The amount of inhalation or demand effort is actually divided into two distinct components. The most obvious is the initial effort or the exertion required to open the valve and start the flow of air. The second effort is the amount of energy required to maintain the air flow after the valve is opened. This is referred to as flow resistance.

To reduce flow resistance, SCUBAPRO developed VIVA (Venturi-Initiated Vacuum Assist) to power assist every breath. A highvelocity air stream pulls air molecules inside the second stage casing, which generates a vacuum behind the diaphragm. As the diaphragm draws further inward, it depresses the lever which maintains the poppet open for a "power assist" breath.

To make this technology even more appealing, SCUBAPRO designed the VIVA system into all of its current regulators to be diver adjustable, so that each diver can adjust airflow for varying circumstances.

The adjustable flow vane can be positioned on the minimum position to minimize free flow in specific diving conditions such as shore dive water entry or practicing buddy breathing.

It is also recommended to use minimum VIVA for extreme cold water diving.

Classic downstream valve second stages are particularly noted for their legendary safety and reliability.

In operation, a downstream valve opens in the same direction as the incoming air flow from the first stage. This incoming air exerts a force on the end of the poppet that pushes the valve into the open position (downstream). This force is counteracted by a spring that pushes the poppet in the opposite direction (upstream) toward the closed position. Inhalation effort required to open a downstream valve is directly proportional to the resistance of this spring.

The SCUBAPRO classic downstream valve has been a major factor in second stage designs for many years.

A regulator second stage must translate natural breathing reactions into mechanical reality. The quality of this human-to-machine interface is one of the most critical elements of the diving experience.

A high-quality second stage can significantly reduce stress and enhance diver safety by providing smooth, low-effort breathing response that results in ample yet controllable quantities of air.

SCUBAPRO's Air-Balanced Linear-Flow Valve technology represents the ultimate in air flow performance.

Based on SCUBAPRO's proven downstream linear valve design, the air-balanced version adds a pneumatic balancing chamber to reduce the force of downstream air entering from the first stage.

Balancing reduces the spring tension required to hold the flow valve closed, while maintaining sufficient pressure to allow for the safety of a downstream override.

Because spring tension in this configuration is one of two causes of inhalation resistance, the smaller, lighter spring used in SCUBAPRO's Air-Balanced Linear-Flow Valve can provide the lowest possible inhalation effort.

The result is an ultra-high air flow that remains exceptionally stable under all breathing conditions.

A revolutionary lever designed in the "S" series acts like an automatic gear box to supply air smoothly to the diver with the slightest sip and can also deliver controlled air flow in specific diving conditions when required.

The new layout drops the diaphragm a little lower and brings it closer to your mouth. Because the diaphragm sits a bit lower, the water pressure is slightly greater, meaning that the water is effectively assisting your every breath. The overall ‘length’ from mouthpiece to diaphragm is far shorter than on other regs, thereby reducing the leverage in your mouth, eliminating jaw fatigue and improving comfort. As the diaphragm is closer to your mouth, the breathing resistance is less affected by different positions, such as upside down or face-up, where other regs tend to experience an increase in breathing effort.

The main internal spring has been moved completely out of the air path. Because the air does not have to flow over the spring as on most other regs, there is less turbulence and drag, thereby improving flow rates and providing very smooth inhalation. In very cold water conditions, the spring is no longer an area of concern for freeze ups.

Co-axial Venturi System – Borrowed from the range topping A700, this feature provides for a pre-dive switch to prevent free flows on the surface but it does so without the need for a vane in the mouthpiece tube, leaving a clear, unrestricted flow path on both inhalation and exhalation.