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 to 50 psig. A wide selection of Dwyer precision-pressure switch models are offered 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 resets 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. 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 resets when pressure drops.

Considerations in Selecting a Pressure Switch

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 set points 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°F (-30°F 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.

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 appropriate 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 MS or MS2 Magnesense® 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 tube 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 Pitot tube array and transmitter.

Glossary of Switch Terms:

Set or Actuation Point - The exact air pressure or flow rate which will cause the electric switch to actuate.

Rated Pressure - The maximum pressure that the actuating components of the switch in contact with the media can withstand continuously and/or repeatedly without risk of permanent damage.

Maximum Surge Pressure - Safe pressure for the switch housing but which may damage the mechanism by continuous or repetitive application.

Normally Open Switch - A switch in which the contacts are normally open. Actuation closes the contacts.

Normally Closed Switch - A switch in which the contacts are normally closed. Actuation opens the contacts.

Single Pole Double Throw (SPDT) Switch - A switch combining both normally open and normally closed switch contacts.

Null Switch - A floating contact switch with a zone of no contact. Often used to operate reversible motors.

Repetitive Accuracy - The ability of a pressure or flow switch to operate repetitively at its set point under consistent conditions.