This switch senses a change in the (differential) pressure (either velocity pressure or pressure drop across a restriction) as the airflow changes. The (differential) pressure is applied to the two sides of a diaphragm in the control. The spring-loaded diaphragm moves and actuates the switch. The series P233A/F can also be used to detect small positive gauge pressure by using only the high pressure connection and leaving the low pressure port open. Or to detect a vacuum by using only the low pressure connection and leaving the high-pressure port open to ambient pressure.
Johnson Controls - P233A-4-AHC DUCT PRESSURE SWITCH, 50 TO 400PA, DIFF: 30PA, 1XNC/NO SPDT OPEN HIGH/2A@250VAC, ACCESSORIES INCLUDED, FOR 4/7 MM PVC TUBE
P233A4AHC