The Second Stage: Construction and the Four Faults Behind Most Complaints
Free-flow, hard breathing, water in the mouthpiece and a hiss at the coupling. What is inside the regulator, and which of those four it explains.
A simple part doing the whole job
The second stage is a demand valve: a diaphragm senses the slight pressure drop when you start to inhale and opens a poppet to deliver from the hose. There is no adjustment inside that changes the air, only the effort it takes to open the valve — which is why almost every complaint about a regulator traces back to one of four mechanical causes rather than to air quality.
The four faults, in the order we see them
- Free-flow — the poppet is not seating. Usually grit on the seat, sometimes a stretched lever.
- Hard breathing — the lever is set too low, or the diaphragm is stiffening with age.
- Water in the mouthpiece — the diaphragm edge is not sealing, or the exhaust valve is torn.
- Hiss at the coupling — the hose O-ring, not the regulator. Check the cheap end first.
Adjusting breathing effort
Breathing resistance is set by the lever height, and a very small change in it makes a large difference in feel — a couple of tenths of a millimetre is the whole adjustment range. Factory setting is verified on a pressure gauge, so if you adjust yours by feel, do it a fraction at a time and test in a pool rather than in open water.
When to stop and send it in
Free-flow that returns after a clean, a poppet that will not seat on a clean seat, or any crack in the lever or body: stop there. The second stage is a small part with a hard job, and building up a regulator from mismatched spares is how people end up with a unit that breathes differently on every dive.
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