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Come Out of the Bloody Cold: The Case for Neoprene with O’Three founder Sean Webb



By Michael Menduno. Images courtesy of Sean Webb unless noted.

There is arguably some confusion in the tech diving community over the advantages and disadvantages of neoprene suits versus the perceived virtues of trilaminates. Accordingly, InDEPTH chief Michael Menduno conducts a deep dive into the case for neoprene with head neo-advocate Sean Webb, founder and president of veteran British suit maker O3. The pair even take a short excursion into hydrogen diving with an O3 ambassador—the Wet Mules’ Dirty Harry. Are you feeling lucky today Punk?

It’s not an exaggeration to say that Sean Webb has been working to keep divers warm for nearly 40 years. The 56-year-old, water baby turned diving business veteran made his first scuba dive at age nine when his father helped him strap on a 10-liter aqualung and attached a rope tether to restrict his depth to two meters/six feet. Webb was hooked. Six years, two dive clubs and numerous dives later, he earned his ocean diver certification at the BSAC’s Weymouth Dorset Club. 

The next year, Webb followed his dreams and joined Ron and Joy Parry’s family dive business, SubAquatics, which they had founded in the 50s, in order to make neoprene wetsuits and drysuits. The teenage waterman never looked back. In 1989, when the family business dissolved, Webb decided to stick with what he knew and formed O’Three. He was later joined by one of the Parry sons. The name is derived from the chemical symbol of ozone, O3, which was one of the big environmental issues of that time.

Webb is surrounded by water on the limestone-tied island of Portland on the UK’s Dorset coast—the Jurassic Coast as it is known—which has been designated a United Nations World Heritage Site. Here, the boutique British drysuit maker quietly built a following and a reputation for making dry, warm, high quality neoprene drysuits that has continued on to this day, some 33 years later—no small feat for a diving business! In recent years, O’Three added wetsuits, and most recently a trilaminate suit in response to market pressure.

O’Three’s ambassadors and friends include Australia’s Wet Mules, who are conducting some extraordinary exploration, and knighted British diver Sir Rick Stanton of Thai cave rescue fame. Then there’s dive training veteran and RAID vice president Steve Lewis, the training officer for the Somerset Section of the British Cave Diving Group Michael Thomas, and YouTube influencer “Adventures With Purpose” principal Jared Leisek, to name a few. 

Full disclosure: I am an ambassador as well, and own an O’Three Ri-2 100 neoprene drysuit. I am also a Fourth Element ambassador and dive an Argonaut 2.0 trilaminate suit. You could say I’m a bit of a Anglophile, when it comes to dry suits—they dive cold waters. In addition, I own an older DUI FLX Extreme, made in the USA. 

Webb and I have been having informal ongoing discussions about neoprene for several years. It’s a subject that is not well understood by many divers. So I thought I’d reach out to this veteran pundit of polymers and ask him to make his case. Here’s what the man had to say.

By my count, you have been in the diving industry for close to 40 years, and you always seem enthusiastic and positive. Where do you find your inspiration?

Ha! I was just speaking to one of the guys who does a lot of graphic design for us. We are not a fashion brand, but we are coming up with a new range of T-shirts. I said to him, “Dave, we really need to tap into where we live. California is a great example. There are so many great brands that are tapped into California and the lifestyle. But we live in an incredible part of the world here in Dorset. We live on the Jurassic Coast and are surrounded by seawater. 

The Jurassic Coast

I think that’s what inspires me; it’s the environment here that helps us design the suits the way we do. If our suits can put up with the rigors of the Dorset Coast and the weather that we have here, then we are going to be making some pretty decent suits. That’s what drives us. You can’t help but be inspired every day when you wake up and traverse the causeway to come to work. We are very lucky where we are.

I’m going to have to come and visit you next time I’m in the UK for sure.

There’s phenomenal diving. I guarantee you! 

Sean Webb in the loop

Before we get into drysuits, I’d like to know what you think the biggest changes are that you’ve seen in the industry over the last 40 years? 

Obviously, rebreathers have been the big change, although they got off to a slow start. I think they’ve revolutionized scuba. It’s been phenomenal. To see people getting into diving and going straight into rebreathers. Ten years ago, people didn’t think that would happen, and now it has. Of course, the continued commercialization of what was predominantly a recreational pastime has been going on the whole time. Has that been good for diving? I don’t know, but the move to rebreathers has been incredible.

Craig Challen of Wet Mules fame

It has. You know, judging from today, when we look back at the “Technical Diving Revolution” that emerged in the late 1980s/early 90s, you could argue it was really all about moving to rebreathers. It has just taken us a while to get here. As a community we first needed to develop and create the infrastructure for mixed gas diving, as a necessary step.

Let me ask you the same question about drysuits. What do you think have been the biggest developments in drysuit technology over the last 30-plus years since you started O’Three? 

One word: neoprene! I think a lot of individuals are stuck in the Dark Ages and think that when it comes to neoprene, we’re still dealing with a fairly thick robust material that was invented back in 1930 and hasn’t changed since then. Whereas the truth is that neoprene compounds have gotten better and better and manufacturers have gotten better at making thin neoprene drysuits that stay dry for long periods of time.

 When we first started it was a 7 mm, or maybe a 5 mm drysuit if you were pushing the boundaries. Whereas now, we can produce a 1 to 2 mm suit that’s strong and robust and divers have a great time in them. Without a doubt, the biggest change has been the neoprene.

Neoprene rules!

Ha! I was thinking you were going to say front zips, dry gloves and P-valves.

Yes, all great developments, especially now that dives run into hours rather than minutes. But if you’re not DRY and comfortable, you’ll not be in the water long enough to warrant using dry gloves or a P-Valve. As for front entry zips, they create as many issues as they solve. But that’s a subject for another day.

We’ve talked about neoprene before. Obviously, there are a lot of new divers who get exposed to neoprene for the first time with their wetsuits. But for many, that’s it. Most tech divers upgrade to a drysuit and go straight to a trilaminate without ever working with neoprene. What should divers understand about neoprene that maybe they don’t?

Whether it’s neoprene or trilaminate, we always say that we spend as much time talking about the under suit, the base layers, the thermal layers, whatever you want to call them, as much as we do the suit. If you get that combination wrong, for example, you get your neoprene drysuit but your under suit combination is wrong, you could have a nightmare combination. 

It’s the same if you get it wrong with a trilaminate. 

So, we tell people: don’t think neoprene is the devil. If you wear the right under suit, which means you wear a much thinner under suit, you can have a terrific time in a neoprene suit and be better insulated, because the neoprene gives you that installation with a much thinner base layer. 

What that means for most people is that the buoyancy change, which takes into account that both the drysuit and the under suit, is minimal. Yes, there is a little buoyancy change in the neoprene drysuit but very little buoyancy changes in the under suit. And people tend to forget that. Whereas with a trilaminate, you’ve got no buoyancy change in the trilaminate but a huge variation in buoyancy in the under suit as it compresses, because it’s that much thicker.

The air, err gas, in the under suit compresses.

Right, it’s the combination of the two—whether it’s shared between the neoprene and the under suit—or the trilaminate and a much bigger under suit. So labeling neoprene the devil, and trilaminate utopia, is simply misleading. I think you’ve experienced that.

I have. I own an O’Three 2.5 mm neoprene suit, of course, and two trliams and have come to appreciate that it’s the combined system when you’re talking buoyancy. Even though people buy the argument that neoprene is a great insulator, they have in their heads that it compresses at depth, and so they get no insulation at depth, and it will screw up their buoyancy. Which isn’t true with the new materials, correct? 

No, it isn’t true. We’ve had some interesting conversations about this, even with the likes of some of our military users who should know better. If the suit starts off at 3 mm and it goes down to 0.5 mm, you’ve still got some insulation there. Whereas with the trilaminate suit, at the surface, at 30 or 40 m, there is zero insulation. It just keeps you dry. 

Of course, the neoprene expands again during ascent, and again, people think it’s going to send them into a tailspin and they’ll come accelerating to the surface. But if you’ve done your buoyancy checks, there shouldn’t be any issues. The neoprene is going to expand at 10 meters and shallower just where you are going to need that extra bit of insulation for the long part of your hang. 

I know you make several sizes of neoprene. I want to ask you about them: the Ri 100 (1 mm), Ri 2-100 (2.5 mm) and the MSF 500 (5mm). But first, I noticed on the website that the Ri 100 isn’t there anymore. That’s the crushed neoprene, right? You have discontinued the line?

The short answer is, yes we have. And credit where credit is due; that suit was inspired by DUI (Diving Unlimited International) and the materials that they made many, many years ago.

Yup, I used to have a CF200 back in the 1990s. That was “The Tech Drysuit” of the day and the team at Capt. Billy Dean’s Key West Diver all had them. DUI still makes them, but I believe they use a different neoprene these days.

The author sporting DUI’s CF-100 crushed neoprene suit circa 1992. Photo by Bret Gilliam

Exactly, credit where credit is due. I’m not going to get into any politics there, but crushed neoprene is not the material that it once was. And our RI 100 material was brilliant. When it was on point, and our manufacturer was ticking all the boxes, it was great. But to make that material requires too many processes, and there are a host of variables. As a result, the tolerances that we were getting made it difficult, if not impossible, to manage and work with. We decided to put it on the back burner. Ironically, we replaced it with a trilaminate suit, which may seem odd, as we have a long history as a neoprene manufacturer. 

I want to talk about your new trilaminate suits, but first I have a few more questions about your neoprene suits. What is the difference between the Ri-2 100 (2.5 mm) and the MSF 500 (5 mm) in terms of diving characteristics? Obviously, it’s thicker neoprene, but what does that mean operationally from a diver point of view?

The 5 mm neoprene suit has been our bread and butter. Think of it as a modern version of the good old-fashioned 8 mm suits, which were great in their way, but things have moved on. You’ve still got a nice piece of neoprene wrapped around you. It can fit a little bit better because the neoprene is the thing that’s giving you the insulation. You only need some very thin base layers underneath it. Nothing big and bulky. And people, shall we say from a more traditional neoprene background, will like that type of fit and feel. Again, because the insulation is in the neoprene, we can cut it slightly closer, err tighter. 

The result is that because it’s fitted better and only needs a thinner base layer, the buoyancy variation of the 500 is likely less than that of a loose fitting trilaminate with really thick under suits. It’s more streamlined, so you’re pushing less bulk through the water. It does change at depth. It will lose its thickness at 30-odd meters. It will be half the thickness. But again, if you’ve done your buoyancy checks and you’ve got that nailed, it shouldn’t be an issue. 


The fact is, over the last 33 years, you’d be surprised by the number of people who’ve come to us and said, “Do you know what? I’m wearing less lead with the 500 than I was with my big trilaminate and under suit which was bulky underwater.” They’re wearing less lead with it. The proof is in the pudding, you know? 

What about the 2.5 mm suit?

It’s a completely different rubber compound, so it compresses much less. It is going to be colder because it’s not five mil, but you have to put a thicker under suit on underneath it, and you make up that difference in the insulation. It’s a nice combination between under suit and neoprene, and you share the responsibility of the insulation between the two elements of your drysuit. 

In the warmer months, for people who still want to wear a drysuit, we sell a lot of those to people who dive and instruct in the Red Sea, and they’ll have a two point five mil neoprene. It’s not so onerous when it’s a little bit warmer. They’ll wear some Lycra leggings underneath it and a T-shirt and carry on diving and then throw maybe a slightly thicker under suit under it. In the winter, they will bring it back to the UK and dive with the appropriate undersuit. It’s a bit more versatile across a wider range of temperatures.

My first drysuit, back in the day, was an SAS neoprene suit with a back zip and all neoprene seals. Remember those? I loved it compared to a wetsuit. OMG! California has cold water. I forget if it was a five or an eight mil. It was probably an eight. Because it was old and that was what people did back then.

We are still making the odd eight mil neoprene drysuit today. And we still make a 7-8 mil hood-attached jacket and long john or farmer john for commercial scallop divers. They love it. They love that feel of a wetsuit. Okay, they’re not diving in January and February in the UK, but…

Come out of the cold! Brrr

Do new UK divers go to drysuits right away? Is that pretty typical?

It’s becoming more typical. I’d say it’s probably 50-50. But I’ve got no statistics to back that up. Again, 10 years ago there was definitely, “get yourself into a wetsuit or a semi drysuit, get a year under your belt and then get a drysuit.” But I would say there’s definitely more push to go straight into a drysuit. And why not? If you’re taught properly and you understand things, then great. 

I’m still a bit old school and would rather see people dive for a year or so in a wetsuit to just focus on their skills rather than have to worry about a drysuit. Because they can bite you in the bum if you drive them wrong. Fine-tune all your skills and then jump into a drysuit.

That makes good sense. Over the last five years, O’Three has added trilaminate suits to their product line. What was your motivation?

First and foremost, we don’t necessarily always make decisions on the best business practices, because we’re passionate about it. I’m not going to pull any punches; it was a business decision. Neoprene can’t be everything to everybody. We came back from DEMA after listening to many of your fellow tech compatriots saying, “Do you make a lightweight front entry trilaminate?” 

After saying “no” to a not-insignificant number of people, we came back on the plane and, after a few whiskeys, said, “We’ve got to master this trilaminate.” And that’s what we did. 

Video by O’Three

It’s been really successful. We didn’t run before we could walk. We knew about trilaminate, but we’d never put one together until four to five years ago. It’s been an interesting journey. In fact, we are just about to expand the product line—the production facilities here at our base in Portland, Dorset. It gives us another bite of the cherry, or slice of the pie, so to speak, and they are backed up with the same customer care and product detail and attention to detail that we’ve done with the neoprene. People are starting to respond positively.

Have you done anything special with them? Anything unique to O’Three?

We can offer many customizations. Gear and suits can’t be all things to everybody, but we can offer many options. In order to attach neoprene seals directly to the suit, a lot of people are moving to ring systems, which we can do as well. We tried to keep the front of the suit fairly clutter free, so we haven’t double-zipped the front entry. We offer an Aquaseal YKK plastic zip running across the front or the BDM medium duty metal zip running across the front, and we’ve come up with a nice clean way of covering the zipper and protecting it without applying another zipper over the top of it. Generally, that is A: a bit of a ball ache to the manufacturer, and B: When they do break, because they will break, they are very difficult to replace. So, we just kept that nice and clean and clutter free.

The O’Three Trilaminate Line

Sounds well thought out. I will take a look at your trilaminate at DEMA this year in Orlando. Will O’Three be coming to DEMA?

To be honest, we’re having a good time selling suits directly to the people in the US. The days of going through a distributor are gone, we feel, and DEMA is quite expensive, as you know. So, the jury is out yet as to whether we are going to be there. We do have a nice following over in the U.S. A couple of years ago, we added an influencer named Jared Leisek, who hosts “Adventures with Purpose,” and that’s really boosted our reach. 

It’s been quite a revelation for our business that had relied on all the traditional ways of advertising and word-of-mouth. So that’s been very, very interesting. He’s coming up with some good results.

O’Three HQ

What would you say is O’Three’s key differentiator, err secret sauce. Is there something that you feel like, “this is our core that we do better than anyone else”?

I’m sitting here and not really racking my brains because I’ve got the answer. I think the main thing with a company like ours is that drysuits are 98% of our business, and if we don’t get it right, we don’t have a business. So all of our focus and all of our energy is on making sure that we make the best possible suit we can. And if things do go wrong, and they do for every company at some point, we’ve got to give people five-star service. If the shit does hit the fan, we’ll move heaven and earth to put things right for people. I think those two things are what sets us apart.

That makes good sense. I see on your website that you stress quality and customer service as your key focus. I know for me; my Ri 2-100 feels like a Mercedes or a Porsche—the quality is just evident. But of course, everyone says, ‘Yes, we make a quality product,’ but what does that actually mean when you say we make a quality drysuit? Talk to me a little about that. 

Again, I think you have to look beyond what’s staring you in the face, which is a sheet of neoprene or a sheet of trilaminate material and some glue and some tape. We’re always thinking about the end-user and how would we want to be treated? What sort of product would we want to buy? And I think that drives us when we’re putting things together.

We do a lot of manufacturing here in the UK. The entire trilaminate line is made here. We have manufactured in the Far East for many, many years, and though other vendors do as well, there are definitely differences.

That’s in the specification, and the fact that we go to the factory during every production period. So, perhaps it’s just a little bit more attention to detail. We are not making units for anonymous users. We slow things down. 

Commercial divers favour neoprene suits .

Putting a wetsuit together is fairly easy, but when you’re talking about drysuits, if the seam is not spot-on from start to finish, we’re going to have failures. Maybe not when we initially leak test it, but six months down the road we’re going to have failures. And so, we need to give that attention throughout the early stages. For us, the tape on the inside of a suit is just there as an insurance policy. It’s not an integral part of making sure that the suit stays dry—that’s been done before we put the tape on. Make sense?

“What sort of product would we want to buy?” Yes. I can totally relate to what you said from my experience making aquaCORPS back in the day, and now InDepth. What sort of story do I want to read!

Let me ask you about neoprene, because as you have alluded to, all neoprene is not the same, right? There are grades of neoprene, levels of quality. 

Exactly. The market starts at the very, very bottom, with neoprene products in Walmart and discount stores; the neoprene is neoprene in name, but there’s very, very little neoprene in it. It’s a mass-produced material that won’t stand up to UV or to compression. It’s great on the surface, but the minute that you put it under water, that is under pressure, it’s a different ballgame. 

It’s important to make sure that the neoprene compound that’s being used reflects that. The surfing industry has been a major driver of neoprene technology. There are many more people surfing and doing surface water sports than diving, and for them compressibility is not an important issue.

When we speak to neoprene manufacturers, we say it has to be a dive-grade neoprene. And there are different grades within that genre as well. You have to choose carefully and make sure that you use certain neoprene on different areas of the body. For example, the Ri 2-100 uses four different neoprene compounds. The nylon linings have an impact as well. There’s more to it than meets the eye.

I know you have quite a few ambassadors. The Wet Mules are one. That’s a group that I follow and they’re doing some amazing diving as you know in Pierce Resurgence and other places. Rick Stanton is another, right?

Rick Stanton has been a customer and a friend for years and years. He quietly does his thing, as you know. Obviously, he’s getting some international recognition now, and I think Ron Howard’s film, Thirteen Lives, is coming out in November. So that’s going to put him and the rest of the guys on another pedestal, and quite rightly so. But he’s one of those people that just gets on with it quietly. As does John [Volanthen]. [Ed note: author of Thirteen Lessons that Saved Thirteen Lives]. John was in the shop last week. They never expect anything and are always happy to pay. And we have to tell them, you’re not paying. They are such great unassuming guys. 

They’re heroes in my book.

They are phenomenal.

Aquanaut Sir Rick Stanton in his 4mm O’Three

I have another technology question. I was just up in British Columbia and I noticed that all the locals up there, or at least most of them, were diving “heat”—electric heating systems. Many used the SANTI system, and there was another company called Versatile Technology that makes a vest with batteries. What are your thoughts on electric heat?

It’s a bit like the Ri 100. We made a heated under vest many years ago. Again, cutting to the chase, we stopped doing it about four years ago because the market was so small back then. SANTI is doing a great job BTW. I would say that they’re probably the market leader.

It seems like, yeah. DUI had a product for a while, Blue Heat, but they withdrew it. I don’t think they offer that anymore. 

It’s such a small niche market. We’re still making our heated vests for the British Special Boat Service (SBS). And we’ve had that contract with those guys for 20 years now, nearly. But the general public, being diplomatic now, just don’t apply the same level of care. It was a difficult product to support because of electricity and water. There’s a lot that could go wrong, and you do need to look after it. So, we decided it’s not our bread and butter, and we decided to leave that to the bigger boys. SANTI is there. 

Irish Army Ranger Wing diver packing their O’Three.

Of course, heat is not just for tech divers but anyone diving in really cold, single-digit water, right?

Going back to the basics: If your drysuit and under suit are really dry, it’s much easier for your body to heat up the dry air. So, if you have that right and you’re still cold, then perhaps you should look at a heated suit. Because, dives are often more than an hour. It can be two, three, four hours. The Pierce Resurgence guys [aka Wet Mules], bloody hell, they’re in the water for 15 hours.

Yeah, in 6°C water. Oh my God. You make heated under suits for them, right?

Yes, some of the Mules are using them, and some are using their own iterations of systems that are out there. They’ve got to bring anything they can. 

Our heated under vest is a loom of wire, and the switch went through the inlet valve. We adapted a standard APEX non-swivel valve, and then we put the on-off switch there. The battery pack is attached outside of the suit. But I believe the Mules also have battery systems in their habitats that they can plug into. They’ve got an array of adaptations, whether it’s some of our vests that are still using and other bits and pieces. They’re quite a clever bunch of guys, and they’ll adapt things. Rick does that with his equipment too. They’ll adapt it and make it suit what they want, and they’re not afraid to change things.

Dressed for success!

I’m glad I asked you about that. Interesting. Just as an aside, I’ve been working with Harry [Dr. Richard Harris]. They are really pushing the limits of physiology on these dives and so we started looking at hydrogen—using hydrogen for the very deep portion of the dives. I organized a small hydrogen working group. Our focus has been to investigate how tech divers could use hydrogen breathing mixes to improve safety and performance. We focused on the Mule’s Pierce Resurgence dives as they were a great example. 

How far off do you think they are from bringing it in?

Good question! They’ve got a ways to go. Ha! The problem is testing. Do you just say, “Okay, it sounds good in principle though it’s never been done before, let’s give it a try for real.” Ha! I have a great video of Harry, Dr. Richard Harris, imitating Clint Eastwood’s Dirty Harry character, going, “Well punk, are you feeling lucky today?”

They are looking at hydrogen, which is, of course, half the molecular weight of helium, because at the depths they are diving (to 250 m/815 ft and beyond), the density of the breathing gas is around seven plus grams per liter (g/L)—quite a bit above Drs. Simon Mitchell and Gavin Anthony’s maximum, do-not-exceed limit of 6.2 g/L. Danger, danger! Any exertion at those pressures and gas density, and you’re liable to buy the farm!

I’ll never forget—I think it was at OZTek maybe ten years ago. They showed footage and were explaining about how heavy the gas was and how difficult it was to push around the loop. And everybody in that auditorium was breathing hard as Harry was talking.

Ha! It’s not a trivial problem to fix. They are actually considering ways to help move the gas around the loop, for example, like those used in Continuous Positive Airway pressure (CPAP) machines for sleep apnea, which pressurizes the gas—well in this case air—slightly above ambient. 

How would they decompress from hydrogen? Are there any tables?

Another good question! According to noted decompression physiologist, David Doolette, who was in our H2 group, that’s the least of their problems! There are a number of critical issues that the Mules are going to have to sort out to dive hydrogen. 

First, it’s explosive when mixed with more than 4% oxygen, hence only useful for deep depths. For example, a 4% oxygen mix at 250 m would yield a PO2 of 1.04—shallower than that and you’re not going to have efficient decompression. Also, H2 has five times the heat conductivity of helium, so they will likely have to heat the gas. Of course, heated breathing gas is standard for commercial and military SAT divers. It’s a crazy and yet intriguing idea.

It sounds like something that Elon Musk needs to get involved with!

Right?! Instead of buying Twitter! Ha! They need a billionaire behind them for sure, if only to afford the testing that will be required. But it’s a real thing. I think at some point it will happen. The idea is to improve the safety and performance of conducting a 300, 310 meter dive, something like that—1000 feet over current methods. And of course, the word “safe” is really relative when you’re talking about doing surface-to-surface bounce dives to those depths. 

Did I ever tell you how we connected with the Mules?


It was through Rick Stanton. Harry mentioned it in an interview in “The Rescue.” He said something like, “This quiet unassuming English guy turned up who we had never met before, with a dry suit that looked like the best part of 20 years old.”

That dry suit was a 4mm neoprene that we used to make and the guy was Rick Stanton. That’s how O’Three came on the radar of the Wet Mules. That was about 15 years ago now. A couple of years into our relationship with those guys, I’m not sure whether it was Harry or Craig, but one of them told me that switching to neoprene (the Ri 2100) brought benefits that they were not aware of at the time; they just wanted reliable, dry, drysuits.

The benefits were the combination of streamlining and insulation that the neoprene suits gave them. The combination of those benefits reduced the effort, the exertion required to carry out those dives. They told me that the switch from trilam to neoprene was a game changer.

That makes sense. I remember Craig talking about the dangers of any exertion at those depths where just moving gas around the loop required an exertion because of the density of the gas. He said if you’re breathing is not calm and controlled, then you’ll soon be “behind the eight ball,” and things could go rapidly out of control. That’s what caused David Shaw’s death at Bushmansgat—respiratory insufficiency. Sadly, his video camera recorded his demise—respiratory insufficiency. 

Sorry to get us off track Sean. I just find this stuff fascinating. So looking into the future, what are your plans for O’Three going forward?

We’ve always been lucky from a staffing point of view. We have a few people that have been with us for a long time, but we have also got a great young team coming up. We’ve managed, not necessarily by judgment, but by luck, to reduce the [average] age of the team considerably over the last couple of years. And that’s a good thing for my partner Marcus and I going forward. We are hopeful that O’Three will continue. With the young team we have now, there’s no reason why the company couldn’t go on for another 25, 30 years. So, from a business point of view, we’re strong. 

I think, being really honest now, that some of our products have been designed to reflect how we think they should be built. But we need to look over our shoulder and realize that many of the new people coming into diving today want a slightly easier ride, if that’s good terminology. They want products that are easier to use. They might not perform as well, but then they are not doing the kind of diving that we expected them to do.

You’re speaking of recreational and or tourist divers.

That’s right. So we’re gearing up some products more suitable for the recreational diver because that is where the biggest part of the market is right now. We probably need to create a line that’s more user-friendly, shall we say, to get on and off. They might not perform as well, but these guys aren’t the Pierce Resurgence guys and they aren’t the Rick Stantons of the world. 

There are a couple of brands out there that have done well in that market, and we need to look at that going forward while not compromising what we do best, which is hopefully creating high quality products that will keep people dry and warm for many hours in temperate waters.

Thank you Sean. I wish you and your team the best of luck!

Michael Menduno/M2 is InDepth’s editor-in-chief and an award-winning journalist and technologist who has written about diving and diving technology for more than 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving” (1990-1996) helped usher tech diving into mainstream sports diving, and he produced the first tek.Conferences and Rebreather Forums 1.0 & 2.0. In addition to InDepth, Menduno serves as an editor/reporter for DAN Europe’s Alert Diver magazine, a contributing editor for X-Ray mag, and writes for He is on the board of the Historical Diving Society (USA), and a member of the Rebreather Training Council. 

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For the Masses not the Classes: Meet the Horizon

Semi-closed rebreathers (SCR) were the first purpose-built breathers for sport divers circa 1995—Thank you Draeger! But while tekkies clearly needed mixed-gas units, many believed that recreational divers would quickly follow suit with SCRs. Fewer bubbles, no troubles, right? Wrong, the migration never came, and the likes of Draeger, Fieno, and the Hollis Explorer ceased production. Now Italian scuba maker Mares is convinced that they’ve solved the problems with the Horizon—a smart eSCR designed for the masses. SSI Training Director Adam Wood explains.




by Adam Wood. Photos courtesy of Mares

In the past, Mares (pronounced Mahr-ehs) was never a brand you would have associated with technical diving. Their market was warm water, entry-level divers who preferred Italian style. Over the years, however, Mares has been a constant innovator, finding its own unique ways to solve problems where others just replicated past solutions. For example, when Apeks and others were adding crack adjustments to their second stages, Mares used their bypass tube system to deliver superior air flow. 

Their high-fidelity testing and development approaches led Mares and Poseidon to become the only regulator suppliers to the US NAVY. 

The Mares development team never stopped pushing the envelope, but these efforts magnified when Head Sports (Mares’ parent company) bought Scuba Schools International (SSI). Both Mares and SSI had long-standing beliefs that training developments and equipment innovations must go hand in hand. 

SSI offers so much under one roof: swim school, lifeguarding, mermaid, snorkelling, freediving, scuba, XR, and CCR. Most of the Mares product lines complemented these beautifully, and as their educational opportunities changed, so did their catalogue. At that time, Mares wasn’t involved in technical diving, and they saw that it would take a team with knowledge and experience to bring the Mares level of innovation to technical diving.

Mares Goes Tech

Mares chose to build a line of technical products from the ground up. They started by training all internal staff—from workshop engineers to product designers and even management staff—in technical diving. This effort began with a three-week training event in Croatia, where they held meetings and delivered SSI technical instruction to help staff understand what technical diving was and what its participants wanted. For Mares, the entry into technical diving meant much more than just delivering a product line; in typical Italian fashion, they had to live it and feel the passion.

From there, the “XR” technical diving equipment line was born. Mares harnessed the potential of its mainstream scuba dealers who now had the option to teach entry level decompression programs through SSI. This project turned out to be a huge success, and what started out as a “soft tech” product line became a complete mainstream choice for technical divers. 

The next logical step in the process was to use that same philosophy and dealer/training center base to increase their market share of rebreather training and unit sales. Mares considered purchasing two other mainstream rebreather manufacturers before reaching a deal with rEvo. There were many clear benefits to this choice, but perhaps the most clear was the chance to learn about the rebreather world from rEvo rebreather “guru” Paul Raymakers. Commitment to learning from subject matter experts and willingness to challenge their own ideas and norms is one extremely positive, if often frustrating, Mares trait. 

The new project had straightforward goals: Create a rebreather that 99% of divers could (and would want to) use that was as simple to operate as single tank gear while offering the added benefits of longer bottom times and reduced gas use. The real kicker was to try, over time, to make it affordable!

This did not look easy, but Mares was thoughtful in its approach and looked to the past before starting development in earnest. Other brands had tried the “recreational rebreather” thing, and it wasn’t all sunshine and rainbows. Some would say the collapse of VR Technologies and the Hollis Explorer could be attributed to heavy investment in this market with little to no return.

The Mares rebreather had to tick a lot of boxes: It needed to be simple, lightweight, quick to assemble and disassemble, easy to train with, redundant and failsafe without adding complexity and, above all else, it needed to be cheap! One problem with the recreational rebreathers of the past was cost. After training investments, there was not a great difference between these recreational machines and full closed circuit rebreathers; this left the consumer thinking they paid a similar price for a less capable device. 

Mares wanted the price to be comparable with OC equipment tailored to the same level of diving. For example, if you could do the same dives on a twinset and two deco stages, then the total cost of the rebreather should be comparable. To keep costs down, mass production was the only option; until now, this was unheard of in rebreather manufacturing. 

Another substantial hurdle was convincing current rebreather divers and instructors that this product was not for them and that it never would be. You will never find a reason to sell your JJ or AP and buy a Horizon. Current CCR divers should not try to compare features. The Horizon carves a totally new path that is unlike anything produced before; keep that in mind while we discuss its features in detail. 

Let’s start this quite geeky tome with an introduction to some of the Horizon’s main components.

Taken from InDEPTH’s Holiday Rebreather Guide.

Gas Addition

Electronic Delivery Valve (EAV) – This is the solenoid that opens and closes to allow supply gas to enter the loop. However, unlike a normal CCR solenoid, this one is normally open and current must be applied to close the valve. When in its open state, the flow rate is 25 L/min. This can be reduced to zero by applying current, or opening and closing the valve as required to achieve any flow rate in the range.

Constant Volumetric Flow (CVF) Orifice – Like the constant mass flow (CMF) valve in an mCCR or hybrid CCR, the CVF enables gas to flow at a fixed rate via a fixed orifice whenever the connected cylinder is open. In the Horizon, this is set to 5 L/min. The internal pressure (IP) of the Mares regulators used in the Horizon is not fixed like some mCCRS and will increase as depth increases; as such, the mass flow rate will also increase slightly. 

The formula for this is:

Adjusted Flow rate = (IP at depth / IP at the surface) * Flow rate

Automatic Delivery Valve (ADV) – Mounted in the top scrubber compartment, the ADV is essentially a second stage regulator that delivers supply gas when loop volume is low enough to activate the diaphragm. 

All three of these gas addition features are supplied from the same gas block. It is important to know that all hoses and gas connections are connected together in one system and are two-way. There are two (double end sealed) quick gas connectors (QGC) for gas on the Horizon: one on the left for bottom gas, and one on the right for decompression gas. The bottom gas regulator has an IP of 10 bar/145 psi and the decompression gas regulator has an IP of 12 bar/174 psi. 

When only the bottom gas cylinder valve is open, all systems, ADV, EAV, CVF, BOV, wing, drysuit and second stages on both cylinders are supplied by bottom gas. When the decompression gas cylinder valve is open, all systems including both second stages on the cylinders will be supplied by decompression gas—even when the bottom gas cylinder valve is still open

This makes gas switching on the Horizon super simple. Confirm personally and with the team that you are above MOD and open the decompression gas cylinder valve, tell the onboard computer that you have switched gas, and that’s it. The decompression gas regulator IP overrides the bottom gas regulator IP, and all systems are now supplied from decompression gas. The computer performs some small tasks in the background to ensure that you’re on the right gas, but more on that later.

A Semiclosed Rebreather

So we have the basics of a two-gas semi-closed rebreather (SCR) at this point. How do we know what we are breathing and that it is a safe gas? There are some basics to understand. If we add nitrox to an SCR, we will be breathing a gas containing less oxygen than that we supplied it with—how much less? Well, that depends on your metabolism (VO2) measured in liters of oxygen per minute. 

A diver at rest might metabolise between 0.6 and 0.8 liters of oxygen per minute at continuous hard work (e.g., swimming in an extremely heavy current). That might rise to 2 L/min, and the most you are likely to see is 3 L/min; however, this rate isn’t sustainable long-term. 

It is important to remember that oxygen consumption is affected by changes in work rate and metabolism, not changes in depth. So the same effort at 10 m uses the same amount of oxygen as it does at 40 m. And the Horizon SCR can supply a maximum of 30 L of supply gas per minute: 25 L from the EAV and 5 L from the CVF. 

To work out what you are breathing on the Horizon (Loop fraction of oxygen, orFO2), remember that whatever gas is vented from the OPV is the same gas that is entering your body since you’re breathing from the same bag:

Loop FO2 = [(Flow Rate * Cylinder FO2) – VO2] / (Flow Rate – VO2)

For the Horizon, the minimum oxygen content for supply gas is 30%, and the maximum is 100%. The upper limit is clear, but where does the 30% lower limit come from? 

If we rearrange the formula above, we can see that a diver with 30% Nitrox as a supply gas and a metabolic rate of 3 L/min requires a flow rate of 30 L/min to sustain a loop FO2 of 22%:

Flow Rate = VO2 * ( 1 – Loop FO2) / (Cylinder FO2 – Loop FO2)

Flow Rate = 3 * (1 – 0.22) / (0.30 – 0.22) 

Flow Rate = 3 * 0.78 / 0.08 

Flow Rate = 3 * 9.75

Flow Rate = 29.25 ≈ 30 L/min
In the table below, you can see what happens with various flow rates and VO2s when using a 30% supply gas:

The next table shows what happens if we were to supply the SCR with air even with a relatively low metabolic oxygen consumption of 0.8 L/min:

Even for very high flow rates, the gas would be hypoxic (less than 21%). At a moderate or high work rate, the loop FO2 would be far too low. For safety reasons and CE regulations, the lowest acceptable fraction of oxygen in the SCR at the surface is 21%. This is why air cannot be a supply gas and the minimum oxygen content is set to 30%.

A return to the FO2 formula confirms this:

Loop FO2 = [(Flow Rate * Cylinder FO2) – VO2] / (Flow Rate – VO2)

Loop FO2 = [(30 * .21) – 3 / (30 – 3)

Loop FO2 = (6.3 – 3) / 27

Loop FO2 = 3.3 / 27

Loop FO2 = 0.12

During the automated pre-dive sequence, the Horizon will analyse the FO2 of both the bottom and decompression gases to determine if they are in range, +/- 1% for bottom gas and +/-5% for decompression gas. The really cool thing about this process is that it shows the FO2 it’s analysing as it goes, so you know if you put in the gases wrong or if you received the wrong gas from the fill station. This process does two things: It confirms calibration as the calibration process is done with air and ensures each regulator is fitted to the right cylinder of the correct gas.

Oxygen Measurement

Another neat feature is that the gas from the EAV is added directly to the cell face of one of the two oxygen cells. This means a periodic gas check procedure can take place to ensure the right tank is open, the cells are performing as expected, and the EAV is opening and closing. The Horizon is equipped with two galvanic oxygen sensors to monitor the gas in the loop. To aid simplicity of training and understanding for the diver, they are only ever displayed as loop “nitrox percentage (FO2) However, in the background, these cells work like any other rebreather, turning oxygen into voltage which can be converted into FO2. 

There is one “T” (test) sensor and one “F” (fraction) sensor. The EAV adds gas directly to the test sensor every 5 minutes during the dive; this process confirms that the correct cylinder is open, and it uses a known gas (analysed while the machine was at the surface) at a known depth (taken from depth sensor) to determine if the right FO2 is generated. This first test confirms the T sensor is accurate. The next part of the test requires 20 to 30 seconds of breathing (something we hope the diver is still doing) to mix the gas in the chamber. The two cells are then compared, and any variation will show an error and ask the diver to either end the dive or bail out. 

In almost all instances, any error or failure of the Horizon will lead to a failsafe mode scenario where the EAVs power will be cut and will flow gas at its maximum rate of 25 L/min adding to the 5 L/min from the CVF orifice. No matter how hard the diver works, the machine will now maintain a breathable loop. The gas consumption will rise significantly and the diver will be prompted to ascend or bail out.

Calibration is often a pain point for CCR divers, and poor calibration can lead to serious consequences. Some machines have automated processes, some are manual, some require additional calibration kits, and some do not; however, it is always critical to perform the steps exactly as directed to get a quality calibration. 

The Horizon simplifies calibration by mechanically integrating the process. The machine will periodically suggest that divers calibrate and will prompt them if the calibration falls out of range. However, it is good practice to calibrate daily. First, the diver selects “Oxygen Sensor Calibration” from the menu on the DC (Dive Controller). The unit prompts the diver to remove the bottom scrubber and remove the cell tray. The calibration confirmation button is located under that cell try so the diver must expose the cells to air or the button cannot be pressed. Like any other rebreather, voltages are stored and values are set.

How Efficient is the Horizon?

Well that depends a little on the setpoint and how hard you’re working, but in practice it is very efficient indeed. With the right choice of gas and setpoint, you can achieve very low flow rates with gentle finning. The setpoint is the FO2 in the loop you would like the Horizon to maintain, obviously this can never be higher than the FO2 of the supply gas and, as a fixed rule, it can never be less than 23%. 

I recently completed a dive in the Maldives: I used a 30% bottom gas with a bottom setpoint of 23% and a 70% decompression gas with a setpoint of 63%. Max depth was 40 m/130 ft, and dive time was two hours. The bottom gas flow rate was calculated at 8 L/min and the decompression gas flow rate was calculated at 3.6 L/min. I used 60 bar from my S80 of bottom gas and 30 bar from S80 of decompression gas. 
You will note an apparent mistake in the above paragraph: How could I have a flow rate of 3.6 L/min when the minimum the Horizon can supply even with the EAV shut is 5 L/min? That’s the difference between my calculated flow rate and real life. In reality, my VO2 was not high enough to metabolise the loop FO2 down to the 63% setpoint (3.6 L/min) as gas was flowing in at 5 L/min. I was actually breathing 65%, slightly higher than my planned setpoint. All the better for deco (i.e., there were higher oxygen levels). I did actually know that 5 L/min was the minimum during my gas usage plan, so took that into account.

CO2 Removal

The Horizon takes its scrubber design from arguably the best feature of its big sister, the rEvo. The dual scrubber design has many top features, including its low profile axial design: once they’re on the diver’s back, the two scrubbers are vertical. This helps the material to settle without the potential for channelling and distributes the weight evenly and high on the divers back. 

Another great feature is the redundancy of two scrubbers. Even with a breakthrough in one scrubber, the second is there to act as a backup. The Horizon uses two 1 kg Sofnolime 797 scrubbers good for: 

> 70 minutes under CE conditions for PCO2 = 5 mbar

(CO2 1. Ll/min, RMV 40 L/min, 2 L tidal volume, 40 m, 4º C)

> 80 minutes under CE conditions for PCO2 = 10 mbar

(CO2 1.6 L/min, RMV 40 L/min, 2 L tidal volume, 40 m, 4º C)

However, we all know CE conditions are not always realistic, so Mares conducted real world tests using the CE apparatus to determine the duration of the absorbent in the Horizon. The test revealed scrubber duration was:

> 180 minutes for PCO2 = 5 mbar

(CO2 1.0 L/min, RMV 22.5 L/min, 1.5 L tidal volume, 40 m, 15º C)

For scrubber durations, the diver has two options based on temperature. If the temperature is less than 15º C, then change both scrubbers after 180 min. If the temperature is greater than 15º C, then change only one scrubber per the scrubber replacement cycle. As the top scrubber is used first in warm water, move the unused bottom scrubber to the top scrubber position, emptying and refilling the now-new bottom scrubber. 

As an additional safety feature, the Horizon tracks the scrubber usage in a unique way, using a process adapted (and simplified) from the rEvo. By telling the Horizon the weight and sex of the diver, it can estimate the CO2 produced by knowing the quantity of oxygen used. As the machine knows the cylinder FO2, the flow rate, and the loop FO2, it can easily determine how much oxygen was metabolised. Oxygen usage is directly proportional to the amount of CO2 produced. The Horizon then uses this figure to provide an estimated remaining scrubber time. If the diver were to work harder (metabolise more oxygen), the remaining scrubber time would diminish. If the scrubber time reaches one hour remaining, a warning is issued. It then repeats at 45 min and 30 min. At zero minutes remaining, the Horizon will go to failsafe mode: opening the EVA, delivering the maximum 30 L/min, flushing the unit with fresh gas, and alerting the diver to bail out.

The Horizon Electronics

The rEvo is fitted with Shearwater Research electronics—these represent a significant portion of the production costs. It was not possible for the Horizon to utilise the same tech and remain within budget. Using their technical knowledge, experience, and growing expertise, Mares decided they could rise to the challenge and create their own controller. Bulhmann had started to be implemented in Mares recreational computers and, looking around the market, it was clear there were very few competitors to Shearwater. Again, after analysing the benefits and drawbacks of the Shearwater products, a Horizon controller needed to be simple to use, with high quality feel and screen layout. It was decided Mares needed to make their own controller.

E1, E2, and DC

Being a life support system, the Horizon needed to have redundant electronic systems to ensure that in the unlikely event of one system failure the other will ensure the power is cut to EAV. These redundant electronics are called E1 and E2 respectively and are located in the electronics module below the bottom scrubber. They are in constant communication with the dive controller (DC) located in the handset. 

If either computer loses communication or detects an error, the machine activates the failsafe mode. Each electronic controller has its own battery: the DC battery is located in the handset and the E2 battery in the electronics compartment. The battery for E1 is conveniently located in the separate battery compartment for ease of access. This is basically a USB-C power pack that can be removed for charging away from the unit for instances like liveaboard diving or when it’s not practical to take your rebreather in the house.

Another cool part of that is you can carry spare E1 batteries in case of the “forgot to charge” scenario or when diving in remote locations. E2 and DC both draw their power from E1 and can go from flat to a state ready to dive (Charge state <30%) in only 30 minutes. If you can get the unit next to a power outlet, you can push charge to DC and E2 while E1 is connected to the unit. Normally this is handled by the auto charge sequence, but it can be overridden as required.

What Is It Like To Dive?

In short, super easy! The exhale resistance is very low and, when in trim, not even noticeable— the work of breathing is <2.75 J/L (joules per litre) with a 75 L RMV, 40 m, 4º C, and 2 kg of 797 Sofnolime. When I explain how it feels to dive, the best way to describe it is, “How you expected scuba to be”—right up until the instructor told you that you go up with inhalation and down with exhalation. You suddenly worked out for yourself buoyancy control is one of the finer points of OC diving. 

Cross that with how you thought CCR diving would be… you do not go up or down on inhalation and exhalation! That is until you tried it for the first time and realised that even small changes in depth cause the loop to expand or contract rapidly, causing waves of buoyancy that come and go. On the Horizon, the loop is almost always full; even a moderately fast descent is often negated by the CVF and EAV so the ADV does not activate. There is no need to vent gas from the loop on ascent as excess loop volume just leaves via the OPV without any “full” feeling in the loop. You continue to breathe in and out while changing depth as required; when you reach your new depth, set the wing to achieve neutral buoyancy, and you will stay there. It really is as simple as that. 

The user interface is intuitive and simple: current gas and setpoint selection at the bottom, current loop FO2 and remaining scrubber on the right. Current depth and dive time are nice and clear in large font at the top. The centre is reserved for NDL and deco stop depth, time, and TTS. Other info on CNS and O2 are available from the next page menu. 

When in surface mode, the whole menu is visible so you can see which item will be selected next, a welcome feature for those used to Shearwater. Menu options show some exciting features still to be launched on the Horizon, including wireless gas integration, digital compass, Bluetooth connection for dive log download and update, plus predictive deco functions. At the time of writing, the menu options are gas settings, pre-jump check, sensor calibration, scrubber settings, gradient factors settings, and dive and DC settings (Brightness, PO2, alarms and warnings, etc.). The only selectable options in dive mode are switch to OC, change setpoint, or change gas; keeping this process simple makes the Horizon feel very familiar very quickly. 

There are two fully integrated weight pockets, one on top and one on the bottom of the case. The top can hold 2.5 kg and the bottom 3 kg. As the scrubber is placed equally on the back and counter lungs run up each side, the trim is pretty much perfect right out of the box. The unit is very short and sits high on the back, so it’s fitted with an integrated buttplate that adds some rigidity and length to the unit to prevent back pain. When you tie this together with a nice loop bungee set of side mounted tanks, even the worst divers trim out super nicely. As the quick loop connectors come from the sides of the top case, you can extend your neck and head back fully without hitting anything, giving good forward visibility in zero trim. 

The BOV is extremely lightweight and with silicone soft loop hoses feels nicely weighted in the mouth. The stainless steel hose weights (stolen directly from big sister rEvo) add a quality, premium feel to the unit. The BOV tested as well as the rEvo DSV in loop mode and as well as a Mares regulator in open circuit: something where many have tried but lots have failed. Clearly the best design for a regulator is not the best design for a DSV, so a compromise is always needed. Where is the compromise here? Well, it’s actually quite a neat one: They have had to position the OC exhale ports slightly vertically, meaning you must tilt your head back when you want to clear water from the BOV in OC mode. It’s not a deal breaker, but must be emphasised during training. 

Training, Anyone?

Horizon training represents the perfect collaboration between agency and manufacturer. The steps for build up, failure drills, and general diving were in part specified by instructors and trainers at the point of design. This means the entry level, 30 m, No Decompression program (SCR Diving) takes only 4 dives and the 40 m Decompression (SCR Extended Range) program only 6 (performance is the only way to pass all SSI programs, so more dives may be required) The Instructor level is also easily attainable with 50 hour prerequisite and 3 day instructor program. As the machine and training are both produced “in house,” the manual relates directly to the machine, giving the best possible information transfer. 

Divers wishing to cross over from CCR need only 1 confined water and 2 open water dives. 

Currently only available through SSI, the prerequisites for training have been purposely made simple with 24 dives, Deep and Nitrox the only requirements for both the 30 m No Deco and 40 m Deco programs. You just need to decide how much time you have and if decompression diving is something you desire. (Upgrade at a later date is also possible by completing only the missing portions).

The academics are short and concise and do not include lots of complicated math as the diver does not need to know this information; these are delivered through SSI’s unique, high-quality digital training platform MySSI and include digital student and instructor manuals, review questions, checklists, skills videos and final exam. This tool allows more skill development time, increasing comfort and safety. 

The Horizon is €3,999 and €4,500 for No Deco and Deco versions respectively. For your extra €500, you get a second decompression gas regulator with QGC to allow connection to the machine. Both units are capable of decompression and do not require software updates or unlock codes to upgrade. Training will cost between €700 and €1,000 depending on the instructor and course. So, for €5,500, a diver will be trained for accelerated decompression to 40 m with dive times up to 3 hours. 

To get comparable training and the equipment on open circuit (twinset or sidemount fully equipped with stages to allow 3 hours diving and training, plus accelerated deco training) will be close to €4,000. Mares hopes that the success of the Horizon will allow further efficiencies in mass production allowing them to recoup the R&D cost sooner and then make the product even more affordable.

Post-Dive Debrief

What is it like to live with? Simple and easy! After diving, you remove the loop, back cover, two scrubber caps, and both scrubbers. The only part that cannot be flushed with fresh water and disinfectant is the cell tray that hangs neatly on the top scrubber lock. You can rinse the entire machine and be ready to start your build checklist in under 2 minutes. There are two OPV-style pull dumps at the bottom of both counterlungs; these are not used during diving at all and are there only to serve as a very efficient way of draining the cleaning fluid. The entire internal system is open to fresh air and you can see the insides of the counterlungs, so drying is much faster than other rebreathers. 

Travelling with Horizon was a significant part of the design brief. At only 12 kg ready-to-travel and being very low profile, it can fit in the bottom of your dive bag comfortably. The loop separates and packs down to almost nothing, you must now put two regulators in, and you are ready to rock! Most of the time, for 1-week warm water diving, I fit all my dive equipment, clothing, and SCR in one suitcase under the 23 kg standard airline weight limit. It’s as simple as travelling with OC. 

However, part of the Mares model is that you might not need to travel with your unit—in fact, you might not even own one. The Horizon is perfectly suited to the rental market. There are cost incentives in place for dive centres to have multiple units on the shelf ready for the travelling diver to roll up and take for a spin. 

Ongoing costs are reduced as much as possible: every six months, you should change one oxygen sensor (€70) and annually service the regulators (€95 each). There is no annual routine maintenance for the Horizon itself until year five. There are very few O-rings in the system, and these are inspected every buildup so will be replaced as required making the unit service interval every 5 years! Using only 1 kg of lime for every 3 hours of diving in warm water, the machine is very efficient. I find that an 11.1 L of 30% (€8) will last 2 to 3 dives and 11.1 L of 70% (€25) will last 5 to 6 dives. 

Because things do not always go as planned—and Mares understood that from the beginning—the design of the Horizon makes it very easy to remove almost any part to send for repair/service. The handset can be disconnected, the HUD can be replaced, and all batteries are replaceable and not potted. All these items need to be completed by a qualified engineer. Note that SSI ensures that all Horizon instructors are able to complete all unit maintenance and all Mares distributors can train Mares Horizon lab technicians. 

I hope this article has shown some of the unique challenges Mares has overcome to make something very different from other options on today’s market. I urge you all to take a try-SCR experience and remember what it is that a recreational diver is really looking for. I believe the Horizon fits that purpose perfectly.

Adam Wood is an SSI International Training Director for recreational diving and SSI Extended Range Business and Training Director. He has written SSI’s market-leading range of digital programs for Open Circuit, Closed Circuit, and Overhead Environments. Adam also owns Divemaster Scuba, a full-service SSIDive Centre in the UK with two arms: one specialising in training recreational diving from Kids Scuba to Instructor and another focused on CCR. As a manufacturer-approved warranty and service centre for most major brands, they lead the way for equipment maintenance and repair. In addition to these roles, Adam is an active member of the dive community through various organisations and professional bodies, including as Rebreather Training Council representative for SSI, CCR ISO committee member, Santi brand ambassador, Reel Diving Ambassador, product design and tester for many manufacturers, BBC safety and support diver, and more. His passion is teaching and passing on knowledge and skills in recreational, technical, and rebreather diving at the instructor/instructor trainer level—to constantly raise the bar of scuba diving and challenge old perceptions and philosophies to make exceptional thinking divers and dive professionals.

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