First Stage
This new regulator second stage is available paired with either the Mk11, Mk17 or Mk25 first stages. I tried it with the Mk17 diaphragm-type unit and compared it to the performance of the Mk11T with a S555 second stage that I had alongside it.
I found that there was very little to choose between them, although the second (titanium) regulator was considerably more expensive. The Mk17 is especially well-designed for use in cold fresh water, although I admit I was using it in the warm Red Sea.

Second Stage
The C300 is intended to be a mid-priced regulator, to which end it has an unbalanced second stage designed such that the user would never know that.
Servicing technicians will appreciate the simplicity of its design, but how do its designers get the high performance from it?
Firstly, they put the spring of the valve mechanism at the other end of a long shaft to the poppet valve, so that air passing is not disrupted by having to flow over and around it. Why did nobody think of doing this before
Instead of a vane placed in the flow to break up the venturi effect (so sought-after at depth but conspiring to give exponential free flows at the surface), a lever to disrupt the path of the gas rotates the valve mechanism within the housing.
There is also a breathing resistance adjustment (BRA), operated by a knob. This tightens a spring to increase the amount of effort need to open the valve and inhale (should anyone want to add effort to their breathing!). There is also a C200 second stage without this feature available, intended for use as part of an octopus rig.
Nowadays, thanks to modern designs made possible by the ANSTI test machine, the work of inhalation has been reduced to minimal levels in most regulators. It is the work of exhalation that has become the major part of the total work of breathing (WOB).
Alberto Belloni, Scubapro’s chief regulator designer, has managed to reduce this by curving the top part of the second stage to give the exhaled air as clean a route as possible to the exhalation mushroom valve. This curved shape is obvious if you look at it.

The purge control is a squidgy bit of the lower half of the front of the regulator second stage held in the mouth. I expected it to be a bit awkward to find, but that was not the case.
The long reach of the second-stage valve-lever meant that the purge control was easy to manage, from a gentle supply to a full gush.

It augers well for a good comfort rating if you can jump into the water and forget that you’re meant to be testing a regulator – and so it was with the C300.
With little disruption from exhaled bubbles, I was able to get on with photographing passing dolphins and co-operative turtles.

Apeks XTX40, £300
Cressi Ellipse Titanium, £262
Oceanic Delta DX4, £255

PRICE £339
FIRST-STAGE (Mk17) Diaphragm
PORTS 5mp, 2hp
SECOND-STAGE Venturi plus/minus, BRA
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