You can typically spot technical divers at a dive site or on a boat because they will be the ones with more equipment than anyone else. Further, technical diving equipment configurations differ substantially from recreational diving. Here we’ll look at the differences between twinsets/doubles and sidemount diving.
Twinset diving
Twinsets or doubles have been classically associated with technical diving and most technical divers trained with them. Many instructors and training agencies use what we call a ‘Hogarthian set up.’
Named after cave diver William Hogarth, it emulates the DIR — Doing It Right — philosophy of technical diving. That means it’s minimalist while also ensuring that divers have backed up every piece of life-saving equipment.
As you’d expect for a twinset configuration, there are two tanks, often the same size as recreational divers’ tanks. A manifold — a type of bridge valve that allows divers to access gas from both tanks through either side’s regulator — connects the tank valves. Each valve has a first stage attached. On the right, this first stage connects to a 7-foot (2.1 m) hose with a second stage and an LPI inflator for the wing. On the left, the first stage connects to a second stage on a standard-length regulator hose. The diver uses an attached necklace to place the regulator around his neck. There’s also an SPG hose and any inflator hoses for drysuits.
Why the long hoses?
Why is one of the hoses over 6 feet (2 m)? This goes back to overhead-environment diving, where divers may have to exit behind each other in an out-of-gas situation. The diver with gas would donate the long hose to the victim. He’d then switch to the back-up regulator on the necklace for himself. In this way, the divers can exit single file by swimming behind each other.
A wing and harness with backplate holds everything together. Wings come in different shapes and sizes, but most importantly they must provide enough lift for the amount and type of tanks the diver is using. Divers can choose between having a single bladder in their wing or a second one to use in case the first one fails. Classic harnesses consist of a single piece of weight-belt webbing plus a crotch strap. They also offer D-rings to provide attachment points for regulators and other gear.
The goal is to achieve stability, allowing the diver to control the twinset and potential stage tanks underwater, effectively turning the twinset into a platform from which the diver is suspended.
Sidemount diving
Sidemount, on the other hand, is more about flexibility, allowing divers to attach and remove tanks underwater when necessary. This configuration has become hugely popular within technical diving — some even call it a fad. We can trace the origins back to U.K. sump diving, where divers maneuvered through restrictions too small for twinsets, sometimes even small enough that they had to remove their tanks and push them through the restriction ahead of their body. Sidemount is still closely associated with cave diving, although depending on the size of the cave, twinsets may be just as suitable.
For sidemount configuration, divers mount tanks at their side rather than on their back, starting at the armpit and ending below the hips. Tanks connect to the harness with a cam band or jubilee clip attached to the lower part of the tank and equipped with a bolt snap. Tank valves remain in place on the diver’s body with bungee cord, typical for aluminum tanks, or a combination of bungee and clips, more typical for steel tanks.
Twinset and sidemount differences
One of the main differences between sidemount and twinset diving is the degree of standardization. Twinset configurations are highly standardized whereas sidemount configurations are not necessarily. This has to do with sidemount history as a means to an end initially, which gradually developed into a full-fledged diving configuration. Divers in sidemount courses will learn a variety of setups but will have to spend time to perfect their own.
As a consequence of sidemount flexibility, different divers might also set up regulators differently. Most sidemount divers prefer regulators with swiveling first stages to allow flexible hose movement. They usually prefer a fifth low-pressure port as well, allowing them to mount hoses at a 90-degree angle. Sidemount divers can also configure regulators by largely following the twinset configuration described above. This means having a first stage, long hose and second stage with an SPG on the right side and a first stage, short hose, second stage with necklace, inflator hose and SPG on the left side.
Harnesses, hoses and bladders
Many sidemount harnesses invert the corrugated inflator hose. This means that it runs from under the diver’s left arm across the upper body, rather than over the left shoulder. This is to keep as low a profile as possible in order to navigate restrictions. This is where the fifth low-pressure port comes in handy, allowing divers to install a short inflator hose across the body rather than over the shoulder.
Speaking of hoses, divers usually mount sidemount SPGs on 6-inch (15 cm) high-pressure hoses. This is because longer hoses are not practical and can become an entanglement hazard. Inflator hose lengths vary according to diver size and configuration.
Sidemount harnesses and bladders come in many shapes and sizes. Most are adjustable to fit different-sized divers, but some come in size ranges. Which one works best will depend on your size and the type of sidemount diving planned. Just like with twinset wings, it’s important to pick one with sufficient lift for the tanks. For example, those who want to do recreational sidemount dives with no more than two tanks will find a small, super-streamlined harness adequate. Technical dives with more tanks will require more lift capacity.
Most tech divers start their journey in either of these open-circuit configurations. Rebreathers add another option — take a look at this post for the basics. Paramount for safely diving in either configuration is instruction by a qualified teacher. Sidemount especially may not look that different from other configurations, but the devil truly is in the details. If you get it wrong, you’ll compromise comfort, streamlining and trim — some of the main reasons to dive sidemount. Technical-diving configurations may seem intimidating, but with the proper knowledge, they open up a whole new world underwater.
