Rebreathers are a great option for experienced recreational divers looking to take their diving further and enjoy longer dives. One of the first questions we often get asked by our diving customers when they see a rebreather on our boat is “How does it work?”.
Rebreathers are a great option for experienced recreational divers looking to take their diving further. One of the first questions we often get asked by our diving customers when they see a rebreather on our boat is “How does it work?”. The rest of this blog provides a simple overview of the workings of the Closed Circuit Rebreather (CCR), with a focus on the APD (or Ambient Pressure Diving Limited) Evolution and Inspiration models. It gets a little technical (as a closed circuit rebreather is more technical than diving on open circuit) – but once you’ve learned how to dive on a rebreather you’ll have the knowledge to prepare and dive safely on a rebreather and enjoy many long dives
So, how does a Closed Circuit Rebreather work?
The CCR has several key elements. The diver puts the mouth piece of the breathing loop into his/her mouth. The diver breaths out and the expelled air leaves the breathing loop and enters the exhale counter lung which is located on the right shoulder. Any moisture or water droplets are removed by a baffle plate fitted into the entrance of the exhale counter lung. The air now passes from the exhale counter lung to the base of the scrubber unit which is contained in the back mounted case. As is passes through the scrubber the CO2 (Carbon Dioxide) is removed / converted into heat and moisture. The cleaned air passes in front of three oxygen cells. These cells measure the level of oxygen in the gas compared with the amount of oxygen that should be there according to the integrated computer (see below). An oxygen solenoid adds a precise amount of oxygen according to the computers requirements and the gas passes to the inhale counter lung, mounted on the left shoulder. The diver breaths in and draws fresh air from the inhale counter lung via the mouth piece and breathing loop. The idea is to have either one lung full in you or one lung full in the unit, never both. This is called loop volume management.
Diving on a Rebreather
Managing buoyancy when diving on a Closed Circuit Rebreather – With conventional scuba you breath in and go upwards, breath out and go downwards. However, with a CCR it has the correct amount of loop volume and the integrated BCD set to suit the current depth. Then, it will not matter how fast or slow the diver breaths he/she will stay still – Cool:-)
Diving on a Rebreather. CCR’s work on fixed Set Points or Partial Pressures of Oxygen (PPO2). The two standard Set Points on the APD units is 0.70 and 1.30 however they can be fully adjusted above or below the surface. A Set Point of 0.70 means that at the surface (Sea Level) the diver would be breathing the equivalent of 70% Oxygen. If the dive was at 10 metres he would be breathing 35% Oxygen because at 10 metres there are 2 Atmosphere (ATA) of pressure (0.70 / 2 = 0.35). So at 60 metres there is 7 ATA the diver would be breathing 10% Oxygen – not good, this will not support life but you get the idea.
This is why we have two Set Points. On a typical 40 metres dive the Diver travels down to 40 metres on a 0.70 Set Point. Change to 1.30 at depth and continue all the way to his/her Safety Stop at 6 metres. Then they would change back to 0.70 prior to leaving the Safety Stop. However, this all changes when doing decompression diving.
Normally, for dives to a maximum depth of say 45 metres they would have two cylinders fitted into their rebreather. One of pure 100% Medical Grade Oxygen and one of Air or 21% Oxygen. This cylinder is referred to as the Diluent, so this set-up would be an Air Diluent CCR. For deeper dives the Air Diluent would swapped out for different mixtures of Trimix (Helium, Nitrogen and Oxygen). Cylinders for the APD Evolution and Inspiration are either 2 or 3 Litre in capacity filled to 200 Bar.
Longer dives on a CCR – Depending upon depths and the particular user the units are capable of diving to 150 metres (correct training required) for durations of 2 to 6 hours.
These are amazing pieces of equipment and great to use for scuba diving.
Find out more about learning to dive on a Rebreather in the tropical waters of Sabah, Borneo.
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