The Barksdale 0403-105 mechanical pressure switch is a device used to monitor fluid pressure within a system and control safety or regulating devices based on predefined pressure values. The Barksdale 0403-105 mechanical pressure switch operates via a pressure-sensing element, such as a diaphragm or Bourdon tube, which in turn acts on an electrical or pneumatic contact. When the pressure reaches a certain level, the sensing element deforms and activates the contact, triggering an electrical or pneumatic switch. The Barksdale 0403-105 mechanical pressure switch can be used in various industries, such as plumbing, the chemical industry, and heating and air conditioning systems. In short, the mechanical pressure switch ensures that fluid pressure within a system remains within specified limits and activates safety or regulating devices if those limits are exceeded.