Pressure switch operating principles

Pressure switch can be modified so as to make a low pressure contact, In addition to a high pressure contact. It is adjustable. The contacts in a pressure switch may be normally closed when the pressure is below the set point for example; the contact in a normally open switch remains opened until the pressure rises above the set point. Then the sensing element makes the contacts snap to close position. The contacts open again when the pressure falls below the set point. The contacts in a normally closed switch remain closed until the pressure rises above the set point. Then the contacts snap open and remain open until a pressure drops below the set point again. Most switches contain the two sets of contacts, one normally open and other normally closed.

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).