Pressure switches are actuated by a change in the pressure of a liquid or gas.  They activate electromechanical or solid-state switches upon reaching a specific pressure level. The most important parameter to consider is the pressure range that is needed to be switched at, or the range over which linear output is needed.  Choices for pressure switches measurement ranges include absolute, gauge, vacuum, differential, and sealed. Absolute switches are used where pressures are to be measured independently of the natural fluctuations in atmospheric pressure. The pressure of the media to be measured is compared against a reference pressure of absolute zero (absolute vacuum) in a sealed reference chamber.  Vacuum measurement switches measure vacuum pressure (negative pressure).  Differential pressure switches give the relative pressure between two points. If both operating pressures are the same, the measuring element cannot move and no pressure will be indicated. A differential pressure is indicated when one pressure is higher or lower. Low differential pressures can be measured directly in cases of high static pressures.  Sealed gauge pressure measurement is similar in concept to an Absolute Pressure switch, except that the pressure of the media to be measured is compared to standard atmospheric pressure (at sea-level).