Pressure Variation Indicator
To monitor breathing gas pressure in the diving cylinder, a diving regulator usually has a high pressure hose leading to a contents gauge (also called pressure gauge). The port for this hose leaves the first-stage upstream of all pressure-reducing valves. The contents gauge is a pressure gauge measuring the gas pressure in the diving cylinder so the diver knows how much gas remains in the cylinder. It is also known as submersible pressure gauge or SPG.
Government Printing Office The Official Gazette of the United States Patent Office (Washington, DC: Government Printing Office, 1895)