The OMEGA PGD Sequence is constructed with two impartial Bourdon tubes. The opposing Bourdon tubes are connected to an individual pinion gear which rotates a pointer for immediate pressure readings. Through the use of two independent Bourdon tubes, the gauge can handle liquids or gases on both or each ports. http://pressuregauge15470.blog-a-story.com/15895845/a-review-of-pressure-gauge