The Zeo, as befitting a high-end second stage, is pneumatically balanced. A second stage supplies air only when you want it by using a spring to keep a valve closed until you inhale.
The problem with an unbalanced second stage is that as you descend and water pressure increases, so does the pressure of gas leaving the first stage. It tries to push the second-stage valve open, so the spring must be strong enough to hold it closed, even on deep dives, until you inhale.
To overcome the spring resistance, you must use lung power – basically, sucking. Anything that increases breathing resistance is unhelpful, and becomes more problematic the greater your demand for air.
At the very least, you’ll use more air simply because of the energy expended just to inhale.
A pneumatically balanced second stage uses a weaker spring than an unbalanced model. It harnesses air pressure by allowing air to surround the spring. This is used to supplement the spring’s strength as needed. On deeper dives, air pressure increases to balance the air pressure coming from the first stage.
A lighter spring can be used, just strong enough to hold back the incoming air at low surface pressure, so the second-stage valve should be easy to open with the least amount of effort on your part, regardless of depth.
The lung effort you expend to open the valve and get the air to flow is called cracking effort. The Zeo has user-adjustable cracking effort. It can be dialled in at any point during a dive by turning a large scalloped knob.
Once you have cracked the valve and the air has started to flow, the venturi takes over most of the inhalation effort for you.
The second stage ball joint.
Most of the regulators I’ve owned have adjustments, but I’m not a fan of using them. Increasing cracking effort might be something you need to do if strung out on a reef hook or shotline and facing into a roaring current, but I’ve never experienced a freeflow in those circumstances myself.
Swimming hard against high current, I want the easiest-breathing, highest-performance reg I can get my hands on, and a better set of lungs.
A simple lever lets you turn off the venturi to prevent freeflows when the Zeo is not in your mouth. Venturi switches are also known as dive/pre-dive controls for a reason.
Under water, they should never be in pre-dive mode. A flexi hose links the first to the second stage, where it attaches via a ball joint.
The Zeo second stage is very light and there’s no noticeable weight on your jaw, even on dives exceeding an hour.
The mouthpiece bite-tabs are very soft which, I suspect, also adds to the feeling of comfort. The flexi-hose is unnoticeable when you turn your head.
Whether or not the ball-joint really helps, I wasn’t sure. I couldn’t feel it moving when I rested my hand on it and looked left and right and up and down. The diminutive design allows you to get your eye close to a viewfinder, and the exhaust T does an excellent job of diverting bubbles from your field of view, both benefits for underwater photographers.
Put this reg in upside-down by accident and it’s unlikely it will dislodge your mask. Clearing it inverted by purging or exhaling is easy.
The Zeo cracking-effort adjustment knob.
I’m no fan of breathing-adjustment controls, but most high-end regs have them. It’s easier to compete by fitting them, even if they go unused.
The cracking-effort control is easy to grip.
A nice feature prevents you overtightening it. The venturi lever is easy to flick open or closed. Leave the Zeo wide open and it is a high performer with a very smooth breathing action.
My deepwater, high-demand reg-sharing exercise aims to simulate a situation in which two divers are breathing hard from a single first stage using both a primary and secondary or octopus simultaneously.
No octopus was supplied, so I fitted another manufacturer’s top-of-the-line pneumatically balanced second stage, adjusted for the easiest breathing possible. With experienced instructor Nick Balban, I descended to 32m.
To get our breathing rates up, we finned as hard as we could against our shotline anchor. Once I felt I’d reached my maximum breathing cycle, Nick came onto the octopus, and as his work-rate increased, I tried to detect any increase in breathing resistance from the Zeo. I could not.
What we can’t subjectively assess is work of breathing – this is something measured on a breathing simulator as part of the EN certification process. But I feel our real-world test is reassuring and worth the effort.