What is a Pressure Gauge? Pressure Gauge is an instrument that indicates the pressure of a process medium relative to a reference pressure, commonly atmospheric pressure. Depending on the application, pressure gauges are available in different sizes, pressure ranges, materials, connection types, and operating mechanisms. Common pressure gauge designs include Bourdon tube pressure gauges, diaphragm pressure gauges, capsule pressure gauges, digital pressure gauges, glycerin-filled pressure gauges, and stainless steel pressure gauges.