A pressure gauge operates by sending a low amperage current through the gauges's 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, ect. As the sender's resistance varies, the amount of current allowed to flow through it to ground changes and the meter deflects, showing the change in pressure. Features: Anti-scratch glass lens, Plastic Cases for corrosion resistance and long life, 12 vDC negative ground, Can be adapted for 24 vDC systems, Available in standard or Metric values.