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?
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 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.