Euro Black 2 Water Pressure Gauge 30 PSIPerimeter-lighted Black dial with bold White graphics, Black aluminum bezel, contoured White pointer and flat glass lens.A pressure gauge operates by sending a low amperage current through the gauges meter to ground via a sending unit with variable resistance. The resistance of the sending units increase or decrease with the changes in pressure, temperature, etc...As the senders resistance varies, the amount of current allowed to flow through it to ground