Pressure
Pressure is a fundamental physical quantity defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed. In metrology and industrial applications, precise pressure measurement is critical for ensuring safety, process efficiency, and structural integrity across various engineering disciplines, from aerospace fluid dynamics to deep-sea exploration.
The Pascal (Pa)
The derived unit of pressure in the International System of Units (SI) is the pascal (Pa). It is defined as one newton of force applied over an area of one square meter.
Because the pascal is a very small unit relative to everyday atmospheric pressures, it is common to use multiples such as kilopascals (kPa) or megapascals (MPa). Other non-SI units frequently encountered in industry include the bar (exactly equal to 100 kPa), pounds per square inch (psi), and the standard atmosphere (atm).
Historical Context
The understanding of pressure evolved significantly in the 17th century. Evangelista Torricelli invented the first mercury barometer in 1643, demonstrating that the atmosphere itself exerts a measurable pressure. Blaise Pascal further developed this concept, showing that atmospheric pressure decreases with altitude, definitively proving that a vacuum could exist in nature. The SI unit of pressure was named in honor of his contributions to fluid mechanics and hydrostatics.
Realizing the Pascal in Practice
The realization of the pascal for calibration purposes relies on fundamental physical principles rather than arbitrary artifacts. Primary standard instruments are used to generate a known pressure from base quantities of mass, length, and time.
- Deadweight Testers (Piston Gauges): These are primary standards that define pressure directly from the equation $p = \frac{F}{A}$. They operate by balancing an unknown fluid pressure against a known force produced by calibrated masses loaded onto a piston of a precisely known effective area.
- Liquid Column Manometers: Historically, these devices measured pressure by balancing it against the hydrostatic head of a fluid column, typically mercury or water. While highly accurate, environmental concerns and the toxicity of mercury are driving a shift toward electronic standards.
Industrial Measurement Instruments
Secondary and working standards are used in the field and on the factory floor to provide reliable, continuous measurements.
- Bourdon Tube Gauges: A classic mechanical device consisting of a flattened, coiled tube that tends to straighten when pressurized. The small mechanical displacement is amplified by gears to move a pointer across a dial.
- Strain Gauge Transducers: These sensors utilize a diaphragm that deforms under pressure. Strain gauges bonded to the diaphragm change their electrical resistance proportionally to the deformation, providing an analog or digital output signal.
- Piezoelectric Sensors: These utilize specialized crystals that generate an electrical charge when subjected to mechanical stress. They are exceptionally well-suited for measuring highly dynamic, rapidly changing pressures, such as those inside combustion engines or shock tubes.