SIDE-EXHAUST REGULATORS were very popular back in the day. The Poseidon range of Cyklons, Jetstreams and X-Streams were favourites with technical- and cave-divers around the world.
They offer two main advantages: they can be used in either left- or right-handed configurations without you needing to use a set of spanners, and they push the exhaled gas bubbles away from your field of vision.
With a recent trend in recreational as well as technical diving being the sidemount system, this style of regulator is enjoying something of a comeback. From across the Atlantic the dive-gear manufacturer Hollis has introduced its own version, the 500SE. I took one with me to the Caribbean to put it through its paces.

The Design
The 500SE’s pneumatically balanced servo-valve is in fact a two-stage valve. Just a small sip opens the servo mechanism or control valve lever.
Gas-flow from this servo creates a vacuum that immediately opens the main valve diaphragm, allowing the gas to flow through the regulator to the mouthpiece.
The first stage is an overbalanced diaphragm design, which provides progressively greater intermediate pressures as the depth and gas density increases. It features a fixed barrel with four low- and two high-pressure ports around the circumference. The port configuration is fixed but has been designed to put the hoses exactly where they’re needed.
The first stage is environmentally sealed to prevent internal corrosion, contamination and freezing. It meets the rigorous CE standard for coldwater performance, and comes only with
a 300bar DIN connection.
The second stage is built from high-grade plastics and polymers and features the balanced servo valve system. You purge this regulator by pressing a small honeycombed pad at the rear of the exhaust port. Exhaled gas bubbles are sent towards the rear and away from your face.
The second stage can be disassembled under water without the need for tools, using a large-diameter handwheel.
The two stages are connected via a lightweight and flexible Maxflex hose.
An orthodontic mouthpiece and a bolt-snap tie point complete the set-up. The whole thing is compatible with 40% nitrox out of the box.

In the Water
I used to dive exclusively with side-exhaust regulators at one time, and I’d forgotten just how good they are.
The second stage gave a loud hiss when I initially turned the tank valve on, something that happens with most servo-type valves. It took me back to my regular UK Channel-diving days on trimix and servo-style Swedish valves.
I’ve been diving with some seriously good regulators recently and I have to admit that
my definition of a smooth breathe has changed immensely.
Under water the gas delivered from the 500SE seemed a little raw in comparison, as the servo tended to over-deliver after each breath.
This was especially true in the shallows, when I had my head tilted and my face pressed into my camera viewfinder. A change in my head position quickly sorted this out.
I found the second stage light and unobtrusive, although initially it looked somewhat bulky. In use I started to forget that
it was even there, the mouthpiece being of a design that fits my mouth perfectly.
I didn’t have to bite down, so didn’t suffer any jaw fatigue during what were mostly 90-minute dives. The Maxflex hose helped in the comfort department too.
The purge was stiff to operate. The semi-flexible plastics used for the second-stage cover had also been used for the purge cover, and although this area has honeycombed cut-outs it still takes a firm push to move the servo lever.
I found that I had to grip the second stage with my thumb on the button and my index finger at the front and squeeze quite hard to make it work.
Once I had sorted out this technique I found that I could control the gas flow better.
And I needed to, because the purge could be quite brutal if I overdid it, and would lash my oral cavity with hurricane force if I pressed too hard. Once familiarised, the purge worked well, delivering gas progressively and clearing any unwanted water with ease.
Hose-routeing was neat and tidy. The port layout put my second stage, gauge, octo and low-pressure inflator just where I wanted them.

This 500SE may have looked old-school but it felt new-age, with its modern materials and construction. I found it very comfortable in use, and once I’d got my head round the different way that a servo-assisted valve delivers gas and changed my technique to embrace the purge and slight over-delivery, I was more than happy to be diving with it.
The big plus was that my exhaust bubbles were never seen in front of my face, and for a photographer that’s a big issue. For side-mount and twin-set fans this would be a great addition to your armoury.

PRICE £450
FIRST STAGE Overbalanced diaphragm
SECOND STAGE Pneumatically balanced servo
PORTS 4lp, 2hp
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