REGULATOR

Scubapro Mk25 EVO/
S620 Ti

Appeared in DIVER July 2020

Scubapro was formed in the USA in 1963. Its founders were drawn from entrepreneurs who had, in the ’50s, done well working for other scuba equipment manufacturers and now wished to break out on their own.

One was an Italian count, Gustav Dalla Valle. The other, Dick Bonin, had deployed with the USN Underwater Demolition Team, forerunner of the Navy SEALS.

Early on Scubapro recruited top engineers such as Dick Anderson, equipment technician on Disney’s 20,000 Leagues Under the Sea, to develop its own line of regulators. They tested these against their competitors’ models by making air dives to 75m off the California coast, in the brazen suck-it-and-see spirit of the times.

Subsequently, many key innovations that have greatly improved regulator ease of breathing have come out of the Scubapro labs.

Two of the most important are found in the Scubapro Mk25 EVO / S620Ti regulator. This combination sits towards the top of Scubapro’s regulator line-up. It’s not only functional but designed to appeal to those who like to feel a flurry of pride of ownership on the dive-deck.

In many ways it illustrates the continued development of design principles Scubapro established decades ago. Its original balanced-piston first stage, for example, was introduced in the 1960s, the adjustable cracking-effort second stage in the ’70s and pneumatic balancing in the ’80s.

Scubapro can fairly claim the resulting Mk V piston first stage, adjustable second stage and pneumatically balanced G250 second stage as classic regulators.

So, more than a quarter-century after the G250 topped the US Navy Experimental Diving Unit’s regulator test charts, is the Mk25 EVO first stage and S620Ti second stage pairing another Scubapro icon in the making?

First Stage

Scubapro contends that a balanced-piston first stage can deliver higher volumes of gas more rapidly than balanced-diaphragm models, but says this difference is normally noticeable only on very deep dives.

Many technical divers favour diaphragm-driven regulators, so this point is probably moot. Besides, Scubapro offers balanced-diaphragm first stages for those who prefer them. It takes a mix-and-match approach to first and second stages, so the combo tested here is only one of those on offer.

The Mk25 EVO has four mp ports arranged around a swivel turret, while a fifth leads straight off the piston.

The Mk25 EVO has four mp ports arranged around a swivel turret, while a fifth leads straight off the piston.

The first-stage main body is made from chromed brass. There are two hp and five mp ports, four of which are arranged around a swivel.

The fifth is mounted on the end of the first stage. It should provide the easiest inhalation under high demand because the air flows straight out of the piston opening and into the hose, rather than turning a corner, which disrupts the air flow.

However, the other four ports equal each other in performance, so you can use any of them for your primary and safe second.

Scubapro uses standard 3/8th mp outlets. Its regulators consistently score highly on breathing-machine tests because its hoses have wider internal bores than some other 3/8th whips, which isn’t obvious from the outside. This helps speed flow-rate.

The hose is Kevlar-lined inside for durability. It’s not a flexi type, so it won’t coil up as well.

A piston first stage is simpler than a diaphragm model, which requires two springs to the piston’s one, for example.

The piston is basically a hollow tube linking two air chambers. The first chamber is filled with high-pressure air from your tank. At the start of your dive, the pressure inside it could be as high as 300 bar.

The hp end of the piston sits on a seat that seals the piston’s opening, much like putting your fingertip over the top of a straw. The other end sits in the second chamber, which contains mp air. The air in this chamber is only around 9 bar above the water pressure surrounding you as you descend and ascend.

Between the two chambers is a free-flooding space through which the piston runs. It contains a spring that has to try to force the piston off its seat in the hp chamber so that air can flow through it to the mp chamber.

This spring is set to an opening force of about 9 bar, which maintains the correct gas pressure in the mp chamber. It exerts a constant opening force so it can’t account for changes in pressure as we change depth and, even in shallow water, would make inhalation very difficult. The imbalance would be similar to that caused if you tried to breathe through a very long snorkel.

Water enters the space around the spring. The end of the piston in the hp chamber has a very narrow opening, but where it connects to the mp chamber it’s surrounded by a large disc. Water pressure acting on one side of this disc works with the spring to boost its opening force in line with increases in depth.

On the other side of the disc, inside the mp chamber, the air pressure acting on the disc’s dry side is sufficient to overcome the opening force exerted by the spring and water. It keeps the piston closed and the air supply to the second stage shut off until we inhale.

Inhaling causes pressure inside the mp chamber to drop and the joint opening forces of spring and water lift the piston off the seat, allowing air to flow from your tank through the MK25 EVO and along the hose to the second stage where we can breathe it.

When we stop inhaling, pressure backs up along the hose and within the mp chamber and builds up enough to force the piston back against its seat, shutting off the air until we take our next breath.

In an unbalanced-piston design, incoming air pushes directly against the piston in the hp chamber. While the pressure in the mp chamber does not change much during the dive, the incoming air in the hp chamber changes considerably as tank pressure falls.

With this fall in pressure, the forces trying to open the valve become weaker, while those trying to close it remain much the same.

This is why inhalation becomes harder at low tank pressures. In the Mk25 EVO’s balanced design air surrounds the piston, but doesn’t act directly on it, so changes in tank pressure have almost no effect on breathing effort even when your cylinder is near empty.

Moreover, Scubapro claims that the Mk25 can pass 8500 litres per minute – exceeding the capacity of four 10-litre / 200-bar cylinders. These are the reasons balanced-piston first stages are associated with high performance.

In piston designs the piston and spring are surrounded by water, which raises two possible problems. One is from silt, probably more of a concern for a professional diver working on muddy bottoms, and the other is freezing.

For CE purposes fresh water capable of causing a regulator to ice up is regarded as that at 10°C or less. This is because air from your tank cools significantly as it drops in pressure and expands as it passes through your regulator.

In fact, the Mk25 EVO has passed CE EN250 coldwater certification, so it has been successfully tested in fresh water of 4°C at a depth of 50m, where it has been subjected to a moderately hard breathing-rate of 375 litres per minute for five minutes, during which it must not freeflow.

The Mk25 EVO uses a set of fins etched into the body that increase its surface area. The more of the first stage that’s in contact with the water, the more the regulator can draw heat from the warmer water around it to combat freezing.

Being metal, the first stage conducts heat well. Internally, the spring, piston and some other parts are coated with a non-stick surface to which ice can’t easily attach.

It’s ice particles that can block the movement of regulator parts such as pistons, and cause either a freeflow or an air-stoppage.

Second Stage

The S620Ti is compact and lightweight, partly due to the use of techno polymer for the main body. Internally, weight is further reduced by use of a titanium valve-casing. There is some stainless-steel reinforcing that I assume also helps with anti-icing.

The pneumatic balancing is intended to minimise the first part of the breathing cycle, the cracking effort. In unbalanced second stages a fixed-strength spring is used to keep the valve closed, and this must be strong enough to hold back incoming air from the first stage even on very deep dives, to prevent a freeflow.

In Scubapro’s balanced design, the spring is enclosed in an airtight tube. Air enters and helps the spring keep the valve shut until you inhale.

This air pressure can be varied with changes in depth to match changes to the pressure of air coming from the first stage, so a lighter-strength spring can be used and cracking effort is less than with an unbalanced model. It should always optimise inhalation effort regardless of depth.

Cracking effort can be diver-adjusted using an external knob. This tensions the spring pressure bearing on the valve so that it needs more effort to open. This control might be used if the regulator was to freeflow, possibly while facing into very strong currents.

There is also a dive/pre-dive switch. This shuts off the venturi to prevent freeflows when the regulator is not in your mouth. Once you’ve cracked the valve and got the air flowing, the venturi routes the air around the second stage to create a vacuum that holds down the diaphragm, keeping the valve open for you with little lung effort. It should only ever be set to pre-dive when out of the water or snorkelling.

Cracking effort, ease of keeping the gas flowing, gas volume and the speed at which it is supplied are all components of the inhalation cycle, and measured during work-of-breathing machine trials as part of the CE certification process. However, exhalation effort also has to be included and, with the S620Ti, Scubapro claims that a new exhaust-valve and tee has improved this as well.

Freezing in second stages can occur when water caught in the casing or moisture in your exhaled air comes into contact with incoming super-cooled air from the first stage and forms ice on the valve. As with the Mk25 first stage, preventing freezing problems is done with a combination of heat-exchangers and non-stick surfaces on the valve components.

The S620Ti second stage uses titanium components in the air path, so can’t be used with nitrox percentages above 40% or there would be a fire risk. This won’t be a problem for recreational divers but excludes it from some technical dives.

In Use

I used the DIN version and the handwheel, which has a non-slip plastic coating and was easy to do up and remove with wet hands. The position and direction of the Mk25 EVO ports allow for versatile hose configuration.

The swivel collar means that hoses can move with you, within reason, as when you turn your head, so you don’t end up with the mouthpiece uncomfortably pulling at the corner of your mouth when you look a certain way.

The second stage is light and comfortable on long dives. It’s easy to clear, even upside-down, either by blast-clearing or using the purge.

The compact exhaust-T didn’t catch and break the seal of my mask-skirt during the inverted tests, which are done to simulate a stressed diver accidentally inserting an upside-down reg in a sharing situation.

Exhaust bubbles are nicely diverted away from your field of view. You should be able to get your eye up to most SLR housing viewfinders without the second stage interfering.

I set the second-stage controls for the best ease of breathing. Inhalation was easy and smooth, as expected. Next came the important deepwater-sharing divEr test.

EN250A requires that a regulator can provide air to two divers using two second stages and breathing simultaneously, replicating a typical out-of-air, safe-second assist. The standard requires that each diver breathe 250 litres per minute at a depth of 30m.

Mouthpiece stem enlarged to improve air flow.

Mouthpiece stem enlarged to improve air flow.

The test is done on a computerised breathing machine that can accurately measure work of breathing, with set limits to how hard the diver must inhale and exhale. Our manned test can’t measure this, but gives a realistic insight into a valve’s breathing characteristics.

Heavy breathing at depth is no fun and there’s always the concern that you might draw a reg you can outbreathe. My buddy for this task was my mentor Dennis Santos, former Gibraltar SAC Diving Officer and a retired RNVR diver.

We took the Mk25 EVO / S620Ti and its R195 octopus to a wreck at 30m and finned so damn hard I’m sure we moved it.

I had the Scubapro G2 gas-integrated computer on test too, so we had a highly accurate digital pressure display to measure our breathing rate.

Dennis and I finned for two minutes and burned through about 800 litres.

It takes time to reach your maximum breathing rate, so I think it’s fair to assume that we met or exceeded the combined 500 lpm requirement at some point!

Breathing from the 620 Ti primary second stage, I didn’t feel that the regulator’s inhalation effort increased noticeably. It’s more the build-up of CO2 that makes you feel breathless. So, a big win for the Scubapro!

Conclusion

Given the specifications and its heritage, it’s no surprise that the Mk25/S620Ti has received the top EN250A rating, so it’s proven to support one diver to 50m (the EN standard tests no deeper than this) or two at 30m using an octopus, and all in water as cold as 4°C. It’s also practically a given that the Scubapro far exceeds this standard.

It’s certainly a worthy successor to its classic ancestors, and I’m happy to highly recommend the Mk25 /S620Ti.

Specs

TESTER> Steve Warren

PRICE> £619

FIRST STAGE> Balanced piston

SECOND STAGE> Pneumatically balanced

PORTS> 2 hp, 5 mp

WEIGHT> 1.2kg

CONTACT> scubapro.com