Differential Pressure Switch
Differential Pressure Switch
Negative Differential
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A simple vacuum cleaner operates as a powerful blower motor that induces a strong flow in the air moving through it. Since the motor exit is at atmospheric pressure, a vacuum is created at the motor's entrance. There is a negative differential pressure across the motor--low inlet, high outlet.
Positive Differential
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Just ahead of the vacuum nozzle, the dust being collected is at atmospheric pressure. Since the air inside the hose and nozzle is equal to the motor inlet pressure, and lower than the pressure in and around the dust, a positive differential pressure across the nozzle induces a high velocity air flow into it, carrying the dust along with it. The filter traps the dust, but allows the air to continue on its way to the vacuum motor, which then ejects the clean air. Dust accumulating in the filter gradually starts to restrict flow, increasing positive differential pressure across the bag until it is replaced.
Balloons
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Rubber balloons are continuous indicators of differential pressure. Assuming a balloon has no leaks, a slightly higher pressure inside the balloon than outside pushes against the wall of the balloon and overcomes the balloon's elasticity--the more pressure, the bigger the balloon gets.
Lung Breathing
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In the human body, the diaphragm pulling down then up alternates positive and negative differential pressure conditions across the lungs, inducing air flow both in and out of lungs. Elasticity in the lungs, as with balloons, assists with exhalation.
Wind
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Wind is the flow of air due to differential pressure--from a higher pressure area to a low pressure area. There is a large differential pressure condition in a hurricane with the eye being extremely low pressure. This induces a strong flow of air into the bottom of the hurricane wall, which raises the water level inside the eye, creating storm surge as the storm makes landfall. Differential pressure across a sail drives the sailboat on its way.
Measurement
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Differential pressure is used as the primary variable in many instrumentation measurements including flow, level, altitude, velocity, density, composition, and barometric and medical parameters--even the indicators telling you it's time to change that vacuum cleaner bag.
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