SPG (Submersible Pressure Gauge)

The gauge that shows how much gas remains in a scuba tank, connected to a high-pressure port on the regulator first stage.

The SPG is your fuel gauge underwater. Connected to a high-pressure port on the regulator first stage, it reads the actual pressure inside your tank in bar or psi, so a full aluminum 80 shows around 207 bar (3000 psi) and the needle falls as you breathe the tank down. Checking it regularly, not just when someone asks, is one of the most basic habits of a safe diver.

Divers use the SPG to manage the dive: you plan a reserve, commonly surfacing with no less than 50 bar (about 700 psi), and in overhead or dive guide contexts you may apply turn pressures or the rule of thirds. Comparing pressures with your buddy during the dive keeps the team honest about who is breathing fastest and when it is time to head back.

Many divers now read tank pressure on an air-integrated dive computer through a wireless transmitter, but a simple mechanical SPG remains popular as a primary or backup because it needs no battery. Recording start and end pressure in your dive log is also what makes SAC rate calculations possible later.