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Introduction to Precision Low Range Differential Pressure Switches |
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.
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.
How to use Precision Low Range Differential Pressure Switches
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.
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.
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.
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.
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.
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.
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.
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.
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
| Pressure Series Switches |
Pressure or Diff. Pressure |
Special Features and Approvals |
Switch |
Min.-Max. Setpoints |
Set Point Indicator |
On-Off Indicator |
Rated Pressure |
Max. Surge Pressure |
|
1626 1627 1638 1640 1823 1910 1950 1950P Minitactor H3 BB DP PG DA/DS SA1100 A1 PQ 1000 1000E AP CS 1990 H2 |
D/P D/P D/P D/P D/P D/P D/P D/P D/P D/P D/P D/P D/P Pressure Pressure Pressure Pressure Pressure Pressure Pressure Pressure Pressure Pressure |
Sealing Diaphragm Dual Switches UL, CSA, FM Adjustable Null Zone UL, CSA, FM UL, CSA, FM Explosion-Proof UL, CSA, FM Explosion-Proof UL, CSA, FM Factory Set Explosion-Proof UL, CSA, Cen. Adjustable DB, CSA Adjustable DB, dual bellows UL, CSA, FM Adjustable DB, UL CSA, FM Explosion-Proof, WP,UL,CSA 316 SS Diaphragm, UL Mercury Switch, UL CSA WP, UL Explosion-Proof, WP, UL UL, CSA UL UL, CSA, FM Explosion-Proof, WP, UL, CSA |
SPDT 2-SPDT SPDT SPDT, Floating SPDT SPDT SPDT SPDT SPST SPDT, 2- SPDT SPAT SPDT SPDT or SPST SPDT or SPST 1 or 2 SPDT SPDT SPST SPDT SPDT SPDT SPDT SPDT SPDT |
.15 to 24" w.c . .15 to 24" w.c. .05 to 12" w.c. .01 to 12" w.c. .07 to 85" w.c. .07 to 20" w.c. .02 to 20" w.c. .05 to 50 psid 2" w.c. to 7 psid 10" w.c. to 7 psid 5 to 8000 psid 0 to 80 psid 1" w.c. to 5 psid -30" Hg to 5000 psig 10 to 500 psig 2 to 500 psig 0 to 30" w.c. 5 to 1400 psig 5 to 1400 psig -10" w.c. to 125 psig -30" Hg to 150 psig 2 to 20" w.c. 3 to 1500 psig |
- - Yes Yes - - - - - - - Yes Yes Yes Yes Yes - Yes Yes Yes Yes Yes - |
- - - - - - - - - - - - - Yes - - - - - - - Yes - |
50" w.c. 50" w.c. 10 psig 10 psig 10 psig 45" w.c. 45" w.c. 35 to 70 psig - 1500 psig 100 to 8000 psig 50 to 600 psig 20 psig 15 to 5000 psig 1200 psig 750 psig - 600 to 3000 psig 600 to 3000 psig 15 to 160 psig 30 to 175 psig 45" w.c. 1500 psig |
2 psig 2 psig 25 psig 25 psig 25 psig 10 psig 10 psig 50 to 90 psig - - - - - - 3000 psig - 1 to 60" w.c. - - - - 10 psig - |
Dwyer® Pressure Transmitters
| Transmitter Series |
Pressure or Diff. Pressure |
Output |
Pressure Indication |
Circuit Type(s) |
Current |
Min.-Max. Ranges |
Accuracy | Maximum Pressure |
|
603A 604A 607 616 645 604D 605 620 621 7116 634ES 636 637 638 639 642 |
D/P D/P D/P D/P D/P D/P D/P D/P D/P Pressure Pressure Pressure Pressure Pressure Pressure Pressure |
4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA 4-20 mA/1-5 VDC 4-20 mA 4-20 mA 4-20 mA 4-20 mA |
- - - - - Yes Yes Yes Yes Yes - - - - - - |
2, 3 or 4-Wire 2, 3 or 4-Wire 2-Wire 2, 3 or 4-Wire 2-Wire 2, 3 or 4-Wire 2-Wire 2, 3 or 4-Wire 2, 3 or 4-Wire 2, 3 or 4-Wire 2-Wire 2-Wire 2-Wire 2-Wire 2-Wire 2-Wire |
AC/DC AC/DC DC AC/DC DC AC/DC DC AC/DC AC/DC AC/DC DC DC DC DC DC DC |
0-.25" w.c. to 0-20 psid 0-0.1 to 0-120" w.c. 0-0.1 to 0-25" w.c. 0-1" w.c. to 0-100 psid 0-1 to 0-100 psid 0-0.5 to 0-1" w.c .05 -0-.20 to 0-50" w.c. 0-3" w.c. to 0-100 psid 0-3" w.c. to 0-100 psid 0-60 to 0-10,000 psig 0-5 to 0-6000 psig 0-15 to 0-300 psog 0-3 to 0-300 psig 0-100 to 0-500 psig 0-1.5 to 0-300 psig 0-15 to 0-100 psig |
±2% F.S. ±2% F.S. ±0.5% F.S. ±0.5% F.S. ±0.25% F.S. ±2% F.S. ±2% F.S. ±0.5% F.S. ±0.5% F.S. ±2% F.S. ±2% F.S. ±0.3% F.S. ±0.25% F.S. ±0.5% F.S. ±0.08% F.S. ±0.1% F.S. |
35 psig 30 psig 10 psig 5-15 psig varies 30 psig 25 psig 5-150 psig 5-150 psig 1.5 x range 1.5 x range 3 x range 3 x range 2 x range 3 x range 2 x range |
SPECIAL MODELS FOR OEM REQUIREMENTS
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