Pressure Switch for Corrosive

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Pressure Switch for Corrosive Ultra-pure
Liquids Series SWT

• Product Education Course

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Purpose of a Pressure Switch
Imagine having an employee who continuously
monitors a single pressure gauge, and is told to
push a button every time that pressure gauge
rises or falls past a pre-determined point.
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A Pressure Switch Won't Blink
and it doesn't take a coffee break. It simply
reacts to pre-determined changes in pressure.
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Uses of a Pressure Switch

1. As a control element, to actuate a pump, valve or
   other control device, when pressure rises or
   falls to a set point in a liquid piping system.
2. As a safety device, to signal an alarm if
   pressure exceeds a dangerous high or low.
3. As a sensor, to signal a counter, computer
   program, or other recording mechanism.

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How it Works
The Plast-O-Matic Series SWT Pressure Switch
connects to a 1/2" tee on the pipeline, just like
a gauge guard or other pressure sensing
instrument.
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An adjusting screw is used to set force on a
spring, which acts on a piston. The piston is
in contact with a switch. The piston and spring
are acted upon by system pressure, via a PTFE
diaphragm.
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As pressure in the pipeline exceeds the force of
the spring, it begins to deflect the diaphragm
and lift the piston
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When pipeline pressure overcomes the "set point"
determined by the spring force, the piston breaks
contact with the switch and a circuit in the
switch is closed, sending a signal to a pump,
valve, indicator light, etc.
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When pipeline pressure drops below the setpoint,
the spring forces the piston down and back into
contact with the switch, opening the circuit, and
switching off the pump, valve, indicator light,
etc.
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BUT the Switch doesnt re-set immediately at the
set point
Friction, mechanical play in the switch
mechanism, inertia of moving parts, energy lost
in the spring as the spring actuates, and other
physical forces result in deadband, also called
hysteresis. Deadband is the difference between
the switch actuation pressure when pressure is
rising, and actual switch de-actuation pressure
when the pressure is falling.
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• It must exceed the set point by a certain
  percentage before it "re-sets." This range is
  the deadband.

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Without hysteresis, the switch would quickly
end up "hunting." In other words, if actuation
and de-actuation pressures were the same, the
switch would bounce back and forth, not knowing
if the pressure is too high or too low. Soon the
pump or valve would be turned on, off, on,
offthe system would be pulsating, pressure would
oscillatechaos would ensue.
Deadband (or hysteresis) is NOT a negative...
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The Amount of Deadband is determined by Switch
Sensitivity. Plast-O-Matic offers 3 different
switches with Series SWT3 Amp 16 Amp 25 Amp
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Which switch is right for a specific application?
It all depends on how much deadband the system
can withstand before it does become a negative.
In many cases, the 16 amp or 25 amp switch are
acceptable. The 3 amp switch has finer
tolerances lighter weight parts which reduce
inertia. The finer mechanism cant handle higher
currents because the contacts, spring, and
conductors are lighter. With the 3 amp model, you
sacrifice current capability for low deadband.
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Advantages of Series SWT

• Max operating pressure 150 psi.
• NEMA 4X design and patented Fail-Dry enhance
  system safety.
• Electrical switch is UL recognized and CSA
  approved.
• Corrosion-resistant plastic cord grip and 18"
  cable simplify installation.

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Advantages, continued

• Better piston design and piston guidance vs.
  competition enhances reliability and accuracy.
• Tested to 1,000,000 cycles in laboratory
  conditions.
• PTFE diaphragm with no wetted metals or
  elastomers ideal for ultra-pure liquids -- No
  Gauge Guard required!

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Series SWT Pressure Switch
Please click the photo for Series SWT webpage
with complete specifications