TECH: Mares Horizon

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Because rebreathers require specific skills not taught during traditional scuba courses, you must get certified before diving one

Instructor Chris demonstrating skills

Over the

Nicolas Remy explains what it is like getting to grips with the Mares Horizon SCR on a certification course

Photographs by Nicolas and Lena Remy

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The Mares Horizon is an innovative semi-closed

rebreather (SCR), which I found safer, simpler and cheaper to use than established CCRs (closed circuit rebreathers), while providing more bottom time and safety than traditional SCRs. In this article, I will report on the

Horizon certification programme, which I think is a good next step if you’re interested in this rebreather, but not quite sure about owning one (another option is to book a try dive).

For further details on how the Horizon compares to traditional scuba and other rebreathers, read my initial review (Scuba Diver Magazine ANZ 39). It has been a few weeks since I got certified myself, so I will complement this report with some longer-term observations on the Horizon, having spent 26 hours diving it so far.

Courses offering and where to get trained

Because rebreathers require specific skills not taught during traditional scuba courses, you must get certified before diving one. Since every rebreather is different, they all have a specific training programme: being certified on rebreather A doesn’t let you dive rebreather B (although you can ask for a quicker ‘cross-over’ course for rebreather B).

For now, SSI is the only training agency certifying divers on the Horizon. I know of three dive centres who offer Horizon trainings on the Australian East Coast (I got certified at Windang Dive and Spearfishing, NSW), but there are options in many countries, including some liveaboards offering both trainings and rentals. SSI offers two courses: SCR diving and SCR Extended Range. The former certifies you to dive the Horizon down to 30m depth, within no-decompression limits, and you need to have 24 logged dives plus be Nitrox certified and Deep certified before taking the course. The Extended Range (XR) course adds decompression diving (up to 25 minutes safety stops), lets you dive deeper (40m), and use a second ‘deco’ tank (carrying a higher oxygen nitrox, to optimize decompression). The first course takes three days, and most divers choose to do both courses in a row, which takes a total of four days. The courses’ objectives are to understand how the Horizon works, how to plan your dive with it, and the safety procedures if any equipment failure occurs underwater. This happens in two steps: a self-paced online learning, and the inperson training.

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The self-paced online learning

Student Ben performing a loop-clearing drill

Once you enroll, a very comprehensive online course is made available to you, covering the history of rebreathers, the physics behind decompression, the risks and conditions that can occur (decompression sickness and others), how the Horizon works, and dive planning. That last section is longer if you take the optional XR course to do decompression dives, as you will learn how to calculate gas consumption when you switch from rebreather to standard open-circuit diving (bailout). I have found the materials clear and enjoyable to study, with a good mix of text, visuals, videos, and many short quizzes.

What’s more, you can choose to study on a computer or a smartphone (MySSI app), and switch from one to the other as you’re pleased. I started on my laptop, and surprisingly found myself liking the app better and then using it extensively: lessons are bite-sized so can study on the go (even without an internet connection), bookmark any page, and even annotate (e.g. note a question to ask your instructor later). The app also includes all your SSI digital C-cards and air/nitrox decompression tables, plus all your training materials, which remain available to consult anytime.

All-in-one, this online learning is pretty comprehensive, and your knowledge will be tested in a final exam, so do not rush it to the last minute. It took me around a week to complete the online part, studying a bit every day.

Instructor Chris presenting the Horizon’s components The in-person training

After the online learning comes the actual in-person training, where your instructor will deliver a mix of theory and practical training, both in the classroom and in-water.

The theoretical part essentially revisits concepts from the e-learning, a good chance to ask questions, make sure all is clear, and do some exercises around dive planning. Importantly, the instructor presents in detail the various parts of the Horizon, and teaches you how to assemble, disassemble, rinse and store it. If you’re still on the fence about owning a rebreather, this is a good step to understand how much time you would be spending caring for your unit. Personally, I found the Horizon quicker and easier to maintain than the three CCRs I have used.

The in-water practice starts with two pool dives or confined water dives (max 12m), where you get used to diving the unit and practice skills (such as bailing-out), swapping masks, reacting to various types of equipment failures, or assisting an unconscious Horizon diver. On this note, I found the ‘gag strap’ (which keeps the mouthpiece in place) makes it less stressful to assist someone. Also, the breathing loop volume is constant for the assisted diver, so a controlled ascent is easier to achieve, another safety advantage I noticed vs full-blown CCRs.

The SCR Diving course concludes with another two dives: one to 20m and the second one to and 30m depth. During these deeper dives, the drills are repeated, with a focus on buoyancy control, at depth and during ascent, simulating safety stops, including deploying an SMB multiple times.

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Divers hanging together during the 25 minute decompression stops (Extended Range programme)

The optional Extended Range certification adds another two dives, to 30m and 40m, with 25 minutes of decompression. This involves preliminary dive planning, noting the planned decompression stops and their duration on a slate, and following that plan, ascending on a SMB.

Longer-term considerations

Reflecting on the last few weeks I have been using the Horizon for, a few considerations came to mind. Due to Covid, I have essentially dived the Horizon in and around Sydney, from the shore. This means I thought twice about carrying the optional deco tank, considering the walks and stairs standing between the me and the ocean. In most dives, I now take a single tank, a seven-litre steel 232 bar tank, which suffices for a 200-minute dive, but weighs only 7.6kg.

This gets me to weighting and balance: it will take a few dives to get it right, but it’s well worth the effort, granting you the super-stability which rebreathers are known for. My first confined water dive was very uncomfortable. I struggled to stay horizontal and had to keep waving arms around to do so. This is because I had too much lead in the top compartment of the Horizon. In my next dive I took these weights off, put them in pockets attached to the bottom of my tanks, which straight away fixed my balance, and I started enjoying myself. Whenever I changed tanks or moved from drysuit to wetsuit, I check my balance and do small adjustments. For example, when I carry a single tank on the left side, I will slide my weight belt to the right, to

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offset that tank weight, and avoid rolling side-ways.

The Horizon will consume three to eight times less gas than traditional scuba. Choosing the right Nitrox mix and setpoint helps getting the best gas efficiency. As an example, if you carry a Nx45 tank but want to breathe Nx40, with moderate exercise the Horizon will consume around 11 litres/minute (at any depth). With the same tank if you change your setpoint to breathe Nx35, the gas consumption will reduce to 5.5L/minute. If you do want to breathe Nx40 but aren’t diving deeper than 22m, get a Nx50 fill instead of Nx45, and your consumption will be down to 5L/minute (again, whatever the depth).

Whether or not you need the Extended Range (XR) course depends on your planned dives - will you go deeper than 30m, and if so, will those longer dive times get you into decompression? Simply refer to Nitrox tables to find out: usually you will breathe a Nitrox mix 5-10% lower than your tank’s Nitrox mix. Most of my recent dives were shallower than 25m dives), and the SCR Diving certification would suffice for these. Let’s consider one of my recent dives as example: with a Nx45 in my tank, I setup Horizon to breathe Nx36, and dived three hours between 16 and 10m, over one seven-litre tank. If I were on a 60-minute boat dive, I would setup my breathing gas a bit higher (Nx40) and could spend 50 minutes at 25m, then multi-level dive for the last few minutes, still with no deco. The Extended Range certification is very valuable if you often do square profiles (can’t wait to take the Horizon wreck diving!), go deeper than 30m, or want to spend longer at depth.

All-in-one, I found the Horizon training very enjoyable to do, and I would recommend it to anyone considering this machine but not sure yet about acquiring one. If you do take a Horizon rebreather home, you will experience a new type of diving, but give yourself a few more dives to arrange your weighting, balance, and fully enjoy the ride… n

Nicolas and Lena Remy

For more images from Nicolas and Lena visit their website, https://nicolaslenaremy.com/ or follow them on Instagram and Facebook @nicolaslenaremy