Some regulators come with adjustable second stages. This is usually in the form of a knob to rotate or a lever to slide. First of all, this serves two purposes:
- It allows you to adjust the sensitivity of the air flow, making it harder to free flow. If you are in a strong current at the surface, for example, your second stage might normally be inclined to free flow.
- As the service date approaches, the performance of regulators decreases slightly. This may mean it requires more energy to pull air from the second stage. An adjustable second stage allows you to "open it up" during this time, maintaining a steady performance.
You may be tempted to use the adjustment knob to limit air flow in an attempt to save air. This is a bad idea.
First of all, it won't even work. If it requires more energy to breathe, you are going to end up needing more air than you would otherwise. Secondly, limiting your supply of oxygen is always a bad idea.
To use an adjustment to extend your air supply, do just the opposite. Open it up so that air is supplied to you with minimal effort. Remember, relax!