The submersible pressure gauge (SPG, pressure gauge, or air gauge) is the gauge a diver uses to monitor the amount of air remaining in his scuba tank. It is absolutely essential in diving, for it allows divers to avoid running out of air underwater. A submersible pressure gauge is frequently grouped together with other gauges on a console. Some of the common gauges found in a console are depth gauges, dive computers, and compasses.
1. Depth Gauge
A depth gauge has two needles to monitor two different things. A black needle indicates a diver's current depth. A second, in this case red, needle indicates the maximum depth a diver reaches on a given dive. The needle that indicates a dive's maximum depth needs to be reset at the beginning of each dive. The maximum depth needle is useful when logging dives. It is also a good idea to glance at it when ascending from a dive to confirm that the planned maximum depth has not been exceeded. Depth gauges may be in units of feet or meters. The gauge above is in meters. Most depth gauges have standard safety stop depths indicated by red lettering, making it easy for a diver to remember his safety stop. The gauge above has the standard safety stop depth indicated by red lines between 3 and 6 meters.
2. Submersible Pressure Gauge
The submersible pressure gauge (SPG) indicates the amount of air pressure in a scuba tank. The units of pressure may be given in bar (metric), or in psi (pounds per a square inch, imperial). A standard, aluminum 80-cubic-foot tank is full at 3000 psi or 200 bar. Different tanks styles may be full at different pressure ratings. Most pressure gauges indicate a reserve pressure, usually beginning somewhere around 50 bar or 700 psi in red. The reserve pressure is the amount of air pressure with which a diver should begin his ascent to avoid running out of air underwater. Be warned, this “red zone” does not indicated a good reserve pressure for every dive, and it is important to take the dive profile and plan into account when deciding on a proper reserve pressure for a dive.
Learn more about choosing a proper reserve pressure for a dive.
