5
Model 288A Differential Pressure Indicating Switch
Section 1
Standard models can have one or two alarm switches. Each switch can be
connected to operate normally-opened or normally-closed. The direct-set
switch contacts are adjustable over a scale range of 5-95% nominal.
The cam rotates counterclockwise with increased pressure. In Figure 1.2,
page 4, the low switch is set at 25% diff erential pressure, and the high switch
is set at 75% diff erential pressure.
Switches are available in several variations.
•
SPDT: This is the standard model (low, high, or both)
•
DPDT: Two switches are stacked and actuated by a single lever (low,
high, or both)
•
Three or four SPDT: Switches have independent switch points.
Switches 1 and 3 are usually low switches set for decreasing pressures;
switches 2 and 4 are usually high switches set for increasing pressures.
Relays
Relays are actuated by internal switches and are available in several varia-
tions:
•
SPDT
•
DPDT
•
Two SPDT
•
Two DPDT
Wiring
No. 22 AWG is used for internal wiring and No. 18 AWG is used for external
wiring.
Diff erential Pressure Unit (DPU)
For detailed information on the Model 224 DPU, see Appendix A.
Specifi cations
General:
Actuating Unit (DPU) ...................... Model 224 DPU
Dial Size ......................................... 6 inches (150 mm)
Temperature Limits (Ambient) ........ -40°F (-40°C) to +180°F (+82°C)
Switch Repeatability ....................... ±0.25% of full scale
Switch Deadband ........................... ±5% (SPDT); ±6% (DPDT)
Switch Type .................................... Mechanical, Snap-Acting; all switches are SPDT
(DPDT switches are stacked SPDT switches with a
common actuator)
Relay Contact Type ........................ Single Pole, Double Throw (SPDT)
Double Pole, Double Throw (DPDT)
Adjustability .................................... 5% to 95% of factory calibrated scale
Activation ........................................ Increasing or decreasing scale