In gases and liquids, pressure arises from particle collisions with container walls. Higher temperatures or more particles lead to more frequent collisions and thus greater pressure. Atmospheric pressure surrounds Earth due to the weight of air above, varying with altitude and weather patterns.
Pressure affects daily life in countless ways. Car tires need the correct pressure for safety and fuel efficiency. Blood pressure indicates cardiovascular health. Weather forecasts rely on atmospheric pressure changes to predict storms or clear skies.
Common Units of Pressure
Different fields use various units to express pressure. The International System of Units (SI) establishes the pascal (Pa) as the standard - one pascal equals one newton per square meter.
Other widely used units include:
- Atmosphere (atm) - roughly equal to average sea-level air pressure, about 101,325 Pa.
- Bar - slightly less than one atm, often used in engineering and meteorology.
- Millimeters of mercury (mmHg) - traditional in medicine for blood pressure readings.
- Pounds per square inch (psi) - common in the United States for tire pressure and industrial applications.
- Torr - nearly identical to mmHg, named after Evangelista Torricelli.
- Kilopascal (kPa) - practical for everyday atmospheric readings, since 100 kPa approximates normal air pressure.
Converting between these units proves essential when working across disciplines or regions. An online pressure converter simplifies this process.
How Pressure Gets Measured
Humans have measured pressure since the 17th century. Evangelista Torricelli invented the mercury barometer in 1643, filling a glass tube with mercury and inverting it into a dish. The height of the mercury column indicated atmospheric pressure - typically 760 mm at sea level.
Modern devices fall into several categories based on operating principles.
Mechanical gauges dominate many applications. Bourdon tubes - curved metal tubes that straighten under pressure - move a needle on a dial. These appear in tire gauges and industrial systems.
Manometers use liquid columns to compare pressures. U-tube manometers filled with water or mercury show pressure differences directly as height differences. Open manometers measure gauge pressure against the atmosphere, while closed versions handle absolute pressure.
Electronic sensors offer precision and digital readouts. Piezoelectric crystals generate voltage when deformed by pressure. Strain gauges detect tiny deformations in materials. Capacitive sensors measure diaphragm movement affecting capacitance.
Diaphragm and bellows devices work well for corrosive or high-purity applications where fluid must not contact the mechanism.
Types of Pressure Readings
Measurements distinguish several reference points:
- Absolute pressure - measured against a perfect vacuum, including atmospheric contribution.
- Gauge pressure - relative to local atmospheric pressure, showing zero at sea level open air.
- Differential pressure - the difference between two points, critical in flow measurement and filtration.
- Sealed pressure - referenced to a sealed chamber at a known pressure.
Understanding these distinctions prevents errors. Tire gauges typically read gauge pressure, while altitude instruments may need absolute readings.
Devices for Specific Applications
Barometers track atmospheric pressure changes. Aneroid barometers use metal cells that expand or contract with pressure, moving mechanical linkages. Digital weather stations now combine electronic sensors with temperature and humidity readings.
In medicine, sphygmomanometers measure blood pressure. Modern automatic versions use oscillometric methods - detecting pulse waves in an inflating cuff.
Scuba divers rely on submersible pressure gauges to monitor tank contents. These often integrate with depth gauges since water pressure increases about 1 atm every 10 meters.
Industrial processes employ transmitters converting pressure to electrical signals for control systems. These 4-20 mA devices enable remote monitoring and automation.
Altimeters in aircraft calculate height from pressure changes - pressure drops predictably with altitude.
Pressure in Fluids and Gases
Fluid statics explains hydrostatic pressure: P = ρgh, where ρ is density, g is gravity, and h is depth. This formula shows why dams need thicker bases and deep-sea creatures adapt to immense pressures.
In moving fluids, Bernoulli's principle relates pressure decreases to velocity increases - the basis for airplane lift and carburetors.
Gases follow the ideal gas law: PV = nRT. Compressing air in a pump increases both pressure and temperature, demonstrating the interplay between variables.
Real-World Implications
Pressure knowledge proves vital across fields. Engineers design pressure vessels like boilers and gas cylinders to withstand forces safely. Meteorologists interpret isobars on weather maps.
In health, normal blood pressure ranges around 120/80 mmHg. Hypertension strains organs over time.
Automotive systems balance tire pressure for grip and wear. Underinflation increases rolling resistance and fuel consumption.
Understanding pressure fundamentals helps everyone make informed decisions - from checking home water pressure to interpreting weather reports.