Why use psi instead of just pound?

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Discussion Overview

The discussion revolves around the use of PSI (pounds per square inch) as a unit of pressure in compressors, specifically questioning why pressure is expressed in PSI rather than just pounds. Participants explore the relationship between force, area, and pressure, particularly in the context of nozzle size and its effect on pressure output.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that pressure is defined as force per unit area, hence the use of PSI instead of just pounds.
  • One participant suggests that if the total force remains constant while increasing the nozzle area, the pressure would decrease, illustrating the relationship with the formula P = F/A.
  • Another participant emphasizes that in compressors, the output is typically controlled to maintain constant pressure rather than constant force.
  • There is a discussion about how pressure changes with respect to the area over which the force is applied, using examples to clarify this concept.

Areas of Agreement / Disagreement

Participants generally agree on the definition of pressure and the relationship between force, area, and pressure. However, there are nuances regarding the conditions under which pressure remains constant versus when force changes, indicating some level of disagreement or differing perspectives on the implications of these concepts.

Contextual Notes

Some assumptions regarding the constancy of force and pressure in different scenarios are not fully explored, and the discussion does not resolve the implications of varying nozzle sizes on pressure outputs in all contexts.

david90
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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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david90 said:
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.
 
david90 said:
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.
 
Last edited:
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.
 
Right, and the OP was describing a compressor, in which output is controlled to constant pressure, not constant force.
 

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