Pressure
Types of Pressure and Pressure Units Explained
Pressure fundamentals
Pressure is a basic mechanical quantity used to describe how strongly a force is distributed over a surface. It appears in fluids, gases, solids, vacuum systems, weather data, hydraulic equipment, pneumatic circuits, and many measurement instruments. Before comparing the main types of pressure, it is useful to separate the physical idea of pressure from the reference used to report a pressure reading.
Pressure as force per unit area
Pressure is the force applied normal, or perpendicular, to a surface divided by the area over which that force acts:
\[ P = \frac{F}{A} \]where:
- \(P\) is pressure,
- \(F\) is the resultant force acting normal to the surface,
- \(A\) is the surface area receiving that force.
The same force produces a higher pressure when it is applied over a smaller area. This is why a sharp point can penetrate a surface more easily than a blunt object under the same load.
A simple fluid example is atmospheric pressure acting on the surface of a liquid. Air above the liquid has weight, and that weight produces a force over the liquid surface. The liquid does not need to be moving for pressure to exist; the pressure comes from the force distributed over the area.
How matter produces pressure
In solids and liquids, pressure can often be visualized as the result of weight acting through material onto a supporting or enclosing surface. A block resting on a table has mass. Gravity gives that mass weight, and the combined downward force is transmitted to the table over the contact area. A deeper layer of liquid similarly carries the weight of the liquid above it, so pressure increases with depth.
Gas pressure is produced differently. A gas does not behave like a stack of fixed layers in the same simple way. Gas molecules move continuously in many directions. When they strike a wall, piston, sensor diaphragm, or container surface, they transfer momentum during each collision. A single molecular impact is tiny, but the combined effect of enormous numbers of impacts creates a measurable pressure.
Only the component of these impacts normal to the surface contributes to pressure on that surface. This is why pressure is treated as force per unit area even though, at the molecular scale, the process is a collection of individual collisions.
Main pressure reference types
Most confusion about pressure readings comes from the reference point. A pressure value is not fully defined unless the reader knows what it is being compared against. The three core pressure reference types are absolute pressure, gauge pressure, and differential pressure.
How pressure types differ by reference point
The main pressure reference types are:
- Absolute pressure
- Gauge pressure, also called relative pressure
- Differential pressure
The practical difference is the zero or comparison point used for the measurement.
Absolute pressure is referenced to a perfect vacuum. Gauge pressure is referenced to local or standard atmospheric pressure. Differential pressure compares one pressure point with another pressure point in the same or a related process.
These reference choices change the meaning of the same numerical value. For example, 2 bar absolute and 2 bar gauge are not the same physical pressure. If atmospheric pressure is approximated as 1 bar, then 2 bar gauge is about 3 bar absolute.
Differential pressure should also be interpreted carefully. It may be stated as a signed value, such as \(P_1 - P_2\), or as a magnitude, depending on the convention. A differential pressure value is not automatically positive unless the direction or high-side/low-side convention has been defined.
Absolute pressure
Absolute pressure is pressure measured relative to a perfect vacuum. In this reference system, perfect vacuum is zero pressure.
A perfect vacuum is a theoretical closed volume with all particles removed. It is used as the zero reference because it represents the lowest possible pressure state. Since pressure below perfect vacuum is not physically meaningful, absolute pressure is nonnegative.
Common notation examples include:
- Pa absolute
- bar absolute
- psi absolute, often written as psia in some industries
The notation is not merely decorative. It tells the reader that the pressure value includes atmospheric pressure if the measurement point is open to the atmosphere. For example, standard atmospheric pressure is about 1013.25 mbar absolute, or 14.696 psia.
Absolute pressure is especially important when gas laws, vapor pressure, boiling point, vacuum quality, and density calculations are involved. These relationships depend on the total pressure relative to vacuum, not simply on pressure above atmosphere.
Gauge pressure, or relative pressure
Gauge pressure is pressure measured relative to atmospheric pressure rather than perfect vacuum. A gauge pressure reading of zero means the measured pressure is equal to the atmospheric pressure used as the reference.
At sea level, standard atmospheric pressure is commonly given as:
- 1013.25 mbar
- 14.696 psi
Because gauge pressure uses atmosphere as its reference, it can be positive, zero, or negative.
Positive gauge pressure is called overpressure. It means the measured pressure is higher than atmospheric pressure:
\[ P_\text{gauge} = P_\text{absolute} - P_\text{atmospheric} \]For example, a compressed-air receiver operating above ambient pressure is normally described in gauge pressure.
Negative gauge pressure is called underpressure or partial vacuum. It means the measured absolute pressure is below atmospheric pressure. When expressing vacuum as a magnitude, the sign may be omitted if the reference is clear. For example, if atmospheric pressure is approximated as 1 bar and a vessel is at 0.8 bar absolute, it may be described as 0.2 bar vacuum or 0.2 bar underpressure. That wording means the vessel pressure is 0.2 bar below atmospheric pressure, not below absolute zero.
Gauge pressure is widely used in industrial pressure gauges because many practical systems are concerned with pressure relative to the surrounding atmosphere: tire pressure, hydraulic pressure, pneumatic pressure, and many process pressure readings.
Differential pressure
Differential pressure is the pressure difference between two measurement points:
\[ \Delta P = P_1 - P_2 \]Neither point has to be a perfect vacuum or standard atmosphere. Both may be process points, such as the upstream and downstream sides of a filter, valve, pump, heat exchanger, or flow restriction.
Absolute and gauge pressures are also comparisons in a broad sense. Absolute pressure compares a point with vacuum, and gauge pressure compares a point with atmosphere. In common technical use, however, differential pressure usually means a comparison between two process connections.
Differential pressure alone does not reveal the absolute pressure at either point. A differential pressure of 50 kPa could occur between 300 kPa and 250 kPa, between 1000 kPa and 950 kPa, or between many other pairs of pressures. The differential value only tells the difference.
It also does not identify which side is higher unless direction is specified. A drawing, sign convention, high-side/low-side label, or connection convention is needed. Without that information, “50 kPa differential” may be only a magnitude.
Named pressure cases and fluid-pressure terms
Some pressure terms describe physical situations rather than measurement references. Vacuum pressure, atmospheric pressure, hydrostatic pressure, and dynamic pressure are common examples. These names are useful, but they do not replace the core reference types of absolute, gauge, and differential pressure.
Other pressure names versus formal pressure types
The formal reference types are absolute, gauge, and differential pressure. Other names usually describe where the pressure comes from or how it behaves.
Common named pressure cases include:
- Vacuum pressure
- Atmospheric pressure
- Hydrostatic pressure
- Dynamic pressure
Each can still be reported on an absolute, gauge, or differential basis when appropriate. For example, a vacuum chamber may be reported as 50 mbar absolute or as 963 mbar below standard atmosphere. Hydrostatic pressure in a tank may be treated as a gauge pressure due to liquid head, or the total pressure may be expressed on an absolute basis by adding atmospheric pressure above the liquid.
The key is to distinguish the phenomenon from the reference. “Hydrostatic” describes pressure caused by a fluid column. “Gauge” describes the reference used to report the value.
Vacuum pressure
A perfect vacuum is zero absolute pressure with no particles present. It is a theoretical ideal rather than a normal practical condition. In real systems, vacuum usually means pressure below atmospheric pressure.
A high vacuum corresponds to very low absolute pressure. The word “high” in this context refers to the quality or degree of vacuum, not to high pressure. A high-vacuum system has removed most of the gas particles from the volume.
Vacuum pumps reduce pressure by removing gas particles from a closed or partly closed volume. As particles are removed, fewer molecular collisions occur against the internal surfaces, so the absolute pressure decreases.
One common industrial design is the liquid ring vacuum pump. At a high level, this pump uses an eccentric impeller and a sealing liquid ring. Rotation forms a moving liquid ring that creates changing volumes between the impeller blades. Gas is drawn into these spaces, compressed as the volume decreases, and discharged. The sealing liquid helps maintain the pumping action and remove heat, depending on the design.
Vacuum can be expressed in several ways, so the reference must be clear. A low absolute pressure, a negative gauge pressure, and a positive “vacuum magnitude” may describe the same condition using different conventions.
Atmospheric pressure
Atmospheric pressure, also called barometric pressure, is the pressure caused by the weight of air in the atmosphere. Air has mass, and gravity pulls it toward Earth. The weight of the air column above a location produces pressure at that location.
Atmospheric pressure is not constant. It changes with altitude, temperature, humidity, air density, and weather systems. At higher elevations, there is less air above the measurement point, so atmospheric pressure is lower. Weather patterns also cause local barometric pressure changes.
Atmospheric pressure matters directly for gauge pressure because it is the gauge-pressure reference. A gauge instrument vented to the atmosphere measures pressure relative to the surrounding air pressure. If the atmospheric reference changes, the relationship between gauge pressure and absolute pressure changes as well.
For clarity, standard atmosphere is commonly stated as:
- 1013.25 mbar absolute
- 14.696 psia
These values are reference values, not a guarantee of the actual atmospheric pressure at a particular place and time.
Hydrostatic pressure
Hydrostatic pressure is the pressure at depth in a fluid caused by the fluid column above the measurement point. It is especially important in tanks, reservoirs, wells, and level measurement.
The hydrostatic component depends on:
- fluid density, \(\rho\)
- gravitational acceleration, \(g\)
- liquid-column height, \(h\)
For a stationary liquid, the hydrostatic component is commonly written as:
\[ P_\text{hydro} = \rho g h \]When this expression is used by itself, it normally represents the pressure due to liquid head above the point. In an open tank, that is often treated as a gauge pressure because the liquid surface is exposed to atmosphere.
To express total pressure on an absolute basis, atmospheric pressure above the liquid surface must be included:
\[ P_\text{tot} = P_\text{atm} + \rho g h \]This distinction is important in level measurement. A transmitter near the bottom of a vented tank may measure gauge pressure due to liquid height. In a sealed or pressurized tank, the gas pressure above the liquid must also be considered, often by using differential pressure measurement.
Hydrostatic pressure is therefore a named pressure case, not a separate reference type. It can be interpreted as gauge, absolute, or differential depending on the measurement setup.
Dynamic pressure
Dynamic pressure is the kinetic-energy-related pressure term in Bernoulli’s equation. It is associated with fluid motion, such as liquid or gas flowing through a pipe, duct, nozzle, or around an object.
For steady, incompressible flow along a streamline under the usual simplifying assumptions, Bernoulli’s equation relates pressure energy, kinetic energy, and elevation-related potential energy. The dynamic pressure term is:
\[ q = \frac{1}{2}\rho v^2 \]where:
- \(q\) is dynamic pressure,
- \(\rho\) is fluid density,
- \(v\) is flow speed.
Dynamic pressure increases with density and with the square of velocity. Doubling flow speed increases dynamic pressure by a factor of four, all else equal.
Dynamic pressure is not another reference type like absolute, gauge, or differential pressure. Instead, it is a pressure term used in flow analysis. In instruments such as Pitot tubes, a measured pressure difference can be related to dynamic pressure and therefore to flow velocity.
Pressure units and reference notation
Pressure units vary by region, industry, and historical practice. The unit tells the scale of the pressure value, while the reference notation tells how the value should be interpreted. Both are needed for unambiguous technical communication.
Pressure units used in different regions
The SI unit of pressure is the pascal:
\[ 1 \text{ Pa} = 1 \text{ N/m}^2 \]Because one pascal is small for many industrial applications, derived units such as kPa and MPa are common. The bar is also widely used in industry even though the pascal is the SI pressure unit:
\[ 1 \text{ bar} = 100{,}000 \text{ Pa} \]A useful approximate relationship is:
\[ 1 \text{ bar} \approx 14.5 \text{ psi} \]Regional and industry usage often follows established practice:
| Unit or unit family | Common use |
|---|---|
| Pa, kPa, MPa | SI-based engineering, scientific, and industrial measurement |
| bar, mbar | Industrial pressure, process systems, vacuum ranges, meteorology |
| psi | Common in the United States and in some UK legacy or industry contexts |
| kg/cm² | Still encountered in some Asian industrial contexts and older equipment |
| atm | Reference and scientific use related to standard atmosphere |
The European industrial environment commonly uses SI units and bar. The United States still uses psi widely for pressure gauges and equipment specifications. The United Kingdom often encounters both psi from legacy practice and bar in modern industrial use. In Asian industrial settings, MPa and kg/cm² may be encountered depending on country, sector, and equipment origin.
When converting pressure units, the reference type must remain attached. Converting 5 bar gauge to psi does not turn it into absolute pressure; it remains gauge pressure unless atmospheric pressure is added.
How to show the pressure reference with a unit
A pressure value should state whether it is absolute, gauge, or differential. Without that information, the same number can be misread.
Clear written forms include:
- 2 bar gauge
- 300 kPa absolute
- 15 psi differential
- 50 mbar absolute
- 10 kPa differential across the filter
Common suffix conventions are also seen in drawings, datasheets, and instrument displays:
- g for gauge, as in barg or psig
- a for absolute, as in bara or psia
- d for differential, as in kPaD or psid
These suffixes are conventions rather than a substitute for clear documentation. In formal specifications, written labels such as “bar gauge” or “kPa absolute” reduce ambiguity, especially when multiple pressure references appear in the same system.
Relying only on context can cause errors. For example, a vacuum process specified as “100 mbar” may mean 100 mbar absolute, but a reader could mistakenly treat it as 100 mbar gauge if the reference is not stated. In compressed-air systems, “7 bar” usually means gauge pressure in everyday plant language, but calculations involving gas density may require the corresponding absolute pressure.
A complete pressure statement should therefore include both the unit and the reference. The unit gives the scale; the reference tells what zero means.