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Why use psi instead of just pound?

by david90
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david90
#1
Dec27-12, 12:56 PM
P: 303
Why does the pressure in a compressor have a unit PSI instead pound? If the pressure of a compressor is 10 PSI, would a 10 sq in nozzle cause the pressure to drop to 1 psi?
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mathman
#2
Dec27-12, 05:43 PM
Sci Advisor
P: 6,037
Quote Quote by david90 View Post
Why does the pressure in a compressor have a unit PSI instead pound? If the pressure of a compressor is 10 PSI, would a 10 sq in nozzle cause the pressure to drop to 1 psi?
It depends on what the total force is. If the total force is 10 pounds and you went from a 1 sq. in. nozzle to a 10 sq. in nozzle, the psi would then drop to 1. This assumes the total force is unchanged.
xodin
#3
Dec27-12, 07:53 PM
P: 45
Quote Quote by david90 View Post
Why does the pressure in a compressor have a unit PSI instead pound? If the pressure of a compressor is 10 PSI, would a 10 sq in nozzle cause the pressure to drop to 1 psi?
Yes, pounds is a force in this context and PSI is pounds per square inch, which is called a pressure, or force per unit area. The formula for this is P = F/A and therefore PSI = (Pounds)/(Inch^2). The reason it is PSI instead of just pounds is because of how the pressure changes with respect to the amount of area the force is applied over. Imagine your body weight (a force) being applied over both feet (an area), or while standing on one foot (a smaller area). If, for example, you were standing on a thin piece of plywood or something that could barely hold your weight with both feet, it's quite possible that transitioning to standing on one foot would be enough to snap the board due to the increased pressure--double the pressure to be exact assuming each foot has the same area.

If you were using a compressor, then as mathman pretty much said, the force would almost certainly be what stays constant and the pressure would therefore decrease if you increased the nozzle area. Increase the nozzle area by 10 times, you reduce the pressure by 10 times, and that relationship is implicit in the math of the P = F/A formula.

Lsos
#4
Dec28-12, 04:41 AM
P: 774
Why use psi instead of just pound?

In a lot of instances, the pressure stays constant while the force changes depending on the area, so it's appropriate to have a unit describing this phenomenon.
russ_watters
#5
Dec28-12, 10:46 AM
Mentor
P: 22,237
Right, and the OP was describing a compressor, in which output is controlled to constant pressure, not constant force.


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