Why use psi instead of just pound?

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In summary, the pressure in a compressor is measured in PSI (pounds per square inch) instead of just pounds, because of how the pressure changes with respect to the amount of area the force is applied over. If the pressure of a compressor is 10 PSI, using a 10 sq in nozzle would cause the pressure to drop to 1 PSI assuming the total force remains unchanged. This is because the formula for pressure is P = F/A, and as the area increases, the pressure decreases.
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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.
 
  • #3
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.
 
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  • #4
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.
 
  • #5
Right, and the OP was describing a compressor, in which output is controlled to constant pressure, not constant force.
 

1. Why use psi instead of just pound?

Psi stands for "pounds per square inch," and it is a unit of pressure commonly used in scientific and engineering applications. It is used to measure the amount of force exerted on a surface per unit area, which is an important consideration in many fields. Pounds, on the other hand, are a unit of weight and do not take into account the area over which the force is distributed. Therefore, psi is a more accurate and precise unit for measuring pressure.

2. Is psi a standard unit of measurement?

Yes, psi is a standard unit of measurement in the International System of Units (SI) and is commonly used in the United States. It is also recognized by the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI).

3. What are some common applications of psi?

Psi is commonly used in industries such as automotive, aerospace, and construction to measure tire pressure, engine compression, hydraulic systems, and building materials' strength. It is also used in meteorology to measure atmospheric pressure and in medical devices to measure blood pressure.

4. How do you convert psi to pounds?

Psi can be converted to pounds by multiplying the psi value by the area over which the force is applied. For example, if a force of 100 pounds is exerted over an area of 10 square inches, the psi would be 10 psi (100 pounds / 10 square inches). Conversely, to convert pounds to psi, you would divide the pounds by the area.

5. Why is psi a more practical unit for measuring pressure?

Psi is a more practical unit for measuring pressure because it takes into account the surface area over which the force is applied. This is important because the same amount of force can have a different impact depending on the area it is distributed over. For example, a small nail and a large hammer may both have the same amount of force, but the nail will exert more pressure on a surface due to its smaller area. Psi allows for more accurate and precise measurements in various applications.

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