Submersible pressure gauge
The submersible pressure gauge (SPG) is used to monitor the air supply in your tank. It attaches to the high-pressure port on your regulator. SPGs are mostly analog devices, but some computers now have an integrated SPG the displays digitally. Analog SPGs are designed with a Bourdon movement, which is a helical tube that unwinds when internal pressure is applied. Most of the analog SPGs on the market today are designed with an accuracy range of 35 to 100 psi when there is 500 psi or less in your tank.
Cost: $80 to $150
Analog depth gauges are similar to the Bourdon tubes in the SPG. Today, most depth gauges are incorporated in dive computers, which measure pressure electronically.
Cost: $50 to $150