Faria Water Pressure Gauge 30 PSI Chesapeake Black - 14712Faria Water Pressure Gauge Kit (30 PSI) A pressure gauge operates by sending a low amperage current through the gauge's meter to the ground via a sending unit with variable resistance. The resistance of the sending units increases or decreases with the changes in pressure, temperature, etc. As the sender's resistance varies, the amount of current allowed to flow through it to the ground changes and the meter deflects, showing the change in pressure.