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.
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.
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.
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.
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.
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 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.
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.
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.
As said before, with two separate tanks you have options when dealing with gas emergencies.
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.
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:
- 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.
- Remember which hose/tank you're breathing.
- If you forget, sweep by checking the shoulder first. The long hose is ether there or in your mouth.
- 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.