Mares Horizon: Learning how to rebreathe

As a mad-keen scuba diver, a major frustration is having to surface because I am getting low on air. As an ‘air hog’ I find that even with a 15-litre tank, I always have to surface long before my dive buddy needs to.

On the other hand, I hate ‘fuss’, and the thought of the complicated setup and strip-down associated with conventional rebreathers has always made them a non-starter for me. Not to mention their cost.

Last spring, I was lamenting this problem at my ‘local’ (the Dive Doctor in Penrose) when Anthony showed me the brochure for the new Horizon Semi-Closed Rebreather. I was soon interested, and after some online research, it convinced me this was the way to go.

Mares designed the Horizon with one overriding directive – simplicity. Targeted squarely at the upper end of the recreational market, their solution is easy to operate, safe, economical to run and provides extended dive times at recreational depths. By setting clear boundaries of what it was and was not (maximum depth of 40m, single gas source and no specific tech diving skills required), they came up with the Horizon. Operating off a standard nitrox gas tank, the Horizon can be used anywhere this mix is available, generally at all major dive centres.

Anthony made it quite clear I would not be able to get my hands on an actual unit because they had not yet arrived in New Zealand. Similarly, there were as yet no certified instructors avail- able locally. Nevertheless, I stretched the old plastic and paid around $7000 for a unit – one of the first sold in the country.

Of course, our second Covid lockdown played havoc with everyone’s plans, and it was well into this year before the first batch of local instructors could even commence their training. All this time, my unit sat safely in the Mares warehouse while I chomped at the bit. As soon as the borders opened, I looked at the feasibility of popping over the ditch to get my training, but flight restrictions made it too complicated and expensive.

It was May this year before the SSI instructor trainer from New South Wales could get here and begin certifying some New Zealand-based instructors, including Shane Housham of Northland Dive. Eventually, I could book a course and take delivery of my unit – the first such device and training in Northland, apart from Shane’s own Horizon.
Of course, I had already absorbed the manuals and viewed numerous videos of people diving with them, so I wasted no time getting my unit configured and the necessary eLearning out of the way. On Friday, 4th November, I drove up to Northland Dive’s fabulous lodge in Whangaruru to start my SCR journey.

A semi-closed rebreather (SCR) fits in between a conventional open-circuit scuba system and a full rebreather. In an open circuit system, we only consume our tank’s air one breath at a time. When we breathe out, the gas is expelled from our regulator even though we consume only around 3-4% of the oxygen per breath. This is why CPR and mouth-to- mouth breathing allows us to revive a patient with this ‘second-hand’ breath.

A rebreather system, in contrast, captures our outgoing breath into a bag called a counter-lung, removes the carbon dioxide from it, and then tops it up with oxygen. We then rebreathe this enriched air back in. In the case of a full rebreather, that top-up is from a small cylinder of pure oxygen, and complex gas mixes, including Helium, are also possible. That gives a rebreather the ultimate performance regarding dive duration, depth and minimising decompression obligations. But with the downside of considerable complexity and cost.

The Mares Horizon differs by using nitrox (air with a boosted oxygen percentage) to top up the loop containing the air we are breathing. Every time we exhale, the system captures the used gas, removes the carbon dioxide, analyses the remaining oxygen and inserts the appropriate quantity of nitrox. Our next breath is of this boosted mix while a small fraction bubbles away. A semi-closed system like the Horizon can provide up to eight times the dive time we would get from the same volume of ordinary air. While it is not entirely bubble-free, there are dramatically fewer bubbles than from an open-circuit scuba regulator.

A couple of other features and benefits of using the Horizon need to be learnt (or, in some cases, unlearned). First, you do not rise up as you take a breath in or sink as you breathe out. Because the volume of air in the breathing loop (your lungs and the counter-lungs of the system) is constant, breathing in and out simply moves the air between them. Hence buoyancy is consistent, and one can hover absolutely still, which is a boon for underwater photography or catching critters!

Diving with any rebreather is also warmer than conventional scuba because every breath is already warmed up and humid, so you are not sucking cold, dry air. That breathing action is also far more natural since you don’t need to create a vacuum in your mouth to enable the air to flow. Also, the unit is almost silent in operation, without the rasping intake and bubbling exhaust from a conventional scuba regulator.

There are some skills to unlearn, one of which is never to remove your mouthpiece underwater without closing the special bailout valve. This is because the scrubber material that removes the carbon dioxide is a dry granule, so water in the breathing loop would be catastrophic. There is an open-circuit regulator for emergency bailout use, and the training covers the procedure for swopping over to this.

There are only about half a dozen specific skills to learn, plus the various setup and pre-jump procedures. These are much less complex than those for full rebreathers, and the entire unit is remarkably simple in operation and use. The primary requirement of the course is to spend 300 minutes (5 hours) actually breathing through the unit underwater. While this sounds like a lot, the purpose is to get you completely comfortable with using the unit.

Apart from the first day’s dives in very sheltered conditions, the rest of the course comprises scenic dives of increasing depth, interspersed with skills practices. Some of these practices are self-initiated (such as draining water out of the loop if it builds up), while instructor Shane prompted others. Because of the generous time allowance, we could temporarily suspend skills training as an eagle ray glided past or literally push away the Sandager’s wrasses that kept coming to investigate, then resume another practice once the wildlife temporarily left us alone. I have to say this was one of the least-stressful training courses I have ever done, thanks to the relaxed pace with which we could cover the required skills.

As an experienced open-circuit diver, the real eye-opener for me was how close you get to wildlife with a rebreather. We could have hand-caught the John Dory swimming alongside the HMNZS Canterbury. We did, in fact, ‘tickle’ the wrasses and leather-jackets and had schools of mackerel swim between us. You never get this close when you are emitting noisy clouds of bubbles. I can’t wait to see what happens next time I have one of the rarer encounters like turtles, dolphins or seals. Of course, we were in a no-take area of the Bay of Islands for the training, but I suspect my crayfish hunting in other regions may be a bit more successful in the future.

After completing the final academic evaluation, I joined the currently somewhat rarefied ranks of certified Mares Horizon divers in New Zealand at the end of the three days. Covid has played havoc with the supply chain worldwide, resulting that these units are only now starting to arrive in New Zealand in greater numbers. I hope to see more users soon.  Otherwise, my usual dive buddy will frustrate me every time we have to call a dive because HE runs low on air!

Thanks, Shane, for a great course combined with the usual great hospitality and food.‍

Not to mention thanks to Anthony for initially doing an awesome sales job on me to get me to buy a unit that has taken me a whole year to get my hands on. But I won’t be taking my hands off it in a hurry!