Data Sheet for Product

Technical Instructions SW 251 Pressure Electric Switches
Document Number 155-057P25
Rev. 1, May, 2000
Page 2
Siemens Building Technologies, Inc.
Specifications
(Applicable to all
models unless
otherwise noted)
Operating
Medium Air only
Set Point range 2 to 30 psi (13.8 to 207 kPa)
Maximum pressure 30 psi (207 kPa)
Repeatability ± 0.5 psi (3.4 kPa)
Factory set point accuracy ± 2.0 psi (13.8 kPa)
Scale accuracy ± 3.0 psi (20.7 kPa)
Ambient temperature range 40 to 120°F (4.4 to 49°C)
Position sensitivity - Can be mounted in any position; Maximum set point shift of -0,6 psi
(4.1 kPa) when changed from normal upright position to upside-down position.
Switch action – Single-Pole, Double-Throw (SPDT) (except 251-0006 which is Single-
Pole, Single-Throw (SPST), Normally Open (N.O.) and 251-0009 and 251-0011 which
are SPST, Normally Closed (N.C.)
Terminals are marked - Red - Common
Yellow - Normally Open
Blue - NormalIy Closed
UL Listed Guide XAPX—File E35198
Physical
Set point scale Excluded on models 251-0009
and 251-0011
Differential scale Included on model 251-0001 only
Pneumatic conn Barb fitting for 1/4-inch (6.4 mm)
O.D. plastic tubing
Electrical connection No. 8 binder head screw
with cup washer
Conduit holes:
One 1/2-inch nominal conduit size hole 251-0001, -0002, -0004
Two 1/2-inch nominal conduit size holes 251-0008, -0009, -0011
One 3/4-inch nominal conduit size hole 251-0006
Mounting- Surface mounting with #10 screws
Pressure sensing element Buna N on nylon
Case material Zinc plated steel with bright chromate
finish
Weight
Single input, single stage models 1.0 lb. (0.45 Kg)
Two stage or dual input models 2.0 lbs. (0.9 Kg)
Operation
(See
Figure 1
)
With no air pressure on the diaphragm (6), the snap switch (7) is not actuated. As air
pressure increases, the lever (5) moves upward, pivoting about point "A", until the snap
switch is actuated. Set point is changed by turning the set point adjustment screw (1) to
change the loading of the set point spring (2). A factory adjusted stop (not shown) on the
lever (5) prevents excessive overtravel on the snap switch. On adjustable differential
models, the differential is changed by turning the differential adjusting screw (4). This
combines the differential spring [not shown, located in differential adjustment block (3)]
with the set point spring to obtain a new spring rate.
Discontinued