The technopolymer second stage is lightweight, compact and pneumatically balanced. The latter feature makes inhalation a little easier than with unbalanced mechanisms.
When you begin your inhalation, you first have to crack open the second stage, which means pulling open the valve that closes off the air supply from the tank with lung power. Without this valve, your regulator would constantly freeflow. A spring is used to keep the valve closed, its default position.
The valve is like a spring-loaded door, and the stronger the spring, the harder you have to pull to open it. An unbalanced second stage needs to use a stronger spring than a balanced version, so cracking effort can be a little higher.
Sherwood uses balanced second stages throughout its range, including the budget Brut Pro that I reviewed a year ago.
Once you crack the valve and get the air flowing, the venturi takes over and does most of the work of breathing for you.
The SR2 second stage has a single control that allows the user to increase breathing effort by simultaneously stiffening the cracking effort and reducing venturi performance.
The rationale for detuning a high-performance regulator to make it breathe harder is mostly to prevent freeflows when facing into current.
I’ve done a lot of that, ridden a few DPVs and never needed to dial down a reg. Mike Ward and I both advocate setting any regulator for the easiest possible breathing performance.
I ran a pressure-gauge off one hp port and a transmitter off the other. I fitted the primary and safe second stages and my BC hoses to the swivel turret. I didn’t use the end outlet.
The all-metal DIN handwheel is poorly thought through. It’s very hard to undo with wet hands to take it off your cylinder. You can get a slightly better grip if you remove the dust-cap, but it really needs a non-slip finish.
Second-stage rear view.
The SR2 second stage is near-neutral under water and, even on long dives, I was largely unaware of it. There was no restriction when I turned my head.
The low-profile second stage is good for photographers needing to get their eye close to their camera viewfinder.
The soft purge is easily used with gloves, though a simple exhalation is just as effective. Put the second stage in upside-down by mistake in a sharing situation and you still get all the water out with a gentle exhalation.
The small exhaust T isn’t likely to dislodge your mask either, and does a great job of diverting bubbles from your field of view.
Ease of Breathing
The SR2 is an easy breathe – virtually a given these days, even with budget regs – but how would it perform when simulating emergency sharing?
CE standards for diving regulators set minimum performance criteria for ease of breathing. The original EN250 rating specifies the mass of air the regulator must supply to a single diver at a depth of 50m, and how hard that diver must inhale and exhale to breathe from the reg – the work of breathing.
This is tested using a breathing machine that takes 25 breaths per minute, with the volume of each breath set at 2.5 litres. At 50m, the mass of gas that must be shifted in and out of the artificial lung every 60 seconds is 375 litres.
Experts I’ve asked regard this as a moderately high demand for air, so a hard-working diver might well exceed it.
EN250 should not therefore be seen as a cast-iron guarantee that a regulator carrying its approval can’t be outbreathed.
Nor does EN250 certification tell you by how much a regulator might have surpassed the work-of-breathing test. All regulators must pass EN250 to be sold legally.
For decades, octopus rigs have been standard issue for recreational diving, but until EN250A was introduced, there was no real guide to how a regulator would perform with two divers breathing heavily from a single first stage feeding two second stages.
EN250A tests work of breathing with the depth limit reduced to 30m. The regulator must supply a mass of air of 500 litres per minute, at which breathing rate a full 200 bar 12-litre tank gives you less than five minutes to sort your life out.
That’s why I conduct these tests with the test reg on a stage cylinder. I take safety on these tests seriously.
Dennis Santos, former Diving Officer of the Gibraltar SAC and a veteran of 50 years’ sport, light commercial and Navy diving, and my own diving mentor, was my buddy on the SR2 test.
We descended to 32m and settled in to fin as hard as we could against some rocks, which quickly gets your breathing rate up. It’s highly subjective, because I’m trying to detect breathing resistance from the test reg as I start working, then figure out whether, when my buddy comes onto the octopus and gets his breathing rate to soar, I can detect a decrease in ease of breathing.
This test typically takes around two minutes to complete. It’s exhausting, because your own physiology is working against you, causing CO2 to build up and breathing to become laboured.
However, while I can’t determine work of breathing as a machine does, I can say that the reg was passing at least 500 litres per minute at maximum load.
I couldn’t detect any increase in breathing resistance, although Dennis felt that, momentarily, there was a stutter in the air-flow to the octopus. I have no explanation for this.