. Paul Raymaekers took a standerd rEvo to 212 metres in Dec 2009 and later this year he is planning on going deeper. He believes in his rebreather and he trusts it to bring him back.