Open Circuit Technical vs Closed Circuit Technical – And Why I Teach on the Hollis Prism 2

Technical diving has never been more accessible. Deeper wrecks, longer bottom times, and safer gas planning are now part of many divers’ progression—from Advanced Open Water and Rescue, into Tec, and eventually into rebreathers.

But at some point, every aspiring tec diver hits the same question:

Should I stay on open-circuit technical, or move to closed-circuit (CCR)?

In this guide, I’ll break down the practical differences between open-circuit (OC) technical diving and closed-circuit (CCR) technical diving, and explain why I choose to teach on the Hollis Prism 2 here in Malta.

Quick definitions: what do we mean by OC and CCR?

Open-circuit (OC) is what most divers learn on first:

• You breathe gas from a cylinder.
• Exhaled gas goes straight into the water as bubbles.
• Gas mix is fixed (e.g. air, nitrox 32, trimix 21/35), so your gas and decompression plan is built around a fixed mix.

Closed-circuit rebreathers (CCR) recycle your breathing gas:

• You breathe into a loop; CO₂ is removed by a scrubber.
• Oxygen is added automatically or manually to maintain a set partial pressure of oxygen (PO₂).
• You exhale back into the loop—very few bubbles, very efficient gas use.

The Hollis Prism 2 is a back-mounted, fully-featured eCCR designed specifically for technical and expedition diving. It’s widely used for wreck, cave, and deep trimix diving because of its robustness, redundancy, and stable work-of-breathing at depth.

Why most divers start with open-circuit technical

Almost every technical diver begins on open circuit for good reasons:

1. Familiar kit, clear progression

If you already dive twinsets or sidemount, PADI Tec 40/45/50 and Tec Trimix build straight on that foundation. You’re:

• Using regulators, wings, and cylinders you more or less recognise.
• Learning solid gas planning, decompression theory, and problem solving without adding a complex machine on day one.

For many divers, this is the most sensible progression:

1. Master buoyancy, trim, and situational awareness on recreational kit.
2. Learn staged decompression and mixed gas on open circuit.
3. Then decide if CCR is right for your long-term goals.

2. Simpler mental model when things go wrong

On open circuit, the options are very clear:

• Out of gas? Share gas and ascend.
• Free-flowing regulator? Shut down and switch.
• Lost gas? Switch to backup gas and adjust the plan.

The complexity is in planning and logistics, not in the life-support system itself. For a lot of divers, this makes OC a great environment to build confidence and discipline before they move to rebreathers.

Where open-circuit technical starts to hurt

Open circuit can absolutely take you deep and far—but it has trade-offs.

1. Gas cost and logistics

Deep trimix on open circuit is expensive and heavy work. For deeper dives you may be:

• Filling large volumes of helium-based trimix.
• Carrying two back-mounted cylinders plus multiple deco stages.
• Doing short bottom times because gas is the limiting factor, not your decompression tolerance.

On a place like Malta’s deeper wrecks, a single OC trimix dive can mean:

• High gas bills
• Multiple deco bottles
• Limited repetition during a week-long trip

2. Limited bottom time

Because your breathing rate is constant and gas is vented into the water, every minute at depth has a real cost in gas. Even with big twinsets and multiple stages, you’re often working within tight limits.

If your ideal dives involve:

• Long exploration passes on wrecks
• Multiple deep dives in a week
• Filming or photography where time and stability matter

…then open circuit starts to feel restrictive.

Why technical divers are moving to CCR

Closed circuit rebreathers like the Hollis Prism 2 were designed to solve exactly those problems.

1. Massive increase in gas efficiency

On CCR, you’re only adding oxygen to replace what your body metabolises. Helium and diluent are barely used. The result:

• Much lower gas consumption, especially on deep and long dives.
• Trimix dives that would be prohibitively expensive on OC become realistic.
• Less weight and fewer cylinders to carry to the site.

Over a season of deep diving, the gas savings alone can offset a lot of the cost of CCR training and equipment.

2. Stable PO₂, optimised decompression

Because the unit maintains a constant setpoint, you’re diving with an optimised PO₂ profile throughout the dive:

• More efficient decompression for the same exposure.
• Flexibility to adjust PO₂ for work rate, depth, and deco strategy.
• The ability to extend bottom time without a proportionate penalty.

On wrecks around Malta in the 50–80 m range, this often means:

• Longer time on the wreck at a safer PO₂.
• More flexibility to explore, shoot video, or run multiple passes.

3. Warmer, quieter, more immersive

Many divers underestimate how much this matters:

• Warm, moist breathing gas vs cold, dry open-circuit gas.
• Minimal bubbles, which is huge for photography, marine life, and wreck ambience.
• Less noise and mask flushing, more time focusing on the dive itself.

Once you get used to that experience on a Prism 2, open circuit often feels like a step backwards for certain types of dives.

Why the Hollis Prism 2 specifically?

There are several CCRs on the market. I teach on the Hollis Prism 2 because it’s:

Built for serious technical and wreck diving

The Prism 2 is designed as a full technical platform, not a recreational toy:

• Back-mounted counterlungs keep the chest clear for stages and scooters.
• Solid work-of-breathing performance in the depths and positions we use on wrecks.
• A configuration that integrates well with deco/stage bottles, DPVs, and long penetrations.

Proven, supported, and standardised

The Prism 2 has:

• A long track record in cave, wreck, and expedition environments.
• Good manufacturer support and a global user base.
• A clear training pathway from entry-level CCR to trimix and advanced levels.

For students, that means:

• You’re not on an obscure or orphaned unit.
• There’s a clear progression from first CCR dives to serious technical projects.

Excellent platform for Malta’s wreck diving

Malta offers high-visibility, easily accessible wrecks in the 40–100 m range. The Prism 2 fits that environment perfectly:

• Efficient gas use makes multiple deep wreck dives in a week affordable.
• Minimal bubbles respect the wreck environment and marine life.
• Configuration is ideal for wreck penetration techniques you’ll learn as you progress.

So should you stay on open circuit or move to CCR?

Here’s a simple way to think about it.

Open-circuit technical may be better if:

• You’re just starting out with technical concepts: gas planning, redundancy, decompression.
• You want to build experience step-by-step without adding a complex machine yet.
• Your typical dives are:
  • 40–50 m range
  • Limited decompression
  • Shorter, high-focus training dives

In that case, a PADI Tec 40/45/50 or Tec Trimix course on open circuit is an excellent foundation before CCR.

Closed-circuit (Hollis Prism 2) becomes compelling if:

• You know you want to dive deeper, longer, and more often.
• You’re drawn to wreck exploration, filming, or project diving.
• Gas cost and logistics are starting to limit what you can realistically do.
• You’re ready to commit to regular practice, unit maintenance, and ongoing training.

Then a Hollis Prism 2 CCR course isn’t just a toy—it’s an investment in how you’ll dive for the next decade.

Training pathway: combining OC Tec and Hollis Prism 2

A smart progression for many divers looks like this:

1. Solid recreational base
   • Advanced Open Water (or equivalent)
   • Rescue Diver
   • 50–100 logged dives with good buoyancy and situational awareness.
2. Open-circuit technical training
   • PADI Tec 40/45/50 to learn:
     • gas planning
     • staged decompression
     • emergency management
   • Optional: Tec Trimix for helium-based deep diving.
3. Transition to CCR on the Hollis Prism 2
   • Entry-level CCR course focusing on:
     • unit setup and pre-dive checks
     • loop management and buoyancy
     • dealing with CCR-specific failures.
   • Progress to deeper/trimix CCR levels as your experience grows.

This way, when you climb onto a Hollis Prism 2 in Malta, you’re not also trying to learn basic decompression and team skills from scratch—you can focus on mastering the unit.

Common myths about CCR (and the reality)

“CCR is only for extreme depths.”

Not true. Many divers use the Hollis Prism 2 for 40–60 m wrecks specifically because:

• It reduces gas costs.
• It keeps them warmer and more comfortable.
• It allows longer, more relaxed bottom times.

“Rebreathers are too dangerous.”

Any life-support system is dangerous if used poorly. CCR demands:

• Strict pre-dive checklists.
• Regular maintenance.
• A commitment to ongoing training and practice.

With proper instruction, solid procedures, and conservative planning, CCR can be at least as controlled as open-circuit technical diving—often with more redundant information (multiple PO₂ displays, integrated computers, bailout options).

“I’ll lose my OC skills.”

A good training programme keeps you current on both:

• You’ll still plan bailout and deco as if you might need to go OC at any time.
• Many CCR divers retain a twinset for skills, backup, or specific dives.

On my Hollis Prism 2 courses in Malta, we emphasise maintaining core technical skills alongside CCR-specific procedures.

Why train Hollis Prism 2 CCR in Malta?

Malta is one of the best classrooms for both open-circuit technical and Hollis Prism 2 CCR:

• World-class wrecks in the 30–100 m range.
• Typically clear, warm(ish), blue water with easy boat access.
• A mix of:
  • training-friendly sites
  • deeper, more advanced wrecks for later in your progression.

For CCR specifically:

• You can get multiple quality dives in a week without burning through obscene amounts of trimix.
• Conditions are consistent enough to focus on your skills and the unit, not just surviving the environment.

Bringing it together: choosing your next step

Open-circuit technical and closed-circuit technical are not rivals—they’re stages in the same journey.

• Open circuit technical gives you the foundation:
  gas planning, redundancy, decompression discipline, team skills.
• Closed circuit on the Hollis Prism 2 gives you the range:
  longer times, deeper wrecks, and more ambitious projects—especially here in Malta.

If your long-term goals involve:

• Regular trips to deep wrecks.
• Serious time underwater on each dive.
• Project-style diving, filming, or exploration.

…then the Hollis Prism 2 is very likely your next logical step.

Ready to talk about your path?

Whether you’re:

• An experienced OC tec diver wondering if CCR is worth it, or
• A motivated recreational diver looking at the full pathway…

I can help you map out a training plan that fits your experience, budget, and goals—from PADI Tec courses on open circuit through to Hollis Prism 2 CCR training and guided wreck dives in Malta.

Get in touch to discuss:

• Which course or level makes sense for you.
• How to plan a Malta tec week around your training.
• Options for guided Hollis Prism 2 CCR wreck dives once you’re certified.