Second stage of the regulator: how it works and how to choose one
When choosing a regulator, attention is almost always focused on the first stage: balanced or unbalanced, diaphragm or piston, number and arrangement of ports. The second stage is often considered a consequence, a component that accompanies the first.
However, the second stage is the element that is held in the mouth throughout the dive, the one that determines the perceived breathing effort, and the one that is duplicated multiple times in a technical configuration. It is worth understanding how it works before choosing it.
What a second stage does
The first stage reduces the high pressure from the tank to a constant intermediate pressure, typically between 9 and 11 bar above ambient pressure. The second stage performs the next step: it brings that intermediate pressure to ambient pressure, i.e., the pressure the diver is at that moment.
The difference from a simple reducer is that the delivery is not continuous but on demand. Gas is released only when requested, and the request is the act of inhalation itself.
The mechanism consists of a diaphragm connected to a lever. Inhalation reduces the pressure inside the chamber, the diaphragm flexes inwards and actuates the lever, which lifts the valve and allows intermediate pressure gas to enter. When inhalation stops, the internal pressure rebalances with the ambient pressure, the diaphragm returns to position, and the valve closes.
The same principle explains why a second stage does not free-flow even when open in water: as long as the valve is closed, the diaphragm separates the chamber from the external environment.
Pneumatic balancing
In an unbalanced second stage, the force required to lift the valve also depends on the pressure acting on the valve itself. Since that pressure varies with the tank's contents, the breathing effort changes during the dive: breathing at 50 bar remaining is not the same as breathing at 200 bar.
Pneumatic balancing introduces a chamber that compensates for that force, making the opening essentially independent of the upstream pressure. In practice, the breathing effort remains constant from the beginning to the end of the dive.
The difference is barely noticeable in short, shallow dives. It becomes significant when gas density increases with depth and when the dive is long: these are the conditions in which breathing work accumulates and affects fatigue and consumption.
The technical range models in the catalog — the TEC1 and the TEC2 — adopt this solution, while the REC1 falls into the recreational category.
Venturi adjustment
Once the valve is open, the gas flow through the chamber creates a vacuum that tends to keep the diaphragm flexed, and thus the valve open. This is a useful effect, as it reduces the effort required to continue inhalation once it has begun.
However, the same effect can trigger a continuous flow when the second stage is not in the mouth: on the surface, with the regulator left free, or during donning. This is why there is an adjustment control, often indicated with dive and pre-dive positions.
The position that limits the effect should be used out of the water and when entering; the one that exploits it should be used during the dive. This is an adjustment that is easily forgotten, and the cost of forgetting it is gas lost on the surface.
EN 250 markings
Regulators for underwater use fall under standard EN 250, which governs open-circuit compressed air breathing apparatus and defines their testing.
Within the same standard, there are different markings, and the catalog displays them: the REC1 states EN 250, while TEC1 and TEC2 state EN 250A. The distinction concerns the intended use conditions of the certification, particularly use in cold water.
This is an element worth checking with the manufacturer for the specific model before purchase, especially if the intended use includes lakes, inland waters, or winter dives. The marking is shown on the product and in the technical documentation.
How many second stages, and how they are arranged
In technical diving, each independent gas source has its own first and second stage. The criterion is not redundancy in abstract, but the ability to isolate a failure without losing access to gas.
Twinset with manifold. Two first stages and two second stages. The primary is mounted on a long hose, typically 210 centimeters, routed along the body; the secondary remains on a bungee around the neck, immediately below the chin. The long hose exists so that if gas is shared, the two divers can move in a line, not side-by-side.
Sidemount. Each side cylinder carries a complete regulator, with direct access to the valve. The management is different but the principle remains: two independent sources.
Stage and decompression cylinders. Each cylinder has its own second stage, stowed with bungee and clip to remain out of the hydrodynamic flow and available at the planned depth.
The reversible version
A reversible second stage can be configured to receive the hose from the right or left, by reversing the inlet side.
This is not an aesthetic feature. In a long hose configuration, the routing must follow a precise path, and the inlet side determines whether the hose runs cleanly along the body or if it makes a curve that exposes it. On some sidemount configurations, the same need arises symmetrically on both sides.
The TEC2 in the reversible version addresses this need without requiring two different models in stock.
Versions for oxygen-rich mixtures
Decompression cylinders often contain mixtures with high percentages of oxygen, up to pure oxygen. A second stage intended for that use requires specific preparation: gaskets made of compatible material, suitable lubricants, and a cleaning procedure that eliminates hydrocarbon residues.
In the catalog, these versions are identified by the color green and the designation O2, such as the TEC2 O2. The chromatic distinction is not decorative: it serves to make it immediately recognizable, on deck and in the water, which equipment is intended for which gas.
It should be noted that oxygen preparation is not permanent. It is maintained through the use of clean gases and through maintenance performed with the same procedure, and it is a matter that falls within specific training on the use of mixtures, not derivable from an article.
Cover and maintenance
The front cover is the most exposed component of the second stage: it protects the diaphragm, houses the purge button, and absorbs impacts against valves, ladders, and structures. It is also the most frequently replaced component, and its cost is a fraction of that of the regulator. There are dedicated replacement covers for each model.
For the rest of the maintenance, a few constant rules apply: rinse in fresh water after every dive, without pressing the purge button while the second stage is immersed and not pressurized, because water would enter the hose; store in a dry place away from heat sources; periodic servicing at an authorized center.
Service intervals depend on the manufacturer, model, and actual use, and should be read in the product documentation rather than assumed by convention.
Conclusion
A second stage is evaluated on three elements: whether balancing is necessary for the intended use, whether the marking covers the conditions under which it will be used, and whether the hose configuration intended requires the reversible version or specific preparation for mixtures.
These are three questions to be resolved before looking at the price, and they narrow the choice down to a few models.