The pound per square inch (psi) is a unit of pressure or stress based on avoirdupois units. It represents the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6,895 pascals.
There are different variations of psi used to specify pressure readings: pounds per square inch absolute (psia) and pounds per square inch gauge (psig). Psia indicates pressure relative to a vacuum, while psig indicates pressure relative to atmospheric pressure. For instance, atmospheric pressure at sea level is around 14.7 psi (101 kilopascals), which is added to any pressure reading made in air at sea level.
The SI system, known as the International System of Units, uses the pascal as the fundamental unit of pressure. The pascal is defined as the pressure exerted by a force of one newton perpendicularly on an area of one square meter.
Understanding the difference between psia and psig is crucial in various applications. For example, when a tire is pumped to 65 psig in local atmospheric pressure at sea level, it will have a pressure of 79.7 psia (14.7 psi + 65 psi).
In technical writing, it is recommended to use modifiers like “gauge” or “absolute” when denoting pressures in kPa to avoid confusion. For instance, instead of using kPa(g) or kPa(a), it is advised to write gauge pressures as 220 kPa and absolute pressures as 321 kPa. This distinction ensures clarity in communicating pressure measurements across different contexts.
Overall, the pound per square inch is a vital unit for measuring pressure and stress, commonly used in various fields such as engineering, physics, and meteorology. Understanding its different forms and applications is essential for accurate pressure measurements and calculations.
Citations:
[1] https://en.wikipedia.org/wiki/Pound_per_square_inch
The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in2), is a unit of measurement of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately 6,895 pascals.
| Pound per square inch | |
|---|---|
 A pressure gauge reading in psi (red scale) and kPa (black scale) | |
| General information | |
| Unit system | Imperial units, US customary units |
| Unit of | Pressure, stress |
| Symbol | psi, lbf/in2 |
| Conversions | |
| 1 psi in ... | ... is equal to ... |
| SI units | 6.894757 kPa |
The pound per square inch absolute (psia) is used to make it clear that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi (101 kilopascals), this will be added to any pressure reading made in air at sea level. The converse is pound per square inch gauge (psig), indicating that the pressure is relative to atmospheric pressure. For example, a bicycle tire pumped up to 65 psig in a local atmospheric pressure at sea level (14.7 psi) will have a pressure of 79.7 psia (14.7 psi + 65 psi). When gauge pressure is referenced to something other than ambient atmospheric pressure, then the unit is pound per square inch differential (psid).