Except for its electric switch , the Dwyer Model 1640 Differential Pressure Switch is identical to the Series 1630 .
Drawing shows the switching action schematically.
As the diaphragm moves in response to pressure changes, it moves the floating contact to cause switching action at two preset points with no switching action between these points. A typical example of usage is to position motorized dampers when static pressure in a duct system reaches a desired maximum and reposition the dampers when the static pressure falls to a pre-established minimum. By using a pitot tube sensing element the Model 1640 switch can serve in the same way to control air velocity and maintain a constant volume of air in a supply duct. Because of the very low electrical rating, the 1640 switch is almost always used with a relay which has suitable current carrying capability. When the floating contact is in the null zone there is no switching action and motor is at rest.