Pressure measurement

Pressure measurement is the process of determining the force exerted by a fluid—liquid or gas—per unit area on a surface. Because pressure plays a central role in science, engineering, industry and everyday life, a wide range of instruments, techniques and reference systems have been developed to measure it accurately under different conditions. The most familiar devices include mechanical pressure gauges such as the Bourdon gauge, but modern practice also uses electronic sensors that relay readings to remote indicators and control systems.
Pressure can be expressed relative to different reference points. Most routine measurements, such as those for vehicle tyres, use atmospheric pressure as the zero point and produce gauge readings. More specialised applications require absolute pressure referenced to a perfect vacuum, or differential pressure comparing two separate points in a system. Understanding these reference types is essential for interpreting pressure data correctly.

Types of pressure and reference systems

Pressure values are always defined relative to a chosen zero. Three main reference systems are used:
• Absolute pressureAbsolute pressure is referenced to a perfect vacuum and equals the sum of gauge pressure and atmospheric pressure. Absolute sensors are required where a constant reference is essential—for example in monitoring vacuum pumps, high-performance industrial systems, packaging processes, or aviation instruments. An absolute pressure transmitter reads barometric pressure when open to the air.
• Gauge pressureGauge measurements use ambient atmospheric pressure as the zero reference. When a tyre pressure gauge reads zero, it indicates that the internal pressure equals the external atmospheric pressure. Most sensors designed for moderate ranges—up to about 50 bar—use gauge references to avoid errors caused by natural fluctuations in atmospheric pressure.
• Differential pressureDifferential pressure expresses the difference between two pressure points. Differential sensors are used to determine pressure drops across filters, measure fluid levels by comparing pressures above and below a liquid surface, or quantify flow rates by detecting pressure changes across a restriction. Many “gauge” devices are technically differential sensors in which one side is open to the atmosphere.
In industrial settings DP cells perform the subtraction mechanically, eliminating the need to read two gauges separately. Differential pressure gauges typically have two inlet ports, one connected to each measured volume.

Gauge variants: vented and sealed

Gauge-reference sensors come in two forms:
• Vented gauge (vg)A vented gauge sensor exposes the rear of the sensing diaphragm to ambient air via a vented cable or opening. This allows it to track atmospheric pressure changes and display readings relative to them. When vented to the air, such a sensor should read zero.
• Sealed gauge (sg)A sealed gauge sensor contains a fixed reference pressure—usually atmospheric pressure sealed at the time of manufacture or, in some designs, a near-vacuum offset. These sensors are used at high pressures such as in hydraulic systems, where atmospheric fluctuation is negligible. Because the reference is fixed, a sealed gauge sensor rarely reads exactly zero.
Absolute reference sensors seal a high vacuum behind the diaphragm so that all readings are measured above total vacuum.

Mechanical and electronic instruments

Traditional pressure gauges rely on the deformation of mechanical components:
• Bourdon gauge—the most widely used mechanical pressure gauge—uses a curved tube that straightens as pressure increases, moving a pointer across a dial.• Vacuum gauges measure pressures below atmospheric pressure and are common in systems needing partial or deep vacuum.• Compound gauges measure both positive and negative (vacuum) pressures on the same instrument.
Electronic pressure sensors convert pressure-induced deformation into electrical signals. These can be transmitted to control systems, enabling telemetry, automation and remote monitoring. Such sensors are standard in industrial control, aviation, and scientific research.

Interpretation of measurements

Pressure readings depend on context. Moderate vacuum readings can be ambiguous without a clear indication of whether they represent gauge or absolute values. For example, a vacuum of 26 inHg (gauge) corresponds to an absolute pressure of 4 inHg, obtained by subtracting the gauge value from the approximate atmospheric pressure of 30 inHg.
Atmospheric pressure varies with altitude and weather. Because gauge measurements are referenced to external pressure, a fluid at constant absolute pressure will show a higher gauge pressure at higher altitudes where atmospheric pressure is lower. This explains why tyre pressures appear to rise when a car ascends a mountain, even though the absolute pressure inside the tyre remains essentially unchanged.
Units frequently include suffixes—psig (gauge), psia (absolute), psid (differential). Although common, such suffixes are discouraged by some standards organisations in favour of clearer notation.

History of pressure understanding

Pressure was often overlooked in antiquity, but early philosophers speculated about it. In the sixth century BCE, Anaximenes suggested that variations in pressure and density of air produced observable changes in matter. Although not a scientific statement in the modern sense, it recognised a link between compression, expansion and material properties.
A major breakthrough came in the seventeenth century. Evangelista Torricelli, working with mercury, devised an experiment demonstrating that air has weight and exerts pressure. By inverting a mercury-filled tube into a dish, he created a partial vacuum at the top, showing that atmospheric pressure supports the mercury column. This was the first known barometric experiment and the conceptual origin of pressure gauges.
Blaise Pascal extended Torricelli’s work by comparing pressures at different altitudes. Experiments on a mountain showed that the mercury column height decreased with elevation, confirming that atmospheric pressure diminishes with height.
These early demonstrations established pressure as a measurable physical quantity, paving the way for developments in gas laws, fluid mechanics, and atmospheric science.

Units of pressure

The pascal (Pa) is the SI unit of pressure, defined as one newton per square metre (N·m⁻²). This designation was adopted in 1971; earlier SI descriptions used N/m² without a special name. Absolute pressure is sometimes indicated explicitly, for example as 101 kPa (abs).
Other common units include:
• pounds per square inch (psi)—widely used in North America, especially for tyre pressures• bar, millibar, and kilopascal for meteorology and engineering• mmHg, inHg, and other manometric units derived from the height of liquid columns in manometers
Manometric units originate from early pressure measurements using columns of water or mercury; mercury’s high density allows relatively short columns for a given pressure.