Series 1640 Floating Contact Null Switch for High and Low Actuation Visual set point adjustment . . . adjustable null zone Except for its electric switch, the Dwyer Model 1640 Differential Pressure Switch is identical to the Series 1630. The Model 1640, however, is equipped with a single pole, double throw floating contact switch (not snap acting) so it functions as a null switch. 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. The "high" circuit will be closed when rising pressure differential reaches the preset level. The "low" circuit will be closed when falling pressure differential reaches the preset level. 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. CAUTION: Because of the very low electrical rating, the 1640 switch is almost always used with a relay which has suitable current carrying capability. SPECIAL MODES AVAILABLE: Environmental (MIL) Construction. Unlisted Model 1645 can be furnished with a special snap switch sealed against the environment for high humidity, exposure to fungus, and/or for military applications. Similar to Model 1638 except dead band is slightly greater and some lower set points may not be available. Specify Model 1645 - (Range No.) - "MIL" and required set point in ordering. Typical Wiring Hook-Up Rising pressure closes the "high" contact which causes the motor to rotate in a counterclockwise direction. Decreasing pressure closes the "low" contact and causes the motor to rotate in a clockwise direction. When the floating contact is in the null zone there is no switching action and motor is at rest. Adjustment of the Switch The "high" actuation point of the Model 1640 switch is indicated on a calibrated scale secured to the transparent range screw enclosure. Resetting is accomplished simply by turning the adjustment screw with a screwdriver to the desired setting on the scale. The "low" actuation point is set by adjusting the span of the null by turning the Span Adjusting Screw on top of the switch element inside conduit enclosure (See image above). PHYSICAL DATA Ambient temperature range:   -30° for dry air or gas to 130°F. Maximum surge pressure:   25 psig Rated pressure:   10 psig Pressure Connections:   1/8" NPT Electrical Rating:   Non-inductive - 2 ½ amp, 110v A.C.; 1 ½ amp, 220v A.C.; 1 amp 24v D.C.; ½ amp, 110v, D.C.; Inductive - 1 amp, 110v A.C.; ½ amp, 220v A.C.; ½ amp, 24v D.C. De-rate 70-80% for very slow pressure changes Switch action:   SPDT floating contact (not snap action) Wiring connections:   3 screw type, common, high and low. Weight:   4 lbs., 13 oz. Installation:   Diaphragm vertical CAUTION: FOR USE ONLY WITH AIR OR COMPATIBLE GASES. MODEL 1640 SWITCHES; OPERATING RANGES, NULL ZONES AND RATINGS Model Number Ranges Inches, w.c. Adjustable Null Span Min. Set Max. Set 1640-0 1640-1 1640-2 1640-5 1640-10 .01 to 0.2 0.2 to 1.0 1.0 to 4.0 2.0 to 6.0 3.0 to 12.0 .01 .02 .03 .03 .04 .03 .06 .12 .18 .20 SUGGESTED SPECIFICATIONS Differential pressure switches shall be diaphragm operated to actuate a single pole double throw floating contact "Null" switch. The pressure differential at which the electrical switch will be actuated on rising pressure shall be adjustable. Set point adjustment shall be screw type with st point indicated on a visual scale. The span of the floating contact switch shall be adjustable. Switches shall be Dwyer Instruments, Inc. Catalog No. ____ for the required operating ranges.