Why use back mounted counterlungs?
By keeping all soft parts in a hard case there is less chance of breathing loop damage and hence the need for an open circuit bailout. Also it means a standard SCUBA harness and Wing can be used to avoid ‘clutter’ on the divers front.
What are the disadvantages of back mounted counterlungs?
Compared to over shoulder designs, they have a higher hydrostatic lung loading and hence the breathing ‘feel’ can be different.
Why is this not a problem with the Ouroboros?
The hydrostatic lung loading is a combination of the resistive work of breathing (WOB) of the breathing loop and the water pressure difference between the counterlungs and the divers lungs in a range of orientations. The Ouroboros has a very low WOB and even when this is coupled with the hydrostatic lung load, the combined effect is still within the European test specification for rebreather's (EN14143) in all normal swimming positions.
Why are the hoses so big?
To allow for deep diving and the increased gas density at depth. This design of hose also contains an armoured spring system to prevent accidental collapse and provide abrasion protection.
Can I adjust the mouthpiece and hoses for comfort?
Yes. All user-interfacing parts have double swivelling O-rings. Simply twist the hose.
Why use a radial absorbent canister?
A correctly designed radial canister is good for the following reasons;
1. The short absorbent path length gives a low WOB.
2. With depth (gas density) an absorbent canister’s efficiency reduces significantly. Axial designs can typically reduce from around 80% efficiency in shallow water (15m) to around 50% in deeper water (40m). Radial canisters are more efficient at depth.
3. Radial canisters can be easier to pack and less prone to channelling.
4. Radial canister can retain more exothermic heat in the canister hence improving the absorbent efficiency.
5. Less absorbent can be used to gain a comparable duration to axial canisters.

Why use 2-liter cylinders?
The Ouroboros matches its canister duration to its oxygen volume. If more gas is required this can be fed into the system via the gas switching blocks optional auxiliary inputs. The system is also designed such that larger cylinders can be attached.
Why have a manual backup PO2 system?
Should batteries be faulty or run out unexpectedly or electronic control or solenoid systems fail, the maintenance of PO2 is vital to life support. This can only be safely achieved by a backup manual system.
How long will the Backup battery last?
The Backup battery will last approximately three times longer than the main electronics batteries. All batteries should be changed at the same time to maintain this safety margin.
Why not just use an active (constant flow) addition?
Tracking of required PO2 is often not as accurate as an electronic control system. Also if the flow jet blocks or fails there may be no alarm indication as to the fall or rise in PO2.
Why use small bore pipe work?
If a valve or addition system fails, a limited amount of gas (potentially oxygen) will be released into the loop and in the event of a failure to atmosphere, a limited amount will be lost before a shut down procedure can be activated. Larger bore gas lines also tend to make the rebreather's PO2 fluctuate considerably during ascents and descents.
Why use stainless steel pipe work?
The tubing in the Ouroboros is rated to ten times its working pressure. It is not affected by the environment and will not perish. It is virtually maintenance free. All rubber hose systems require maintenance.
Why use twin counterlungs?
Twin counterlungs slightly assist the breathing cycle as a small pressure drop is experienced across the absorbent. As one lung inflates a small pressure differential is experienced and gas is automatically drawn across the canister. Twin lungs also provide a very efficient water trapping method. The Ouroboros employs an automatic water removal system in the exhale counterlung.
Why use a vibrating alarm in the Heads Up Display (HUD)?
It has been found that touch is one of the last senses you lose in a gas poisoning event. Visual and audible alarms may not provide enough warning. The Ouroboros has dual vibrating and visual alarms.
Can I recover from a partial flood or mouthpiece loss?
Yes, providing water can be moved into the exhale counterlung it can be removed by simply filling the lung until the over pressure relief valve vents.
What happens if I sever a cable?
All cable systems are flood proof and double armoured.
What happens if I flood or damage a display?
The other displays will continue to function independently.
Why use a ‘wet turn-on’ system?
An analysis of accidents has shown a high proportion of users forgetting to turn their units on prior to entering the water. Some even forgot to turn their gas on. The Ouroboros turn on system not only automatically turns on when immersed, it will always maintain a minimum PO2 of 0.4 and will alarm if HP gas is turned off.
What happens if the wet turn on system fails?
The unit can also be turned on by touching a button on the Primary display it will also automatically turn on at approximately 1.5m or can be dived manually using the Backup oxygen display.
What happens if the oxygen solenoid or automatic diluent addition valve fails?
Firstly alarms will be activated and then you have the option to manually isolate the failed valve and continue diving.
Will the computer system lock out for a decompression violation or any other alarm?
Never, the user is always given the option to make the final decision.
Why have a rear facing PO2 display?
When diving in teams or in a teaching scenario not only can the PO2 be read from the rear but also the complete HUD status including decompression requirement can be read from the rear. If all forward facing displays fail, your partner could actually guide you through the remainder of the dive as the main electronics will continue to function and be displayed on the rear display.
Will the unit keep up its PO2 on fast ascents?
The unit has a proportional solenoid valve fire algorithm which keeps track with variable ascent rates.
How do I calibrate when diving at altitude?
The rebreather automatically senses altitude and with a known gas will automatically calibrate.
Some units have a solenoid fire light, why is the Ouroboros different?
Most rebreathers light the solenoid fire light when the electronics has told the solenoid to fire. This is not an indication that the solenoid has done its job. The Ouroboros will only light the light when feedback is received from the valve to say it has fired. Valve failures will show as alarms in the HUD and on the Primary and rear facing displays.
Can I complete a decompression dive on the HUD only?
Yes, there is enough information for PO2, decompression, general alarms and solenoid fire status.
Will the dive computer track an open circuit bailout decompression?
Yes, tell it you are now on open circuit and it will track. If you recover the rebreather and switch back to it you can put the computer back into closed circuit mode.
What happens if all three cells are reading wrong as confirmed on the Backup display?
You can turn all three cells off and tell the computer to use a fixed setpoint. Then use the manual addition valves and the Backup display to control your PO2. Gas switches can still be performed.
Why keep active electronics and batteries outside the breathing loop?
Electronics of any sort are a fire hazard in an oxygen enriched environment. Major failures of oxygen valves and pipe work in the loop will create a rapid rise in PO2.The Ouroboros has no active electronics or batteries in the breathing loop.
How does the Ouroboros protect against valve failures?
All valves are protected by in-line filters which can be replaced if required.
Is the solenoid valve affected by varying first stage pressures?
No, the valve will work at twice the rated pressure of the first stage. In addition there is an overpressure valve built into the system.
Why have an adjustable diluent addition valve?
This is purely so that the diver can adjust the valve so that it ‘feels’ comfortable for them. The ADV is adjusted by simply removing the canister and turning the ADV adjusting plate.
What items in the unit can be stripped down or removed without tools?
The case
The mouthpiece
The gas blocks
The hose ends
The counterlungs
The absorbent canister
The oxygen sensor tray
The battery compartments
The centre section (can be removed from the unit separately)
All major O rings
If you cannot crush the hoses how do you monitor a negative test?
Either look at the counterlungs or listen for the ‘in-rush’ noise in the mouthpiece when you open it again.
Some rebreathers have a canister life monitor. Why does the Ouroboros not use one?
Our tests have shown that with a comparable weight of absorbent canister durations are within 5 –10 minutes of each other, hence the internal stack timer is accurate enough. No currently available CO2 prediction system can sense a bypass of the stack. The design of the stack is the thing that more than anything else protects against channelling and hence CO2 bypass. Radial canisters are less prone to channelling when used correctly.
If I fit my own wing and harness will it affect the breathing resistance?
If the harness puts a space between the diver and the back of the rebreather greater than the that apparent with the supplied system, then yes. However even this may have an unnoticeable affect while diving. To meet EN14143 the supplied system should be used.
Can I use any absorbent and what affects will it have on duration?
The unit has been tested and rated using Sofnalime 797 grade. Any other absorbent will give different durations. Closed Circuit Research Ltd cannot guarantee the duration of the unit with anything other than the standard absorbent.
Can any style of cylinder valve connection be fitted to the Ouroboros?
The braided HP hose will take a ¼ BSP end fitting. However to remain within EN14143 only CE approved fittings should be used.