Ready for MOD 2? A Practical Guide for Advancing your Rebreather Skills

“You need to go back to Intro to Tech” is not something nice to hear after spending thousands of dollars on a new rebreather.  Despite being on the receiving end of that comment during my MOD 1 course, it inspired me to successfully scrape by in MOD 1, and more importantly, ensure I was adequately prepared for anything that could be thrown at me during an eventual MOD 2 certification.

Unlike MOD 1, the introductory course to rebreather diving where you’ll take your first recycled breaths underwater, MOD 2 differs significantly by expecting basic competency with the rebreather from the outset.  Just as an open water student requires basic SCUBA experience before taking Intro to Tech, a newly certified rebreather diver requires hours on the unit, mastery of basic skills, and real world diving experience before moving to MOD 2.

Preparing for MOD 2 can be a daunting process, but with targeted practice and an understanding of rebreather dynamics, it’s achievable.  This guide shares practical steps and considerations to help newly certified rebreather divers prepare for the next level.

Core Competencies: Practice Makes Perfect

While often overlooked, maintaining basic open-circuit skills remains critical for rebreather divers of all levels.  In the event of a bail-out, having mastery of the ability to perform gas switches, buoyancy control at the appropriate stop depth, deployment of a DSMB, and gas management/sharing procedures significantly impacts the ability to exit the water safely.  These skills should be practiced regularly; though while rare, they will be lifesaving if needed.

As it relates specifically to the rebreather, the minimum expectation is being able to build it correctly and understanding how it works, and while in the water, executing skills learned during MOD 1 quickly, correctly, and with neutral buoyancy.

The key here is neutral buoyancy.

Not only should you master hovering at relatively shallow depths, but skills such as a diluent flush, valve checks, bottle manipulation, and boom recovery ought to be performed without shooting to the surface, rocketing to the bottom, or tumbling out of relative trim.  This should additionally include the ability to deploy and use equipment while neutral, such as wet notes, marker buoys, lights, reels, spools, and a backup mask.

Neutral buoyancy and the ability to perform MOD 1 skills is critical for success in MOD 2, and the course cannot continue if these skills are not apparent during shakedown dives leading up to the course.

I had tremendous trouble with buoyancy throughout MOD 1 and in subsequent dives while performing skills.  The following is how I prepared myself for MOD 2:

First, dive the rebreather and avoid occasions where you’re tempted to dive open circuit.  A charter may be running to your favorite wreck or your buddies may be taking a trip to cave country… if you aren’t qualified or comfortable to perform these dives on the rebreather, stay home.  The skills and mental understanding required to safely dive the rebreather are perishable; consider yourself starting over as a closed circuit diver and focus on mastering the unit during the first year of ownership.  The wrecks and caves aren’t going anywhere.

Next, schedule specific time to practice, and practice often.  As tempting as the trips and dives mentioned above may be, you invest in your ability to perform those dives safely by practicing with the rebreather.  Set time aside each weekend to dive a local quarry or easily accessible dive site; the shallower the better.  This forces you to dive at the most challenging depth for buoyancy control and encourages skill circuits.  I did this every weekend during the warm season my first year with my rebreather, and at least twice a month through the winter.

Finally, develop a skill circuit.  Regardless if I were alone or with my rebreather buddy, I performed a skill circuit every time I was in the water diving for practice.  It included the following in no particular order and was performed between 20 and 30 feet deep:

• Hovering

• Mask flood and clear

• PO2 high > diluent flush, manually bring PO2 to set point high (if able)

• PO2 low > O2 valve on, pressure check, MAV operational

• Boom drill with buddy check (if able)

• Bailout > go back to rebreather

• Shoot DSMB from depth (if ending dive)

• Ascend and hold simulated deco stops at 20, 15, 10, and 5 feet

As you progress in skill, and to prepare for MOD 2, add: 

• Bottle drop and pickup

• Bottle swap

• Out of diluent

• Leaky O2 MAV

If able, find a buddy to practice with and hold each other accountable.

Building Intuition: Deeper Knowledge of Your Rebreather Enhances Decision-Making

Equally as important as practicing skills is developing a deep understanding of how the unit operates.  This will allow you to respond appropriately to different situations presented throughout the course.

During MOD 1, you most likely learned how to respond to certain situations in a methodic and structured way.  In MOD 2, and as preparation for the course, you should begin to understand that there are numerous ways to handle different situations, and that intimate familiarity with the operation of the unit is key in dynamically responding to problems while underwater.  As an example and to help you understand the appropriate headspace, let’s consider an instance of high PO2 where all of our cells are reading in range but above set point high.

You look at your handset and notice that your cells are reading [1.51] [1.49] [1.50].  You perform a diluent flush of the rebreather, monitor your cells, and return to set point high.

Five minutes later, you check your cells and see that they are again above set point high, and reading [1.54] [1.52] [1.53], what do you do?

A MOD 1 certified rebreather diver who has mastered basic skills, but does not understand how the rebreather really works, may re-perform the above skill.  This is a call and response, not dynamic troubleshooting to understanding what is happening with the unit.  While not necessarily wrong, a more knowledgeable rebreather diver may consider the following given the continued increase in PO2 over time:

Consider ADV Off

“Have I descended with my ADV turned off, and therefore no diluent gas diluted my mix?”  They check the ADV shutoff, add diluent to reduce PO2, and continue the dive while monitoring their depth and PO2 for further issues.

Evaluate O2 MAV Leak

“Depth hasn’t changed, so why would PO2 increase slowly over five minutes?”  They check the O2 MAV; a leaking MAV may cause a slow increase in PO2 due to oxygen leaking into the breathing loop.  They unplug the O2 MAV, add diluent to bring the PO2 down, and continue the dive while monitoring PO2.

These are examples of dynamic ways of solving problems that can only be known if the diver has an intimate understanding of how the unit operates.  There may also be other answers and factors, like keeping a close watch on PO2, loop volume, and to simply breath the high PO2 down as opposed to manually adding diluent after resolving the issue.

You may be wondering how to understand the nuance and potential situations you may encounter.  The primary way is to have a knowledgeable instructor willing to engage in ongoing communication throughout your career.  The second way is experiential.

Beyond Practice: Developing Resilience in Real Diving Conditions

Most of us dive for fun, and I’m not naive enough to think that everyone will only focus on shallow practice dives between MOD 1 and MOD 2.  Realistically, you shouldn’t.  Incorporating fun dives to your favorite sites while on the rebreather is equally as important as setting time aside to practice.  This is because it is inevitable that during those dives you’ll be faced with challenges and situations, both above and below the surface, that will force you to develop as a thinking rebreather diver.

The experience of having to manage the equipment on a boat, planning deeper profiles, executing decompression, and dealing with the litany of pre-dive checks under less than optimal conditions builds a confidence that cannot be gained outside of those specific environments.  Structured practice builds skill, but experience in varied real-world conditions helps foster the confidence and resilience needed for MOD 2.

Ultimately, MOD 2 is a relatively straightforward course for the serious rebreather diver putting in the time, practice, and knowledge development.  Understanding the expectations of the course, dedicating time to practice, and putting yourself in real-world situations is invaluable and necessary for success as you move into the next step of your diving career.

Final Steps: Assessing Your Readiness

How can you tell if you’re ready?  Consider the below checklist for readiness.

1: Maintain open communication with your instructor

Communicate with your instructor, understand their expectations, hold yourself to a high standard of performance and accountability, and stay humble regarding your skills.  If you’re open with them, they’ll be able to provide color, point you in the right direction, and make suggestions about your progression.

2: Demonstrate skill mastery with neutral buoyancy

Ensure that you can perform all of the necessary skills for the course while maintaining neutral buoyancy at all depths.

3: Cultivate mental readiness and resilience through real-world experience

Mental preparation is key, and executing experiential dives helps build the confidence and mental resilience necessary to comfortably push your own limits during MOD 2.  Build-ups are extremely important, as are shakedown dives with your instructor prior to the course.

Preparing for MOD 2 requires dedication, consistent practice, and a commitment to mastering the nuances of rebreather diving. By investing time to develop both technical skills and situational awareness, you’ll approach the course with the confidence and ability needed for success. Remember, every dive is an opportunity to grow—stay focused, stay humble, and keep pushing your boundaries safely.