Pressure Reference Page
Under Pressure

Author

DOFPro group

  • Video links go directly to the YouTube video.
  • JTF (Just the Facts) videos are the streamlined versions: greenscreen LaTeX equations, clean graphics, minimal narrative. Think efficient and to the point. Each has a companion TFS video.
  • TFS (The Full Story) videos include interviews, additional explanation, and equations written on whiteboards. Same math, more context, more personality. Each has a matching JTF version.
  • Info Page links lead to definitions, expanded explanations, and related material—because sometimes you really do need to explain it.
  • Visuals links contain the greenscreen or whiteboard materials used in the video, for those who like to see the scaffolding.
  • Wondering about the titles? See Appendix B: If you have to explain it, it’s no longer funny.
  • Videos marked This is NOT a DOFPro video were not produced by DOFPro but are included because they are relevant to the topic. They are shown in red so no one calls the academic integrity police.

Purpose of Under Pressure

After temperature, pressure is one of the most commonly measured and reported quantities in chemical and thermal processes. The concept of pressure is fundamental to modeling, designing, and monitoring those processes.

The Part 1 and Part 2 videos explain most of the background and practical information that a practicing engineer needs regarding pressure.

Part 1 discusses what pressure is—force per unit area—how it is used in engineering calculations, and several common methods for measuring it.

Part 2 continues the discussion with atmospheric pressure, barometric pressure, gauge pressure, absolute pressure, and vacuum pressure.

For those who caught the reference in the title, here is a link to the video.

Part 1 Just The Facts

The Just the Facts video explains the basics of pressure, hydrostatic pressure, the different units used to measure pressure, and how pressure is used in engineering calculations.

Visuals

Part 1 The Full Story

The Full Story video examines common conceptions and misconceptions about pressure before explaining the fundamentals of pressure, hydrostatic pressure, pressure units, and how pressure is used in calculations.

Visuals

Part 2 Just The Facts

The Just the Facts video explains the differences among atmospheric pressure, barometric pressure, gauge pressure, absolute pressure, and vacuum pressure, and shows how these quantities are used in calculations.

Visuals

Part 2 The Full Story

The Full Story video examines common conceptions and misconceptions about atmospheric pressure, barometric pressure, gauge pressure, absolute pressure, and vacuum pressure before explaining how these different pressure definitions arise and how they are used in engineering practice.

Visuals

Examples and Definitions

Definitions

Pressure
Normal force per unit area. The SI unit is the Pascal (Pa), defined as \(1\ \mathrm{N/m^2}\).
Pascal
The SI unit of pressure, equal to \(1\ \mathrm{N/m^2}\).
Torr or mmHg
The hydrostatic pressure generated by a column of mercury 1 mm high. Torr and mmHg differ by less than one part in seven million and are usually treated as equivalent.
\(1\ \mathrm{Torr} = 133.3224\ \mathrm{Pa}\)
Inch of Mercury
The hydrostatic pressure generated by a column of mercury 1 inch high.
\(1\ \mathrm{inHg} = 3386.389\ \mathrm{Pa}\)
Atmosphere or Standard Atmosphere
The standard reference atmospheric pressure at sea level.
\(1\ \mathrm{atm} = 101{,}325\ \mathrm{Pa}\)
Standard Atmospheric Model
A model describing how atmospheric properties vary with altitude. Two commonly used models are the International Standard Atmosphere and the U.S. Standard Atmosphere.
Kilopascal
A unit equal to \(1000\ \mathrm{Pa}\).
Bar
A unit of pressure equal to \(10^5\ \mathrm{Pa}\).
Pound per Square Inch
A unit of pressure equal to one pound-force per square inch of area.
\(1\ \mathrm{psi} = 6894.757\ \mathrm{Pa}\)
Foot of Water
The hydrostatic pressure generated by a column of water one foot high at \(39.2^\circ\mathrm{F}\).
\(1\ \mathrm{ftH_2O} = 2989.98\ \mathrm{Pa}\)
Inch of Water
The hydrostatic pressure generated by a column of water one inch high at \(39.2^\circ\mathrm{F}\).
\(1\ \mathrm{inH_2O} = 249.082\ \mathrm{Pa}\)
Meter of Water
The hydrostatic pressure generated by a column of water one meter high at \(4^\circ\mathrm{C}\).
\(1\ \mathrm{mH_2O} = 9806.38\ \mathrm{Pa}\)
Hydrostatic Pressure
The pressure difference between two points in a fluid caused by the weight of the fluid column between them. For a constant-density fluid,

\[ \Delta P = \rho g h \]

Atmospheric Pressure
The local pressure caused by the column of air above a given location.
Absolute Pressure
Pressure measured relative to a perfect vacuum.
Gauge Pressure
Pressure measured relative to the local atmospheric pressure.

\[ P_{\mathrm{abs}} = P_{\mathrm{gauge}} + P_{\mathrm{atm}} \]

The ideal gas law must always be used with absolute pressure.

Barometric Pressure
The atmospheric pressure measured by a barometer.
Differential Pressure Gauge
A device that measures the difference between two pressures.
Absolute Pressure Gauge
A gauge that measures pressure relative to a vacuum reference.
Gram Mole or g-mol
A gram mole is what chemists call a mole. See What Is a Mole for further explanation.
Troposphere
The lowest region of Earth’s atmosphere, extending to roughly 8 miles (13 km) altitude.