Pounds per square inch

A pressure gauge reading in psi (red scale) and kPa (black scale)

The pound per square inch or, more accurately, pound-force per square inch (abbreviations: psi, lbf/in², lb_f/in², lbf/sq in, lb_f/sq in) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch:

1 psi = $\frac{1\text{ lbf}}{(1\text{ in})^2}$ ≈ $\frac{4.4482\text{ N}}{(0.0254\text{ m})^2}$ ≈ 6894.757 N/m²

Therefore, one pound per square inch is approximately 6894.757 Pa.

Now converting the psi to standard atmospheres:

1 atm = $101325\text{ Pa}$ = $\frac{101325}{6894.757293168}$ ≈ 14.70 psi

Therefore, 1 atmosphere is approximately 14.7 pounds per square inch.

Pounds 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, this will be added to any pressure reading made in air at sea level. The converse is pounds per square inch gauge or pounds per square inch gage (psig), indicating that the pressure is relative to atmospheric pressure. For example, a bicycle tire pumped up to 65 psi above local atmospheric pressure (say, 14.7 psia locally), will have a pressure of 65 + 14.7 = 79.7 psia or 65 psig.^[1]^[2] When gauge pressure is referenced to something other than ambient atmospheric pressure, then the units would be pounds per square inch differential (psid).

Multiples

The kilopound per square inch (ksi) is a scaled unit derived from psi, equivalent to a thousand psi.

ksi are not widely used for gas pressures. They are mostly used in materials science, where the tensile strength of a material is measured as a large number of psi.

The conversion in SI Units is 1 ksi = 6.895 MPa

The megapound per square inch (Mpsi) is an other multiple equivalent to a million of psi. It is used in Mechanics for the elastic modulus of the materials, especially for metals.^[3]

The conversion in SI Units is 1 Mpsi = 6.895 GPa

Magnitude

Main article: Orders of magnitude (pressure)

Inch of water: 0.036 psi
Blood pressure – clinically normal human blood pressure (120/80 mmHg): 2.32 psi/1.55 psi
Natural gas residential piped in for consumer appliance; 4-6 psi.
Boost pressure provided by an automotive turbocharger (common): P_g = 6–15 psi
NFL football: P_g = 12.5–13.5 psi
Atmospheric pressure at sea level (standard): P_a = 14.7 psi
Automobile tire overpressure (common): P_g = 32 psi
Bicycle tire overpressure (common): P_g = 65 psi
Workshop or garage air tools: P_g = 90 psi
Air brake (rail) or air brake (road vehicle) reservoir overpressure (common): 90 psi ≤ P_g ≤ 120 psi
Road racing bicycle tire overpressure: P_g = 120 psi
Steam locomotive fire tube boiler (UK, 20th century): 150 psi ≤ P_g ≤ 280 psi
Union Pacific Big Boy steam locomotive boiler: 300 psi
Natural gas pipelines: 800 to 1000 psi
Full SCBA (self-contained breathing apparatus) for IDLH (non-fire) atmospheres: 2216 psi
Full SCUBA (self-contained underwater breathing apparatus) tank overpressure (common): P_g = 3000 psi
Full SCBA (self-contained breathing apparatus) for interior firefighting operations: 4500 psi
Airbus A380 hydraulic system: 5000 psi
Ultimate strength of ASTM A36 steel: 58,000 psi
Water jet cutter: 40,000–100,000 psi

Conversions

Pressure units
	Pascal	Bar	Technical atmosphere	Standard atmosphere	Torr	Pounds per square inch
	(Pa)	(bar)	(at)	(atm)	(Torr)	(psi)
1 Pa	≡ 1 N/m²	10⁻⁵	6995101970000000000♠1.0197×10⁻⁵	6994986919999999999♠9.8692×10⁻⁶	6997750060000000000♠7.5006×10⁻³	6996145037700000000♠1.450377×10⁻⁴
1 bar	10⁵	≡ 100 kPa ≡ 10⁶ dyn/cm²	7000101970000000000♠1.0197	6999986920000000000♠0.98692	7002750060000000000♠750.06	7001145037700000000♠14.50377
1 at	7004980665000000000♠9.80665×10⁴	6999980665000000000♠0.980665	≡ 1 kp/cm²	6999967841100000000♠0.9678411	7002735559200000000♠735.5592	7001142233400000000♠14.22334
1 atm	7005101325000000000♠1.01325×10⁵	7000101325000000000♠1.01325	7000103319999999999♠1.0332	1	≡ 7002760000000000000♠760	7001146959500000000♠14.69595
1 Torr	7002133322399999999♠133.3224	6997133322400000000♠1.333224×10⁻³	6997135955100000000♠1.359551×10⁻³	6997131578900000000♠1.315789×10⁻³	≡ ¹/₇₆₀ atm ≈ 1 mm_Hg	6998193367800000000♠1.933678×10⁻²
1 psi	7003689480000000000♠6.8948×10³	6998689480000000000♠6.8948×10⁻²	6998703069000000000♠7.03069×10⁻²	6998680460000000000♠6.8046×10⁻²	7001517149300000000♠51.71493	≡ 1 lb_F /in²

References

↑ "Glossary of Industrial Air Cleaning Technology". United Air Specialists, Inc.
↑ "Gage v. Sealed v. Absolute pressure" (PDF). Dynisco.
↑ An example of the use of Mpsi in Mechanics for the elastic moduli of several materials

External links

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