Pressure is a fundamental physical quantity defined as force applied perpendicular to the surface of an object per unit area. In engineering and scientific research, two common units encountered are the pound per square inch (psi) and the Torr.
The psi is a non-SI unit of pressure derived from the avoirdupois system, commonly used in the United States for industrial applications such as tire pressure, hydraulic systems, and gas pipelines. Mathematically, it is defined as \(1 \text{ psi} = \frac{1 \text{ lbf}}{1 \text{ in}^2}\).
The Torr is a unit of pressure based on an absolute scale, defined as exactly \(\frac{1}{760}\) of a standard atmosphere. It is named after Evangelista Torricelli and is the standard unit used in vacuum engineering and high-vacuum physics. The relationship between these units is defined by the conversion factor \(1 \text{ psi} \approx 51.7149 \text{ Torr}\).
PSI Applications: Automotive maintenance, HVAC systems, and pneumatic tools.
Torr Applications: Vacuum chambers, semiconductor manufacturing, and mass spectrometry.
Pound per square inch to Torr Conversion Reference Table
Pound per square inch (psi)
Torr (Torr)
0.1
5.1715
0.5
25.8575
1.0
51.7149
2.0
103.4298
5.0
258.5746
10.0
517.1492
20.0
1034.2984
50.0
2585.7459
100.0
5171.4918
500.0
25857.4591
1000.0
51714.9181
Conversion Example: 10 psi to Torr
To convert pressure from psi to Torr, multiply the given value by the conversion factor of 51.71491812328611. Follow these steps:
Identify the pressure in psi: \(P_{\text{psi}} = 10\).
Apply the conversion formula: \(P_{\text{Torr}} = P_{\text{psi}} \times 51.71491812328611\).
The Torr is preferred in vacuum science because it is based on the mercury barometer scale, which is more intuitive when measuring pressures significantly lower than atmospheric pressure. Using psi for high-vacuum applications would result in extremely small, cumbersome decimal values.
The conversion factor itself is a constant ratio between two units of pressure. However, in practical engineering, gas pressure is often temperature-dependent according to the Ideal Gas Law. Ensure you are accounting for thermal expansion or gas density changes if you are calculating pressure changes in a closed system.
"On fait la science avec des faits, comme on fait une maison avec des pierres ; mais une accumulation de faits n'est pas plus une science qu'un tas de pierres n'est une maison." "Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house." — Henri Poincaré (French Mathematician, Theoretical Physicist & Mining Engineer)