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Differential Pressure Applications

Differential pressure as the name implies, is a reading of the difference between 2 pressures.
Hence a differential instrument will always have two inputs.  A differential instrument can be used to measure ‘gauge’ pressure by opening one port to the air such that the pressure on the second port is referenced against atmospheric pressure.

These application examples are sourced from Dwyer Instruments “Engineering and Applications Handbook”, a full copy of which can be downloaded from our website at this link.
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Gauges indicate pressurisation of special rooms
A zero-center Series 2000 Magnehelic® differential pressure gauge with a 60 Pa range either side of zero makes an effective monitor for proper operation of room pressurisation systems. In the example, differential gauge B has its high pressure port open to room 2 and its low pressure port to room 1; gauge A has its high pressure port open to room 1 and its low pressure port open to the atmosphere. With the makeup air supply damper adjusted properly, room 2 will be a higher pressure than room 1 which is at higher than atmospheric pressure; both gauges will read positive. Should the air supply to room 2 be obstructed, gauge B will read negative. If the air supply fails entirely, both gauges will read zero.
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Zero-center switch/gauge controls the inert atmosphere in glove box
A controlled inert atmosphere “glove-box” is used in the fields of physical chemistry and metallurgy for handling and welding special or hazardous materials. A Series A3000 Photohelic® differential pressure switch/gauge serves as an automatic and readily adjustable pressure control for the helium, argon or nitrogen gas used in the system. The box is first evacuated, then pressurised with the required gas. Therefore, a zero-center Photohelic® switch/gauge is used, permitting both pressure and vacuum to be read and controlled by a single gauge. 
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Filter Monitoring
A Magnehelic air filter gauge instantly reveals when filters need servicing. Select a Magnehelic gauge model with range appropriate to the filter manufacturer’s recommendation for maximum permissible pressure drop across the filter. Install the gauge with a red adjustable signal flag (optional) set at the maximum allowable pressure drop point for the air volume being handled.
Portable kits complete with static tips, tubing and hardware are available.
Instead of the Magnehelic gauge, the Photohelic differential pressure switch/gauge or a differential pressure switch will automatically actuate an audible or visual alarm when filter needs servicing.
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Static Pressure Monitoring
The existence of a static pressure above or below a required level (compared to atmospheric) is easily determined by connecting an appropriately installed static pressure tip to the appropriate port of a differential pressure switch, such as a Dwyer Series 1800, 1900 or ADPS differential pressure switches.
When the pressure reaches the pre-set switch point, the contacts are closed to activate a warning lamp or other alarm device. It is good practice to mount the differential pressure switch with the ports down to prevent moisture from entering the diaphragm chamber. The diaphragm should be vertical and in the same plane as any vibration or anticipated shock movement.
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