Equipment, Recreational, Technical

In terms of the number of tanks you might carry, sure, doubles and sidemount appear similar.

However, sidemount and backmount doubles differ in one crucial area – the isolator manifold!

Why would you ever need more than one tank?

Now, let’s start from first principles.

What are the reasons for carrying more than one tank in the first place?

Some might argue that gas volume is the primary reason. However, redundancy is a more important and valid reason.

On a normal single tank (especially one with yoke valves), O-ring failures (tank neck, tank knob and yoke valve) are relatively common.

The next most common thing to fail in scuba gear would be the regulator setup, as there are many O-rings and hoses that can rupture and cause a catastrophic gas loss.

Single points of failure are dangerous when diving

With single tanks, a gas loss from an o-ring failure in the yoke valve or the regulator will result in an immediate out of gas situation.

In order to deal with this situation in deeper or more extreme dives where a direct ascent to the surface is impossible (a decompression obligation or an overhead environment), we would require a secondary gas supply so that we don’t go into an out-of-gas situation on a single failure.

Double your tanks, double your hoses, double your failures

A quick solution to this was to just carry another tank.

In the early days, people would just carry 2 tanks, with a full set of regulators on each, in essence, doubling up on the gas supply, but also doubling up on the failure points in the system.

However, what if halfway through the dive, you had one full tank and one nearly empty tank and a failure happened on the full tank?

This would leave you with a nearly empty tank, with no backup.

Clearly, this wasn’t an ideal solution.

What about connecting these tanks?

The best solution to this was connecting the tanks with an isolator manifold.

Two 300 bar scuba cylinders connected by an isolating manifold

Using this setup, we could have a single regulator failure but still have access to all of our gas supply.

The isolator is an important part of this setup because it would allow a diver to isolate an unfixable leak to just one tank.

This prevents an immediate out-of-gas situation because you would still have one regulator working.

Some might argue that the manifold itself presents a failure point, but in all my years of diving doubles, I’ve not seen a bad manifold failure happen in the water.

In most cases, if the manifold were to fail, it would fail during the filling or setup process.

So what about sidemount?

So where does this leave us in the discussion on sidemount vs backmount?

Sidemount was created because of a specific need to access ever smaller and tighter holes in cave exploration. Places where backmounted tanks would have limited access.

Diving sidemount in open water negates the benefits of carrying 2 tanks by reverting to the original idea of the independent doubles. This of course comes with the associated negatives mentioned above.

There are some benefits to diving sidemount that proponents argue on; that it’s easier to put on; easier logistically; better for the back, etc.

See my other article about diving sidemount in open water to get an idea of why I think that’s not so relevant.

However, when we come back to the main reasons of why we have multiple tanks, sidemount just doesn’t match backmount when it comes to redundancy.