The submersible pressure gauge (SPG) displays your cylinder's remaining air pressure while you dive. In the United States, the display is in pounds per square inch (psi). In other countries the unit of measurement is BAR.

Air is delivered to the gauge through a hose that's connected to your regulator's high-pressure port. The inside diameter of the hose is extremely small, so if a leak develops you'll have plenty of time to make a safe ascent to the surface.

Your safety depends on your SPG's accuracy, so handle it with care. If you notice water inside the gauge, have it serviced and inspected before using it.