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The Pre-Dive/Dive, On/Off, or +/- Adjustment on a Scuba Regulator
The red arrow shows the "Venturi switch" on my alternate air source. These adjustments may be found on the side or top of the regulator second stage.
Natalie L Gibb
What do you notice about the design of a regulator second stage
? At first glance a diver may notice the size, weight, or color. Perhaps you notice an interesting little knob on the second stage labeled "Dive/Pre-Dive," "On/Off," or "+/-". This switch or knob alters the airflow inside the regulator, making breathing either easier or more difficult. Turning the knob enables and disables something called the Venturi Effect, which regulator designers take advantage of to assist breathing. Click through the following pages to find out how it works, and when you should disable the Venturi Effect.