Oct 04, 2013

What to expect when you're sidemounting

Not long ago I was a regular open water diver who had just read about sidemounting. If you are curious about the details, here are some things I've learned.

When most people learn to dive, a single tank strapped to their back is enough to see the wonders of the sea. Sidemount involves hanging two separate tanks to from each side. There is no agreement on "the right way" to sidemount and many different opinions around, so I'll do the same and present my own take on it.

Why sidemount?

According to popular sources, sidemounting was developed by cave divers in the UK and later in Florida for reasons of logistics and passing through tight restrictions. But first, lets back up a little.

Double tanks

Doubles are safer than singles.

You may have seen technical divers with paired tanks on their backs (called manifolded doubles or simply doubles or twins). In sport diving a diver is trained to rely on their buddy for alternate air, and that there is always easy access to the surface with survivable odds of decompression sickness. A technical diver is often unable to surface in the event of a problem, usually due to an overhead environment or a decompression obligation. Any problem has to be dealt with right there, including gas supply failures. A failure in a single tank almost certainly means an out-of-gas situation. That is why a technical diver carries at least two of everything.

Redundancy, safety and self-reliance

Doubles mean a diver has twice as much gas. More importantly, each tank has its own regulator. In the even of failure a single tank can be isolated and shut off, and the diver still has one good tank and half the gas to safely finish the dive. In effect, the diver carries his/her own alternate air supply.

Sidemount configuration provides this gas redundancy in a simpler configuration.

Configuration benefits

Sidemount is safer than doubles.

Twin cylinders are not without problems. Out of the water they are heavy, the manifold valves are behind your head and they stick up from your back, making it hard to fit in tight spots.

Sidemount cylinders are completely separate from each other, they hang from your sides by means of boltsnaps and bungees. Each has its own regulator and a pressure gauge, there is no connecting manifold and no isolation valve to think about.

The tanks can be carried individually, not together on your back, and put on or taken off in the water. The tank valves sit just under your armpits, always visible and within easy reach. Finally, there is nothing on your back, allowing you to squeeze through narrower passages. You can even unclip one or both tanks and push them in front of you, making your profile even smaller. Swimming in general is easier because of the reduced drag.

Safety again

Having a valve behind the head makes it not only difficult to operate, but potentially dangerous. You can accidentally bump against something hard and break a burst disk, or roll off the valve. In sidemount protecting the valves is easier since you are much more aware of what's in front of you. Also, correcting a valve rolloff is not an issue.

... and again

Since you cannot see behind your head there is an exercise called a "bubble check", where your buddy checks for any gas leaks from your tanks/regs by watching for bubbles. You can also do it yourself by situating yourself face up and waiting without exhaling — any escaping bubbles should be visible as they float up. Of course, you still can't see exactly where they came from.

But in sidemount you can — the valves and regulators are right in front of you at all times. If a tank fails, you'll immediately know which one and can take action.

Finally, even with a failed regulator you can breathe from it by continuously operating the valve (see feathering below).

Sidemount in practice (combined first impressions)

The theory is great, so how does it work? I wanted to know so I found a local instructor and we scheduled a class. The instructor lent me his BC and brought a couple of steel tanks. Then my own set of sidemount regulators and a harness arrived. I rented a couple of steel tanks and spent some time in the pool, to test out the new harness and to practice diving in sidemount. Then I went to Florida for cave training, all in sidemount — but that itself is another story.

Sidemount takes a little getting used to, but the process is easy and it works. I do not see myself ever needing twin doubles.

The regulators

Regulator configuration is based on the Hogarthian system. You have a pair of identical regulators, each with its own pressure gauge, and at least one long hose for sharing air. The short hose hangs on a necklace (always available as a quickly reachable backup in emergencies), the long hose has a boltsnap for clipping to a shoulder D-ring.

There is a disagreement on which tank gets the long hose. Having been taught both ways I can say this: the hose should be on the side where it attaches to the regulator. Unless your reg is a switchback that side is right. This has to do with the specifics of sharing air while exiting a cave: the donor is directly behind the recipient and you want the hose going along the same side, not across.

Steel tanks

Steel tanks were new to me, before I only dived aluminums. Full aluminum tanks sink a little, half full they are neutral and empty they float.

Steel tanks always sink and they are heavy! In fact, they were heavy enough for me to not need any additional weight in fresh water, both wet and dry suits. With Al tanks you need the additional weight just to counter their positive buoyancy at the end of the dive. That was nice.

Tank attachments

Each tank is attached at three points. First, there is a leash with a boltsnap on the valve, that clips on to your shoulder D-ring. Second, a tank band near the bottom has another leash with another boltsnap, that clips on to your waist or buttplate. Finally, a bungee goes over the valve and pulls the tank up and towards you, under your armpit. The valve boltsnap does not carry load after that.


Getting trim is different in sidemount and mainly involves hanging the tanks in correct position. You want them alongside and parallel to your body, and close. Not below, not at an angle, not too far back.

In my first session the heavy tanks were hanging way too low, the inflated wing was floating above and I was suspended in the middle, unable to neither pitch nor roll. Not even to look up. It was not comfortable and a little frustrating.

The second session went much better — I had my own tank bands with shorter leashes and my own harness with much better bungees. You see, the bungees need to really pull the tank valve up, otherwise it will hang too far down and too far back. Staying horizontal will be difficult, not to mention poor profile and dangling tanks.

Getting there will take time at first. Once there, however, maintaining the proper prone position is effortless, your body will naturally orient itself that way.

The tanks are just hanging there

In backmount the tank is strapped in place. It's not going anywhere, no matter how you flip. With sidemount, facing up means the tank bottoms flop down towards your butt. However, with properly positioned tanks flipping around in the water is really a non issue.

Getting in and out of the water

Diving fresh water you usually put the tanks on in the water. For this you want to be standing and about chest deep, otherwise it's difficult.

If that is not an option, you'll gear up on land or boat. All the usual water entries and exits are doable in sidemount, just more awkward.

Gearing up

Sticking to a sequence makes gearing up faster and less error prone. An example sequence looks like this. Left tank: shoulder clip, bottom clip, inflator hose to BC, regulator in position (necklace on), bungee. Right tank: shoulder clip, bottom clip, inflator to drysuit, regulator in position. The long hose is always the last thing to go on — that's the thing that comes off in an out-of-gas emergency.

Alternating tanks (gas management)

You want to breathe the tanks down evenly, so you switch regulators at predetermined points (usually every 200-300 PSI). If you don't, one tank will start getting noticeably lighter and you'll begin to list to one side. Also, should you lose the fuller tank you'd have less gas left in the remaining one. Also, calculating fractions for gas planning is harder when tank pressures are far apart.

Another upshot of tank switching is that breathing from both regulators ensures that both are working. This mostly eliminates the chance of switching to a non-working regulator in an emergency.

Gas emergencies

As said before, with two separate tanks you have options when dealing with gas emergencies.

Valve shutoff

Most gas supply malfunctions involve gas escaping from some part of the system. The first step in dealing with leaking gas is to immediately shut off the tank to prevent any further gas loss. If that happens to be your current tank, you obviously have to bail to another regulator.

Breathing down the hose

This is an interesting exercise if you have never done it: learning what it actually feels like to breathe from an empty tank. You do this to both familiarize yourself with the feeling and to learn to recognize what's happening.

To do this, you close the valve on the tank you are breathing from. Depending on your depth, the long hose may still have almost a whole breath left in it. After that, a brief moment of rapidly increasing resistance, then a solid nothing. An amazing feeling.

A partially closed valve delivers gas at a reduced but constant rate. If the rate is below your current consumption rate, somewhere in drawing a breath it will get difficult, every time.

Valve feathering

After shutting off a tank with a problem, you have options. For example, a regulator fails by sticking open, never closed — this is called free flow. You can still breathe from it without losing too much gas. To do this, you put your hand on the valve and crack it open as you draw a breath. The regulator will deliver gas, and some of it will escape. As you're almost finished you can close the valve and breathe down the hose, or wait another second.

Bailing and donating in sidemount

In a traditional Hogarthian configuration you always breathe from the long hose, and the short hose reg hangs on a necklace under your chin. The reg in your mouth is always the one you donate (or lose because someone or something ripped it out), and the backup is always on the necklace. One quick reach and you're breathing again, even in total darkness. Burn that into your muscle memory and you're set.

But in sidemount things are different: half the time you are breathing from your short hose, and the long hose reg is clipped to a shoulder D-ring. A number of what-ifs had bothered me. How do you find the right regulator to donate? What if you don't remember which reg is the long hose? Do you waste time fumbling while your buddy is about to start killing you? What if he rips your short reg and now there is no backup under your chin?

In practice things turned out to be simpler:

  1. There is always a spare regulator nearby, either on the necklace or on the shoulder (you can start breathing from it without unclipping). Never let it dangle unclipped.
  2. Remember which hose/tank you're breathing.
  3. If you forget, sweep by checking the shoulder first. The long hose is ether there or in your mouth.
  4. The long hose is attached to the bolt snap by a zip tie. Don't waste time unclipping. Push it away hard and it will break off.

Oct 01, 2013

Diving sounds awesome, where do I start?

A friend recently asked: What is the least I need to spend to start diving? Diving is not a cheap hobby. However, it does not have to be terribly expensive either.

To dive, you need three things: training, equipment and experience. What that will be really depends on what you do and where you do it. Where will you be diving? Truth is, you probably don't know. Diving is such a vast thing that you can only explore it little by little, trying different things, going to different places, tinkering with equipment and so on.

Naturally, my advice is biased towards my own approach — watch for a later post on my take on gear choices.

The first step

Your first step will be to take an Open Water course, and there are two choices: find one nearby or go to a tropical resort. The tropical resort course is likely to be more enjoyable (more on that later), you can get your OW card in 2-3 days and do some more nice tropical dives while you're there. The local course will likely take longer, the training will be harder, the water will be colder, etc.. In both cases expect to spend: $200-400.

Word of advice for the local course: find and join a local dive club. Heavily discounted classes may be available for members.

Word of advice if you go the tropical route: do not put off your training until the end of your vacation. In the likely event that you discover you really like diving you will still have time left for it! Expect to spend $60-80 for every two tank boat dive.

Also, there are such things as "discovery" or "introductory" dives — for $50-$100 you can get a couple of dives with everything provided, no cert required. I don't see a point in those unless you really have no intentions of diving more.


So you've decided to take a class and become an Open Water diver. Maybe you even decided to work a tropical vacation into it. Maybe you are going to a place where buying gear is problematic (ahem, Mexico). You walk into a dive store. What do you get?

First and foremost, remember you have absolutely no idea how a piece of gear will work for you in the water. You may get decent advice if you're lucky, but more often you'll get a salesman wanting to upsell you on crap (more on that later). Either way, you won't know until you try and get some experience! So either hold off on purchases for now, or do serious research and only get the most basic things, starting with a mask.

Therefore, your first best option is to simply wing it and go to a popular diving resort with nothing but cash in your hand. They will have everything for you to rent. Sometimes rent is included in the price of the class and even the regular dives.

Rent or own?

This is not much different from other "equipped" sports like skiing. Renting is fine in the beginning and is a good way to familiarize yourself with all kinds of gear, how it actually works and what you actually need. Once you do, however, you will probably want to kit up. Some things to consider for the future:


I have my own gear and I bring it. It is a huge and heavy bag, but I do it for reasons listed below. Many people travel with nothing but their certification card, some carry only small essential things, like mask and computer. Diving locally you might want to get equipped sooner.

Fit and hassle

This concerns wearables in particular: finding rentals that fit is a game of chance. Suits and jackets are all made differently and some will fit you better than others. Some will have specific features you might like. Being randomly uncomfortable in rentals, getting used to new gear every time, wasting time picking them out — all of that is fine at first, but will get old fast. Once you get that one suit that really fits you'll be set for a long time.

Your gear is your own. You can splurge a little and get the nicer things. For me it's about having my own gear configured, adjusted and set up exactly how I like it.

A fellow diver once said: Diving is so unhygienic — we spit in our masks, we pee in our wetsuits and we throw up in our regulators. All true! While you shouldn't let it deter you from renting or buying a used wetsuit, you may prefer to pee into your own. Not to worry, you will stop caring soon enough.


Dive gear requires maintenance. The rental shop will take care of theirs, that could be a good or a bad thing. If you rent, learn to inspect it. A loose dump valve cap, for example, could be a bad surprise.


Helpful rules to remember:

  • Once it's wet it's yours. No refunds, with very few exceptions.
  • Initial investment will seem high — do not be put off by this. Some of the items listed below are worth it.
  • The price ranges are for new equipment. Used can be had for considerably less.

Mask: $25-$100

This is one thing you can safely buy right away, and the most crucial one to get to fit properly. Few things will make you more miserable than a leaky mask.

Check the fit by gently holding mask to your face, sucking the air out through the nose and closing the back of your throat: the mask should stay on for a while. Still, a mask will fit differently once in the water. See if they let you try it in the pool.

Things look unusual in the water: bigger, closer and not where you expect to find them when turning your head. The BIG masks don't really add anything, because of water-to-air refraction (ever stand in front of a huge aquarium?). If anything, the larger mask confuses me. You think you are seeing more while your field of vision is still just as narrow as with a smaller mask, and much narrower than it appears. That is why, after losing my Hollis M-1 (a great mask), I am now using a tiny Technisub Micromask.

Smaller volume masks are better. Large masks will bounce on your head and are more prone to getting ripped off by the current (or your buddy's fin!).

Black vs. clear silicone: black. With the clear skirt the light spillage from the sides makes it harder to see the things in front you. This is especially noticeable in low light situations. Also, some people say black silicone is softer — I don't know personally.

Framed vs. frameless: I have spent too much time thinking about that, don't. For you it won't matter much. The difference is chiefly that a frameless mask folds flat for storage in a pocket, as a spare (you probably won't need one just yet).

Purge valves: stay away, they are useless.

Bottom line: fit is everything. If the only mask that fits is the most expensive one in the store, get it.

Computer: $200-$1000+

While you might not need one at first (the first few dives will likely be shallow and well within the no decompression limits), this is the thing that tracks your theoretical nitrogen loading/oxygen toxicity and tells you much play time you have left until your theoretical NDL. They also log your dives electronically, nice.

Why do you need a dive computer? If you haven't heard the soda bottle analogy yet, your tissues dissolve gas (mostly nitrogen, oxygen is metabolized) proportional to ambient pressure. As the pressure changes slowly this on-/off-gassing happens gradually. But, should the pressure drop too low too fast, the dissolved gas can form expanding bubbles (otherwise it leaves the body dissolved via the bloodstream and through the lungs). This is Decompression Sickness (DCS). It can cause pain, injury and death; and must be prevented.

Exactly how body tissues behave in changing pressures is the subject of Decompression theory. When divers talk about dive profiles they are describing pressure exposure. The profiles are used to calculate DCS risks. A dive computer constantly monitors the diver's profile and is able to calculate more accurate and longer safe bottom times.

Your computer stays with you and remembers what you did yesterday, and the day before, and the day before. This really starts to matter on consecutive dives, especially deep ones. Your recent dive history and current residual load will be unique and different from everyone in your group — that is why you should not share or borrow them. Do 2 afternoon dives and the next morning blowing past your NDL is not so hard, all on a typical rec dive.

Dive computers come in multiple forms and feature combinations. The most basic one will suffice, a used one can be had for even less. Do your research and start learning about decompression theory and various decompression models available in different units.

Features to consider:

  • Audible/visible warnings: nice features for the entry level diver, but do not rely on them, they are very easy to miss. You will have to learn to monitor your instruments yourself.
  • Wrist or console: wrist.
  • Wireless air pressure monitoring: if you want, I don't. This adds air pressure to your computer's screen (but you already have it on your SPG) and your computer's dive log (can be nice). Some computers can also sound alarms if your pressure gets low or consumption raises too much.
  • Upgrade path: there is a good chance your OW certification is just the beginning. At some point you might need a feature your computer does not have, meaning an upgrade.
  • Nitrox: definitely yes. You will very likely certify for this next.
  • Gas switching: probably not yet. By that time you will probably want a different computer.
  • Gauge/bottom timer mode: yes.
  • Decompression algorithm: yes, you do need to learn this stuff. Some models are more conservative than others — that means shorter bottom times but also lower DCS risk. DCS risk is not to be taken lightly and there are many factors at play which you won't even know right away (and some are still undiscovered!).

I currently dive a Suunto D4i. It's a fantastic machine but if I had to choose again I would have gone with something simpler, cheaper and user serviceable.

Bottom line: nothing wrong with fancier units, but the cheaper/uglier ones work just as well. Also, when you are ready for the next big upgrade you'll probably need a backup. This is where that gauge mode comes in handy.

Exposure: varies

Exposure protection keeps you warm and scratch-free. You may develop a different preference, but I want as much of myself covered as possible.

  • Wetsuit: can't go wrong with 3-5mm for the tropics ($100-$150), thicker for colder waters. Do not be deluded by the warm tropical water: if it's colder than your body it will draw heat away. Full length suit is good — the stuff on the bottom scrapes, cuts and stings. Some people do a thinner suit and a shorty or a hooded vest on top. Much of my early diving has been in +29C water and not once have I regretted having a 5mm suit.

    My first wetsuit was a Cressi Castoro, it was the cheapest thing in the store. It worked well, but after getting in and out 50 times I regretted not spending a little more on a suit with wrist and ankle zippers.

  • Drysuit is a whole other story ($1000-$4000), beyond the scope of this post. If you don't dive cold (below +23C/74F) and/or don't chill easily, you don't need one.

  • Booties: for your open-heel fins to go on to.

  • Hood and gloves. A surprising amount of heat is lost through the head, so a hood makes a world of difference. Also helps soften the bump that boat sometimes gives you. And gloves, did I mention things sting?

Trying it on: it really has to fit tight, possibly tighter than you feel comfortable. The suit especially. Neoprene does loosen up considerably in the water and under pressure.

Regulator: $300-$800+

Regulators deliver breathing gas at ambient pressure. Rental regulators are usually dependable, but they might breathe harder than you'd like. The mouthpiece might not be as comfortable as the one you might pick out out yourself.

If buying, you will need at least one first stage, one or two second stages and a Submersible Pressure Gauge. Lots of variety on the market, do your research. I used to own an Aqualung Legend LX, it was very nice and very expensive. Now I have a pair of HOGs that are even nicer, for 1/3 of the cost. I did my research.

In sport diving the alternate second stage is called an octopus. Many package deals include an octopus of lower quality than the primary first stage, the rationale being that the octopus is something you rarely use, so why spend more. I believe both second stages should be identical. In a real out-of-air emergency switching to a harder breathing regulator will mean additional stress.

Regulators must be serviced annually ($30-$40).

Gas analyzers: $250-$400 each

Yes, they are expensive and yes, you want them. Breathing wrong or bad gas will easily kill you. Chances of getting bad gas are slim, but mistakes and malfunctions do happen. Dying from something entirely preventable because you didn't check what's in your tank is a stupid way to go.

For diving Nitrox you are required to test the oxygen content of your own tanks, for this you need an O2 analyzer. Often the dive shop will have one to lend you, but you cannot always rely on that. Even if you don't dive Nitrox, every gas has a maximum operating depth exceeding which can lead to drowning. You want to always ensure you weren't handed a wrong bottle by accident.

A carbon monoxide analyzer will alert of you of dangerously high CO levels in your gas. There are several ways for CO to end up in your gas, and there is always a chance. Breathing CO causes headaches, confusion and blackouts leading to drowning.

Analyzers will require replacement sensors eventually. Mine are made by Analox.

Fins: $100

Many innovative designs to stay away from. Some classic designs to consider:

  • Mares Avanti Quattro: great all around fin. My first fins with 70 dives on them (I have the Superchannel version). With heavy use the rubber insets can start tearing.
  • DiveRite EXP and XP: stiff and powerful. My next fin, for handling stronger currents (the Mares are a little too floppy in comparison).
  • OMS Slipstream, Turtle fins, Jet fins, etc.: highly regarded fins, don't see these often in recreational diving.

Spring straps are an amazing investment.

BC: $200-$1000+

Buoyancy Compensator Devices (BC or BCD) are one of the tougher choices, and probably the one thing you can put off the longest — until you really know what you want. They are not cheap, the choices are many and their differences are not easy to evaluate.

The common styles are:

  • Jacket BCs are most common, that's what you usually get in rentals. They are the easiest to learn and use. They are bulky, hard to trim and they squeeze you from the sides when inflated. They are designed keep your head above the water on the surface, with zero effort on your part (good if you're unconscious).

  • Back inflate BCs are a crossover between jackets and a proper wing harness. Compared to jackets they offer more freedom in the water, they don't get in the way and they are much easier to swim in. They are easy to trim perfectly, when your center of buoyancy matches your center of gravity and nothing is flipping you into any particular orientation — you are truly weightless. On the surface they will tend to flip you face down, because the lift is in your back. This is easily countered by laying on your back, but it's a task you have to perform.

  • Harness, backplate and wing is a modular and highly configurable system consisting of a rigid backplate, nylon webbing and a wing which provides lift. Used mostly in technical diving. The system is highly adaptable and can support single tanks, doubles, stages, etc.. The components are individually replaceable and the system will last forever. You can put one together on the cheap. It can be very light and compact for travel.

  • Sidemount BC is a soft harness with a wing designed for sidemounting tanks. Some sidemount BCs are hybrid, capable of back-mounting single and double tanks.

If you never ever venture past OW diving, the first two options will work and the choice is determined by your comfort (I would stay away from jackets just because back-inflates dive much nicer). A BP/W system will still dive nicer, last longer and grow with you, but it does take some effort and skill to adapt to and nobody teaches them in OW training. Unless, of course, you decide to sidemount, then a sidemount technical harness is your better option.

After diving rental jackets my first BC was an aluminum backplate with Oxycheq wing with 30 lbs of lift, it's fantastic. Going to sidemount required an upgrade — Hollis SMS100, modified by Edd Sorenson of Cave Adventurers.


There is always more gear to have, but this list should cover the most essential and costly things. If you look for deals and buy used, you could be diving in your own gear for under 1K.

There is nothing wrong with sexy gear, just remember to not make that the deciding factor. DIY has always been the way of diving, and there is still much of that today. The most functional dive gear is usually unsexy and unglamorous: boltsnaps, zipties and scratched up fins.