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Spray bottles are very simple machines with very few parts, yet they are enormously useful in spreading water, fertilizer, pesticide and other important liquids. They are small manually operated reciprocating piston pumps with an attached reservoir. Like many pumps, spray bottles operate by creating a difference in pressure through the addition of energy to a static system.
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In the case of the spray bottle, the energy added to the system is the force applied to depress the trigger. Through a spring, this force drives a piston, which, as it pushes inward, decreases the volume inside the pump mechanism. The same amount of air and liquid is in the system, however, so the result is an increase in pressure that forces out of the nozzle any liquid that has been sucked into the plastic barrel extending into the reservoir. The first pull on the trigger doesn't usually result in spray because at first there is usually not any water in the barrel.
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What siphons water up into the barrel is the decrease in pressure created when the trigger is released. The expansion of the volume inside the pump draws liquid up the barrel toward the lower pressure area. Thus, one or more cycles on the trigger is required to get water into place before anything comes out the nozzle. Generally, liquid only moves in one direction through the spray bottle, but any liquid left in the barrel will slowly move back into the reservoir if not used.
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The nozzle is a crucial part of the spray bottle mechanism that allows the rest of the pump action to work properly. If air entered as the trigger was released, no change in pressure would occur, and liquid would not rise into the barrel. If an opening was not created for the release of liquid as the trigger was depressed, the increased pressure would only drive the liquid back into the reservoir. A moving valve in the nozzle reacts to the changes in pressure, opening and closing to prevent inflow of air and to allow outflow of sprayed liquid. Screwing the nozzle on tightly locks this valve in the closed position and prevents liquid from spraying out.
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The pump system in a spray bottle is similar to the technology used to pump oil and water from underground. It is also very similar in principal to a heart, which is made of muscular fiber that expands and contracts to create pressure differentials known as systole and diastole that push blood through a circulatory system.
