Comparison
Sidemount vs Backmount Twinset
Two ways to carry redundant gas, and what actually drives the choice.
Once a diver needs redundant gas — for decompression diving, overhead environments or simply for comfort — there are two mainstream configurations: a backmounted twinset with an isolation manifold, or cylinders mounted at the sides.
Both work. The choice is driven less by ideology than by what you dive, what your body can carry, and in Australia specifically, whether you intend to dive the Mount Gambier cave systems.
Backmount twinset
Two cylinders manifolded together on the back, with an isolator valve allowing either to be shut down.
Strengths
- Both cylinders breathe as one, so gas management is simpler
- Familiar to anyone used to a single backmounted cylinder
- Streamlined profile in open water
- Well established with abundant instructor availability
Limits
- Heavy and awkward to carry to the water as a single unit
- Cannot be removed in the water
- Impractical in restricted cave passages
- Valve drills require flexibility some divers do not have
Best for: Open-water technical diving, wreck diving without significant restrictions, and divers who find the simpler gas management preferable.
Sidemount
Independent cylinders mounted along the diver's sides, rigged to a dedicated harness.
Strengths
- Cylinders carried separately to the water — much easier on the back
- Can be removed and passed through restrictions
- A failure costs half your gas rather than all of it
- Valves are in front of you and easy to reach
- Lower, more streamlined profile in restricted spaces
Limits
- Requires regular regulator switching to keep cylinders balanced
- More rigging to get right; configuration varies between systems
- Fewer instructors than backmount in some regions
- Trim takes longer to dial in initially
Best for: Cave diving, restricted wrecks, and divers with back or shoulder issues who cannot carry a twinset.
Mount Gambier is the decisive factor for many Australian technical divers. Systems like Tank Cave include restrictions that make a backmounted twinset impractical, and sidemount is effectively the standard configuration for the region's more demanding caves.
If cave diving in South Australia is anywhere in your plans, learning sidemount early saves relearning later.
A twinset is heavy and is carried as one unit. Sidemount cylinders are carried separately and clipped on in the water.
For divers with back or shoulder problems — and for anyone facing a long walk down a beach or a pier — that difference is frequently the deciding factor, and it has nothing to do with the diving itself.
With independent cylinders, a sidemount diver must switch regulators regularly to keep the cylinders balanced, both for gas reserves and for trim. It becomes automatic quickly but has to be learned deliberately.
A backmount twinset removes that entirely — both cylinders feed the same system, so there is nothing to manage until something fails.
The honest answer
For open-water technical diving in Victoria or Sydney, a backmount twinset is the straightforward choice and has the widest instructor availability.
For cave diving, restricted wrecks, or if carrying a twinset is physically difficult, sidemount is the better answer. Many Australian technical divers end up competent in both, and the order they learn them in is usually determined by what they want to dive first.
Frequently asked questions
Is sidemount only for cave divers?
No. It originated in caves but is now widely used in open water and wreck diving, and many divers prefer it purely for comfort and redundancy.
Can I use sidemount recreationally?
Yes, and many divers do. The configuration provides redundancy that a single cylinder does not, and some find it more comfortable.
Which should I learn first?
Whichever matches the diving you intend to do. If Mount Gambier is on the list, sidemount. If you are diving Port Phillip wrecks, a twinset is the conventional route.