Three Reasons to Pre-Breathe


By Tim Snape

Do you prebreathe in the prayer position? Is the standard 5-minute prebreathe a good diagnostic for loop problems and could we do our prebreathes more effectively?

I was a late starter to rebreathers, converting from open circuit only two years ago. The suggested protocol to ensure that your rebreather was working correctly came as a bit of a revelation to me. The thing that interested me the most was understanding the physiological mechanisms and how breathing exotic cocktails of gas at depth affect your body.

Prayer Position vs Fully Kitted

My entry into CCR 2 years ago was with a non-TDI agency, completing their 40m entry course. This agency and its training materials reinforced the idea that you should prebreathe in the prayer position – knelt in supplication in front of your unit, with your nose firmly pinched so you do not accidentally breathe through it and invalidate the test. 

After a year of excitement, diving my APD Inspiration in the English Channel and surviving some very silly mistakes using this prayer position approach, I went on to complete my 60m mixed gas certification with TDI and the amazing Mr. Mark Powell.  When I raised this subject with Mark, he was very clear that you should prebreathe fully kitted up and go straight from prebreathe to dive.

Mark’s training and guidance combined with my interest in using prebreathing as an opportunity to reduce deco obligations made me completely rethink how and why I prebreathed.

In the end I was able to identify three reasons to prebreathe:

Testing the loop

Running a PreBreathe Checklist

An opportunity to relax before the dive

Prebreathing – A bad experience 

As every CCR diver will know, checklists are drilled into us and we must always prebreathe to double check everything is working.

But after completing the CCR entry course, my checklists failed me on a few occasions. Most were minor issues easily remedied such as failing to connect my dry suit inflator. On one occasion though, when we were doing a wreck, everything was going fine, I’d completed all my checklists and performed a good 5-minute prayer position prebreathe. When I was just about to put the loop in my mouth and jump off the boat into the water, I checked my unit’s handset and discovered my CCR was switched off!

The APD Inspiration would have compensated for this stupid mistake by automatically turning itself on when I entered the water. However, I was shocked because I was absolutely certain my unit was switched on. I don’t know exactly what happened or why the unit was off, but it was off – almost certainly it had been turned off by me!

My mistake was doing a prayer position prebreathe, before sometime later putting it back on again to do the dive. I had always treated the prebreathe as a check to see if my unit was working, so for me it was just another tick box test.

What I have learnt 

These mistakes caused me to rethink how I dive and how I perform the prebreathe. Between my mistakes and my completion of the TDI 60m certification with Mark and what he taught me I have come to a fairly simple set of conclusions on how to properly prebreathe.

The purpose of this article is to explain my rationale and how I now use my prebreathe as an opportunity to make my diving safer.

Here are my 3 key reasons for prebreathe and why these work for me… 

Reason 1 – Does my loop deliver a breathable gas? 

The first and most obvious reason we prebreathe is to check if our loop is working correctly.  I would like to refer to Dr. Simon Mitchell’s research on prebreathe effectiveness. In summary, he demonstrated that for a group of divers prebreathing for 5 minutes:

If the scrubber was completely missing, 25% of the sample did not detect a problem.

If the scrubber was compromised (in the experiment they removed both the O-ring and the compression ring), 90% did not detect a problem

The divers failing to detect hypercapnia induced problems, typically experiencing dramatic physiological changes – without being aware!

Dr. Mitchell conducted a randomized single-blind study, which was peer reviewed by leading experts. The conclusion seems clear: a 5-minute prebreathe is too short for divers to reliably detect faults that could result in high partial pressures of CO2 in the loop. 

The longer the prebreathe the better, so my prebreathe starts when I am fully kitted up and continues until I start my descent. I kit up early and use the time waiting for the dive as an opportunity to do an extended and relaxed prebreathe. This helps me reduce my breathing rate, which I monitor. 

Reason 2 – PreBreathe Checklist 

As I explained in the introduction, my checklists have occasionally failed me. When this happens, a smart CCR diver should learn from these mistakes.  When I make a mistake (and especially when I make stupid mistakes) I record it in my logbook. I then reflect and try and identify something I could do or change so I don’t repeat the mistake.

My checklist process is now broken down into 4 types of check:

The night before checklist – I always like to set my unit up the night before and will always run through everything just to make sure everything is ready for diving the following day.  Emphasis is on the kit, do I have enough gas, are the batteries sufficiently charged, are all the valves working and are there any leaks anywhere.

Powering up the unit – When getting ready to dive and powering up the unit I simply follow the checks on the unit itself. I have an APD Inspiration, and it guides me through the basic equipment setup checks.

The PreBreathe – I now use my prebreathe as the opportunity to run through all my operational checks.

The final check is the Go/No Go decision point – Do I dive?

My approach to prebreathe is now very different. Rather than to run through it as just another check it now forms an integral part of my dive.  I only start my prebreathe when fully kitted up and ready to dive. I use it as a time to sit down and relax. Once everything is on, I run across my body from left to right checking each control point. 

Starting from the left side of my body and transiting to the right I perform the following tests. 

Before I put the mouthpiece in my mouth, I check my handset. Is the unit turned on and is it supplying the expected breathable gas? I check to see if the computer is configured with the correct diluent.  If all is good then I mentally record the time and put the mouthpiece in my mouth – my dive has now started.

From this point onwards I am in dive mode and I will regularly monitor the information shown on my computer. During the prebreathe I am particularly interested in the PO2 levels. Is the unit able to maintain correct levels and are the PO2 sensors behaving consistently?  My APD has a temperature-based scrubber monitor, so I also monitor that, as well as battery levels.

I then proceed with the following operational checks.

Lean bailout – Does it have gas? Can I access the 2nd stage?

Dil MAV & contents – Do I have gas and does the MAV button work? When I push the button does the pressure fluctuate on the contents gauge (which would indicate a problem)?

ADV – Is the ADV on or off? Does it give me gas when my loop gas volumes are low?

BOV – I switch from CC mode to OC and test if I can Bail out onto my inboard. 

Suit inflator – Is it in and does the inflation button work?

Crotch strap – Is it looped into my belt?

O2 MAV & contents – Same checks as #2.

Rich bailout – Same checks as 1.

Torch & power cable – Does the torch work and is the cable routed correctly?

My Backup Computer is on my right wrist, so it is my last check. Is it actually on my wrist (or still in my kit bag – this has happened)? Is it on and is it configured with the correct diluent?

Go/No Go check – This is the final and the most important check of all. It is the decision point whether to dive or not dive.

Once the previous 11 checks have been completed and you are ready to jump in the water, due to the relaxed way you are prebreathing, you now have an opportunity to review and decide whether to Turn the Dive.

Mark Powell teaches a methodology where you make decisions based on reviewing a situation at 3 different levels.

I apply this for my final Go/NoGo check:

My inner self. Am I nervous, do I feel fit and strong, am I sufficiently hydrated, is the dive inside my comfort zone, how do I feel about my buddies? A little nervousness is considered a good thing as it keeps you on your toes, equally if you are not nervous enough that can lead to complacency and is when you are most likely to make mistakes.

My immediate surroundings. I perform a summary review of the previous prebreathe checks – were any of the controls slightly out? Individually, small issues might be tolerable, but collectively they rapidly become much more serious. Also consider the rest of your kit, are you confident you are weighted correctly, are you likely to have a problem with the water temperature and what about any new kit you may be using?

My wider surroundings and what is going on around me. What is the sea state? What about the tides and surface visibility? Are your buddies ready prepared and relaxed?

Once you have completed the first 11 checks, followed by the 360-degree, final review and check, you should be ready to make the conscious and calculated decision – I am ready to dive. This is the optimal point to Turn the Dive. 

Go or No Go 

Go or No Go is obviously difficult. Many deaths have occurred in this sport due to people being reluctant to “Let the Side Down” and diving anyway. CCR diving is different and requires a level of competence and discipline where the practitioners are able to say no if they do not feel completely comfortable. 

The above list is not definitive and not intended to be. For example, checks 7 and 10 are particular to me. They are in my personal checklist, because on previous dives, I have forgotten to attach my crotch strap and have routed my torch incorrectly.

Physically transiting my body left to right and checking each item in turn works for me as a strategy. I follow this sequence slowly, I know the prebreathe will take several minutes but by starting early, I also know I can run through these checks in a thorough and relaxed fashion.  As a rule, once I start my prebreathe, I do not take my loop out of my mouth and I continue my prebreathe until I enter the water and commence my descent.

I have yet to reference buddy checks, but your buddies need to understand what you are doing and you need to be familiar with their testing protocol. In our club, with one exception, we are all CCR divers. The skipper will usually be CCR certified and helps the divers kit up and checks that they have correct and sufficient gases.  You might consider formalizing this by agreeing a sign off with the skipper that you have completed all checks and now ready to dive.

Reason 3 – PreBreathe Chillax 

Combining my prebreathe with my final checklist and keeping my equipment on creates an opportunity to chill-out, relax and get my breathing and heart rates to where they need to be.

Referring again to Dr. Mitchell’s prebreathe study, I like to monitor my breathing rate. If after a 10-minute prebreathe and relax your breathing rate has not gone down and is actually rising, that is a very strong indication of high CO2 levels in your loop.

Yet another Dr. Mitchell observation is that when divers exhibit symptoms of hypercapnia and are breathing heavily, in almost all cases they are initially unaware.  So, if you are going to measure your breathing rate, you need to measure it with a watch and count your breaths. This should give you a simple and reliable test.

You should also consider the PO2 level. If the unit is not able to maintain the PO2 level at the low setpoint or the sensor readings are inconsistent, then that would suggest a problem with the O2 supply or the sensors.


Human factors studies in aviation and medical surgery have proven repeatedly that simple checklists can reduce fatalities and serious incidents dramatically. In medical studies it halved fatality rates.   That said, some people view their prebreathe as just another check to tick off.

By making it an integral part your dive, it allows you to use the prebreathe to:

Extend the time spent prebreathing, thereby improving your ability to detect CO2 and O2 problems with your loop

To complete a detailed set of operational tests, with a Go/NoGo decision point

To relax and chill out before the dive

The important thing to remember is that prebreathing creates a great opportunity to perform a thorough set of operational checks of your equipment and to make a calculated Go/No Go decision before you jump in the water.