Owing to the adherence of the United States, Liberia and Myanmar to the imperial system of measurement, scientists around the world find themselves having to convert between units in that system and units in the much more prevalent SI, or metric, system. When measuring air pressure, hydraulic pressure and quantities related to stress tolerance in metals, that means converting between pounds per square inch (PSI), which are imperial units, and Pascals, which are SI units. The Pascal is actually a very small unit compared to the PSI, so when measuring in SI units, it's more commonplace to measure in kilopascals (kPa). One kilopascal is equal to 1,000 Pascals.
What Is a Kilopascal (kPa) Exactly?
The Pascal (Pa) is named after Blaise Pascal, who was a French scientist and mathematician who worked extensively on problems concerning hydraulics. Pressure is equal to force per unit area, and in the metric system, force is measured in Newtons (in honor of Sir Isaac Newton). The unit of length is the meter, making the unit of area the meter2, so one Pascal is equal to one Newton per meter2 (N/m2).
Similarly, PSI is the force in pounds per unit of area in square inches, and so any lbs to PSI conversion calculator would need additional information about the area in order to complete the calculation.
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The Pascal is a small unit when compared with other pressure units, such as the PSI or the bar. The pressure exerted by 1 pound of force spread over an area of 1 square inch is equivalent to 6,895 Newtons spread over 1 square meter. In other words, 1 PSI = 6,895 Pa. To convert PSI to bar, remember that a bar is approximately the atmospheric pressure at sea level. It's equal to 14.6 PSI. That makes 1 bar equal to 100,667 Pa.
Because the Pascal is so small, it's reserved for low pressure measurements, such as the pressure differences in ventilation systems. Most measurements are easier to quantify by using the kilopascal (103 Pa) or the megapascal (106 Pa). The kilopascal is easiest to use for atmospheric measurements, because it provides for readings in the hundreds. For example, 1 bar = 100.7 kPa. The megapascal is more appropriate for systems with extremely large pressures, such as hydraulic systems.
Converting PSI to kPa
To convert PSI to kPa, remember that 1 PSI = 6,895 Pascals. Since there are 1,000 Pascals in a kilopascal:
1 PSI = 6.895 kPa and 1 kPa = 0.145 PSI.
Example Conversion Problems
(1) The air pressure in an inflated tire is about 33 PSI. What is that pressure in bars and in kPa?
To convert PSI to bar, remember that 1 bar equals 14.6 PSI. That means 1 PSI = 1/14.6 = 0.068 bar. It follows that 33 PSI = 33 × 0.068 bars = 2.26 bars.
To convert to PSI to kPa, remember that 1 PSI = 6.895 kPa. That means that 33 PSI = 227.54 kPa.
(2) The air pressure at the top of Mt. Everest is 33.7 kPa. What is that in PSI and bars?
Keeping in mind that 1 kPa = 0.145 PSI, you can find that 33.7 kPa = 33.7 × 0.145 PSI = 4.89 PSI.
Since 1 PSI = 0.068 bar, 4.89 PSI must equal 4.89 × 0.068 bar = 0.333 bar.
The atmospheric pressure at the top of Mt. Everest is 4.89 PSI or 0.333 bar. It's no wonder people who make it there need oxygen masks.