SPG in scuba diving stands for submersible pressure gauge. What is gauge pressure in scuba diving? Gauge pressure in scuba diving is the amount of air or breathable gas left in your scuba diving tank. A pressure gauge measures this gas/air in bar or psi.
Most commonly, dive computers are used to indicate the time remaining for a no-stop or no-decompression limit; this is when divers can safely surface without the requirement to perform decompression stops. In short, dive computers allow you to maximise your bottom time whilst remaining safe both below and above water.