The Newry & Mourne Sub Aqua Club - Technical Issues

Technical Issues
Go back to Technical Diving page


by John Thornton, IANTD Trimix Instructor, CCR Instructor and owner of the technical dive boat 'Karin', based at Scapa Flow.

'Welcome to the world of rebreathers, I would like to introduce you to a fully closed circuit rebreather - a potential killing machine!' I normally start my courses with something along these lines, it certainly gets the students' attention and it is not an exaggeration.

So what is a rebreather?

Basically a CCR (closed circuit rebreather) is a closed loop of gas which can have diluent (the gas used to dilute the oxygen, normally air) manually or automatically injected into the loop and oxygen manually or automatically added. There is an electronic package included which analyses the gas in the breathing loop and can add O2 to the loop to bring up the FO2 and hence the PO2, this includes readouts which are monitored to check the levels of oxygen that you are breathing. The gasses are stored in 3-7 litre diving cylinders. Waste gas (CO2) is scrubbed out of the system by the use of an in line canister called a scrubber, this is filled with sofnalime (a molecular absorbent) and has an in water life of approx 3-6 hours depending on temperature etc. So we have a loop of breathing gas which re-oxygenates itself and scrubs out the nasty stuff, hence no bubbles, no noise, brilliant for diving, if treated with the respect it deserves.

With the different types of rebreather now available, make sure that you get an instructor who is proficient with the unit he is teaching. There is no such thing as a general RB instructor as each make has its own techniques required to fly successfully. I believe that an instructor should own and use the unit he is teaching and that he should use it on the course and be competent enough with it to resolve any student problem in water.

The rebreather course usually lasts about five to six days and I base the emphasis on the in-water side. RB diving is all about firstly understanding the unit and then time in the water to learn the unit, it is so different to OC (open circuit) diving that it is like starting again. As we all know it is difficult to get an old horse to learn new tricks but the instinctive reaction to situations is exactly what I have to teach in just six days.

So what is different with CC diving?
Pre dive checks, buoyancy, breathing, decompression, maintenance and attitude - not much left is there?

Pre-dive checks
The CC unit must be thoroughly checked before every dive, even if you have just come out of the water, then check it out again before re-entry - each type of unit has its own specific character which must be learned and stuck to. First the high-pressure checks on the cylinders and bail out systems and then the positive and negative pressure checks to ensure the gas tightness of the breathing loop followed by the calibration of the gas analysing systems. Pre breathe and away we go. If the unit fails any of these checks then it's a no go until all is well, the key is to give yourself time and take no short cuts.

A diver on OC holds their breath for a variety of reasons, the main one being to fine control the buoyancy side of the dive. To maintain a position in water he will hold or breathe off the tops or bottoms of his lungs to keep the required volume of gas in his lungs, other reasons are to avoid noise and bubbles i.e. looking up or videoing etc. On the CC unit holding your breath has no bearing on the volume of gas in the loop as you are simply transferring from your counterlung (CL) to your own lungs, you are moving the gas but not changing the volume. That means that all buoyancy control has to come from suit or wing. You can of course expel gas from the loop by exhaling around the mouthpiece or from the nose but this quickly results in a potential problem as this is the gas that you are relying on to breathe! The net result is also that you will descend as you vent off the gas so the CL's will compress further giving less gas to breathe again.

Let's divide the dive into three sections and see how the RB diver controls buoyancy:

1 Descent
We leave the surface with a lung full of air in the CL. As we descend the CL compresses so we need to add diluent gas to the CL to maintain volume of gas for breathing, too much could make us buoyant, too little gives a lack of breathing gas and makes us heavy. We also, of course, still have the suit and wing to worry about, hmmm... how many hands do I need for this? Part of the solution is to have an auto dump valve in the suit (run with it open), limit the use of the wing (the only time I use mine is occasionally on the surface). So now we just need to inject a little in the suit to offset squeeze and keep our CLs at the right volume by injecting diluent into our inhale CL. Okay then, we now only need two hands instead of three, the control of the gas inputs has to become instinctive and correct to save problems on descent. Some CC units have what we call an automatic diluent feed, they work off a demand valve type system that will auto inject diluent when needed (is this something else that can go wrong or is it better than manual diluent feed? - I'll leave that to you).

2 Bottom Phase
Not a huge problem here with buoyancy control as the unit replaces the used oxygen (mechanically or manually) and the rest of the gas just goes round and round. Hassles such as mask clearing need the user to replace lost gas as again when you clear your mask you are venting off your breathing mix. It is an unusual feeling when you can't breathe because you've just cleared your mask!

3 Ascent
Expanding gas is the problem here, not only in the suit but the in wing and CLs as well. As on the descent we reduce the problems with the auto dump valve on the suit and by flying with the wing empty. As our CLs expand then we must vent them off through mouth/nose or the next thing we know we are on the surface. The above is a matter of practice and good control only comes through time.

Continual deep breathing is important on both OC and CC, but as said above we are always holding our breath during a normal OC dive. On CC regular breathing keeps the PO2 at a consistent level, holding your breath stops the circulation of gas and hence the analysis is of a stationary gas, injection of O2 becomes localised and the readings become inaccurate when related to the whole gas. Again it's the instinctive reactions that only come with constant practice/use which will lead to better breathing control.

Generally there is less deco for a given exposure with CC than with OC, the deco necessary is also shorter on CC. Why? Because the unit optimises your breathing mixture to a pre-determined set point hence giving the best mix at all stages. On ascent I teach a staged type ascent rate as the CC unit can take a small period of time to generate the chosen set point. Stop every 6-12m on ascent, depending on depth attained, and let the unit catch up, this also helps with the deco (a la Pyle stops).
Deco is about inert gas management. Simply put, the less inert gas we have in our bodies the less deco we have to do. As you know, the way we manage this is by increasing our O2 in our breathing mix. We depend on O2 for life but react badly if we have too much, we are in the compromise situation where a balance must be drawn. Oxygen levels have to be watched!! We do this by regular monitoring of our PO2s on our handsets.
So the deco is shorter on the CCR but if the unit fails what do you do? You have to complete your deco safely or you might not only ruin your day but everyone else's as well. The bottom line is to know what you can do on your OC bailout and keep that as your limit (OC bailout is the OC regs linked to your diluent and oxygen cylinders, I insist on both being there) then in the rare event of failure you can surface safely on your OC diluent to 6m and then O2 to surface. Again this all has to be practiced and repeated.

With the breathing loop being enclosed we will have a build up of fluids in the loop, all the goodies you normally dispense to water will be in the CLs. The scrubber, when doing its job, creates heat as a by-product. Great if you feel the cold. Hence we have on our hands a nice little bacteria machine - heat, humidity and human gunge. The unit needs to be disinfected periodically and especially so if you've let someone else have a play. This is reasonably straightforward but takes a few minutes, I normally do mine at the end of the week but do not allow anyone else to use it. Pressure settings in the HP side need checking etc etc. - again this is all part of a routine that we must follow and learn.

Most important in RB diving is discipline and the correct attitude to the diving. I personally feel that entry grade to CCR is too low, most agencies/manufacturers specify that a nitrox qualification etc is needed. With basic and advanced nitrox divers should not be able to buy oxygen. They have not been taught the relevant O2 theory nor do they have the relevant in water practice using optimum mixes.

The diver who depends on computer run dives is not the diver for CCR diving, they must know how to plan dives and how to generate bailout to surface schedules in case of unit failure. If you are the guy who dives on a single cylinder and hopes for the best by going into deco and relying on your computer then the road to safe RB diving is a long one. We need the diver who can monitor their O2 levels, can act on a failure by going into a preplanned bailout mode, knows how to respond to the hidden signals a rebreather gives you when in use, can happily control three things at once and is patient enough to take the road to RB diving slowly. I have had several enquiries from divers who say they want a short course as they have been diving for so long they don't need the full thing. I am afraid some of last year's casualties testify to this train of thinking being wrong and dangerous.

It is important that all students leave the course with a level of ability that allows them to dive but not over confident. Rebreathers can and will bite, be prepared and in control and your CCR diving will be the best move of your diving career, be complacent and the results can be terminal.

For a full list of Inspiration instructors visit AP Valves website:

The table below describes a typical six day CCR course as taught by John Thornton at Scapa Flow Technical. Course details: IANTD 'nitrox rebreather' course, in water requirements 420 mins. Also available is 'technical rebreather' which takes the student to full mixed gas capability.

Day 1
Introduction to unit; dissemble and rebuild; full run through of unit operation.
Dive to 5m in the sea (pool training is okay but must be done in full kit).
Objectives:- to introduce buoyancy control in a safe environment and establish bailout techniques; emphasise monitoring of PO2s; descend down a weighted line.
Drills - to be completed on every dive on the course:- switch to OC and switch back to CC; ascend with emphasis on control. Repeat if necessary.
All drills are practiced dry- a lot of RB techniques can be learned on the deck or dry land and should first be practiced there. Debrief
Day 2
Dive to 15 to 20m for a no stop time, wreck of the F2 or Carlruhe.
Objectives:- to give in water time at a safe depth, descent and ascent are up and down weighted or fixed lines.
Drills:- switch from CC to OC and back again; elevate setpoint to 1.3bar on arriving on wreck; switch back setpoint to default (0.7) at 6m.
Dive on shallow wreck (V83).
Objectives:- time in water.
Drills:- switch from CC to OC and back; setpoint movement. Open circuit bailout to surface from shallow depth; switching to oxygen at 6m and going to surface.
Debrief discussion is could we stay on CC O2 from 6m to surface? - it depends on the fault which caused the need to bail out.
Day 3
Dive to 25m cruiser with short drop to 35m. Objectives:- simulate a unit failure and go to surface on OC bailout. Drills:- CC to OC to CC; OC bailout to surface from top of wreck (22m) switching to O2 at 6m (OC or CC).
Dive to 10-20m.
Objectives:- to simulate O2 auto addition failure.
Drills:- to run the unit on manual oxygen addition and maintain a good PO2 during dive, emphasis on slow ascent to allow time to control O2 levels; repeat CC to OC to CC.
Hypercapnia test
Remove the sofnalime from the unit and reassemble and one at a time breathe from the unit. The idea is to get an idea of what CO2 poisoning feels like in a safe situation, do not do this test in water!! Observe the reaction in the student - it is interesting that even after observing this test a student is visibly being affected before he realises it. Remember that underwater the effects occur sooner. Keep an accurate log of the scrubber duration remembering to include dry drill time. If in doubt change it.
Day 4
Dive to 25-35m cruiser Coln.
Objectives:- dive time plus deep repeat of yesterday's drill.
Drill:- as per day 3 PM
Dive to 20m.
Objective:- shallow rehearsal of tomorrow's drills.
Drills:- semi closed running of unit to simulate scrubber failure; CC to OC to CC. Simulate 5 mins at 6m deco.
Day 5
Dive to 30-36m on Kronprinz Wilhelm.
Objective:- to complete dive with a simulated 5min stop at 6m.
Dive to 15-20m.
Objective:- to simulate oxygen injection jamming on.
Drill:- shut down O2 cylinder, purge bag, inflate with diluent or go OC diluent to give time to assess situation.
Discussion:- do we do a diluent flush or do we go OC?
Day 6
Dive options:- if student has proved competent and has the right background qualifications then a dive to 40m with a real deco of up to 10mins at 6m. If student not as competent and/or qualifications are not as strong then a dive to 30-35m with simulated deco for 5-10 mins at 6m.
No planned drills.
Dive of choice to shake down, no planned drills.