Unit of pressure · BEGINNER LESSON
Pascal
The pascal (Pa) is the SI unit of pressure. One pascal equals one newton per square meter, and it can also be treated as one joule per cubic meter in some measurement methods.
What it is and what is measured
The pascal measures pressure, which is the amount of force applied over an area. In the SI (International System of Units), 1 Pa equals 1 N m−2, meaning one newton spread over one square meter. Pressure can also be written as energy per volume, 1 J m−3, because energy and force are related through distance.
[2] Quantum-based vacuum metrology at NIST [3] Quantum for pressure - PMC - NIH
How the pascal is realized in practice
A unit can be defined on paper, but it also has to be realized by a real measurement device. Traditional pressure standards include piston gauges, which use known force on known area, and mercury manometers, which use the height of a liquid column. These methods have been important for a long time, but the sources say mercury-based standards have reached a limit in accuracy and are hard to improve further.
[1] physicsworld.com Refraction redefines the pascal - NIST [2] Quantum-based vacuum metrology at NIST [3] Quantum for pressure - PMC - NIH
Optical and quantum-based methods
At low pressure, the pascal can be realized using the ideal gas law, which relates pressure, particle number, volume, Boltzmann constant, and temperature. The gas’s refractive index, meaning how much light bends in the gas, depends on density. Because density depends on temperature and pressure, measuring refractive index can be used to infer pressure. NIST researchers used optical interferometry, a light-based method that detects tiny path changes, in a fixed-length optical cavity (FLOC) to measure gas refractivity. For helium, quantum-chemical calculations help connect refractive index to pressure without relying on a mercury manometer.
[1] physicsworld.com Refraction redefines the pascal - NIST [2] Quantum-based vacuum metrology at NIST [3] Quantum for pressure - PMC - NIH
Limits and why improved methods matter
Different standards work best in different pressure ranges. Manometers can be reliable near atmospheric pressure, while high-vacuum work needs other systems such as dynamic expansion methods. The sources say the optical approach can reduce uncertainty and improve accuracy, and that current mercury-based standards cannot be improved much further. The newer methods are designed to provide traceability to the SI, meaning the measurement can be linked back to the unit definition through a documented chain.
[1] physicsworld.com Refraction redefines the pascal - NIST [2] Quantum-based vacuum metrology at NIST [3] Quantum for pressure - PMC - NIH
SEE THE IDEA
Ways to realize the pascal
The quantity being measured: force per area or equivalent energy density.
Piston gauges and mercury manometers use force or fluid height.
Refractive index changes with gas density, temperature, and pressure.
Light-based measurements and helium calculations connect refractive index to pressure.
Pressure can be measured by older mechanical methods or by newer optical methods that infer it from gas properties.
The pascal is the same unit, but different physical systems can be used to realize it.
TRY IT YOURSELF
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PUT IT TOGETHER
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.