What are they?
A Differential pressure switch
is a device which utilizes a differential air pressure to
actuate an electric switch at a pre-set actuation point.
This may be the difference between two positive or two negative
pressures, one of each, or a positive and atmospheric or
a negative and atmospheric pressure. The electric switch
may be used to start or stop motors or fans, open or close
dampers or louvers, light a warning signal, sound alarms,
etc.
Dwyer® Differential Pressure
Switches are built exclusively for low and very low pressure
differentials ranging from .01" water column (.0004
psig) to 50 psig. A wide selection of Dwyer® precision-pressure
switch models are offred within these ranges.
How
do they work?
Cross section shows a typical
Dwyer® Differential Pressure Switch. Some models
employ other methods of transmitting diaphragm motion
to the electric switch button.
When a change occurs
in the differential pressure between the two sides
of the diaphragm, the spring loaded diaphragm moves,
transmitting a force to a snap switch. The switch
may be designed to actuate on either increasing or
decreasing differential pressure.
Diaphragm
motion is resisted by a calibrated spring. This spring
determines the range of differential pressure within
which the diaphragm motion will actuate the electric
switch. The actuation point is set by adjusting the
compression or tension of the spring.
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What is the Dead Band?
When a change in air pressure (or
pressure differential) occurs, causing the diaphragm to
move and actuate the switch, some of that pressure must
be removed before the switch will reset for another cycle.
The dead band is the pressure that must
be removed before the switch resist for another cycle after
the set point has been reached and the switch actuated.
The dead band is inherent in a snap switch.
It results from two things: The spring rate of the range
spring at the chosen set point and the de-actuation travel
of the switch lever. Thus the dead band is different at
each set point. When the set point is at the lowest end
of the pressure switch range, the dead band is minimum.
Conversely, when the set point is at the highest end of
the pressure switch range, dead band is maximum.
Differential pressure plot,
illustrating dead band.
For example:
A pressure switch is set
to actuate when an increasing differential pressure reaches
3-1/2" w.c. When the differential pressure in the system
being monitored builds up to 3-1/2" w.c., the switch
actuates and causes a corrective action to occur. The differential
pressure then begins to drop. The switch remains actuated,
however, until the differential pressure drops to approximately
2.4" w.c. This difference of 1.1" w.c. is the
"dead band." It is the overlap or pressure difference
between the set point at which the switch actuates when
pressure increases and the point at which the switch resists
when pressure drops.
Considerations in Selecting
a Pressure Switch
Diaphragm size
- Dwyer® Pressure Switches are offered in three series
based on diaphragm size. Series 1600 models have a 7-3/4"
diaphragm diameter which provides the greatest actuating
force for maximum sensitivity and repetitive accuracy. Series
1800 models with 4" diaphragms are compact and low
in cost, yet they deliver excellent performance for OEM
and general service. Series 1900 models have 3-1/2"
diaphragms operating through Dwyer-engineered force-motion
amplifier levers, assuring high sensitivity and repeatability
in these compact, low cost switches.
Range selection
- We recommend selection of the range which has a set point
(actuation point) as close as possible to the middle of
the total adjustment range.
Set point
- For service where the set point is known, the unit can
be set at the installation point or for OEM quantities,
preset at the factory. For service where field changes in
set point are anticipated, Series 1638 pressure switches
with visual set point adjustment offer extra convenience.
Where pressures will be reset frequently, the Photohelic®
Pressure Switch/Gage offers outstanding convenience. Where
two set points are required, choose Model 1627 or the Photohelic®
Series. Both have dual switches. Two separate pressure switches
may also be used.
Dead band
- For OEM applications, special snap switches are available
with an extra wide dead band. Consult factory. Photohelic®
Switch/Gages have the narrowest dead band of all Dwyer®
switches and the low/high setpoints can be interlocked to
provide adjustable dead band control.
Maximum PSI
rating - All models are rated at 10 psig or more
surge pressure except the No. 1626 and No. 1627. These have
the sealing diaphragm and are rated at 2 psig. They should
not be used for higher total pressure.
Temperature
rating - Dwyer® switches are assembled, calibrated
and tested at 70°F. Recommended application temperature
limits are 32° (-30° for dry air) to 130°F.
With reduced electrical rating, the upper limit can be extended
to 180°F. See specifications for each model for details.
Where higher temperatures exist, a coil of copper or aluminum
tubing will often provide adequate heat dissipation.
Installation
- Select a location free of excessive vibration where oil
or water will not drip on switch and where ambient temperature
is as close to 70°F, as possible. See specification
pages for weatherproof enclosures, explosion-proof housings
and environmental switches. As a general rule, differential
pressure switches should be mounted with diaphragm in a
vertical plane. It is essential that set point be adjusted
with diaphragm in the position in which it will be mounted.
Pressure
connection - All are 1/8" NPT. To use rubber
or plastic tubing, order No. A-339 1/8" NPT to 3/16"
tubing adapters. For 1/4" metal tubing, order No. A-324
1/8" NPT to 1/4" tubing compression fittings.
Electrical
connections - Each switch has three screw type connections.
Conduit enclosures have 7/8" hold for a 1/2" conduit
fitting.
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A
Dwyer® explosion-proof pressure switch detects
loss of control room purge air pressure and signals
an alarm.
The flow of conditional air
must be maintained to provide adequate positive room
pressure compared to atmosphere. This ensures that
control or instrumentation rooms located in hazardous
environments are safe for the operation of electrical
devices.
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Dwyer®
switch senses ice build-up on cooling coils and actuates
defrost cycle.
In this cooling application,
the Dwyer® differential pressure switch senses
the increasing restriction to air flow caused by icing
and actuates the defrost cycle. Two static tips are
installed to sense the differential pressure across
the cooling coils. |
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Pressure
switch monitors flow in high pressure system.
In a liquid process
high pressure line, the process may be adversely affected
by flows above (or below) the desired flow. The W.E.
Anderson® Model H3 can monitor flow in systems
with operating pressure to 1500 psig as a function
of pressure drop across a calibrated orifice plate.
The H3 set point is adjusted so that an alarm sounds
or the process is automatically shut down if flow
exceeds (or falls below)the desired rate. |
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Switch/gage
regulates pressure in air-inflated building.
The unit senses overpressure
that may over-inflate the building - or loss of pressure
that may result in collapse - and controls the blower
to maintain correct pressure. The gage is easily reset
to conserve power or provide extra pressure to resist
strong winds. |
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When
paper web breaks on printing press, air jet reaches
pressure switch, which signals controls to stop press.
When the paper strip, or web,
breaks accidentally, a large amount of paper is lost.
To prevent such losses, the integrity of the web is
often monitored by a Dwyer® differential pressure
switch. When the web breaks, the air flow from the
jet causes pressure to rise in the pickup tube, closing
the switch which signals the main control unit to
shut down the press. |
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Tape
drive speed varies by Minitactor™ switch to
control tape loop length. Dwyer®
Minitactor™ differential pressure switches function
to maintain the proper loops in tape drives for computer
systems. The loop columns are subject to vacuum with
pressure switches monitoring the upper and lower ports
in each column. When a short loop senses the vacuum,
the switch signals the appropriated reel drive to
change speed slightly and supply more tape loop. A
long loop senses atmospheric pressure, and the switch
then signals the drive to reduce the loop length.
The system is identical for both the machine reel
and file reel columns in this illustration. |
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Differential
pressure switch actuates roll filter motor when dust
reduces air flow. When
dust build-up on this roll filter increases the filter's
resistance to air flow, the increased pressure drop
across the filter is sensed by the differential pressure
switch, which starts a motor to roll a new section
of filter into place. |
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Transducer
maintains constant level in reservoir by controlling
motor-actuated fill valve.
The air pressure required
to purge the dip tube of liquids is equal to the reservoir
depth expressed in inches of water. Any change in
reservoir level results in a proportional change in
output current from the transducer which, when fed
back through the control circuit, adjusts the valve
to either increase or decrease the fill pipe flow
to maintain the desired level. |
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Dwyer®
Transmitter signals precise air velocity adjustment
to computer-controlled variable-speed fan motor.
In variable air volume (VAV)
HVAC systems, a computerized control provides precise
adjustment of air volume to meet changing system needs
with maximum energy efficiency. A Dwyer® Series
604 differential pressure Transmitter is used with
a Pitot tube array to provide a 4-20 mA signal to
the computer. The signal is directly proportional
to the differential pressure developed across the
Pitot array as a function of air velocity. The computer
reacts to any change in velocity by signaling the
motor control to increase or decrease fan speed to
maintain the required velocity. The computer, taking
inputs from other ambient condition sensors, will
establish a new required air velocity and signal an
appropriate adjustment in fan speed until the new
velocity is achieved as sensed by the Pitit tube array
and transmitter. |
Dwyer®, Mercoid® and W.E. Anderson®
Pressure Switches
Dwyer® Pressure Transmitters
SPECIAL MODELS FOR OEM
REQUIREMENTS
