Key Takeaway
Common units of pressure measurement include:
Pascal (Pa) – SI unit; often used in scientific contexts
Bar – 1 bar = 100,000 Pa; common in Europe and industry
psi (pounds per square inch) – Widely used in the US, especially for tire and hydraulic pressure
atm (atmosphere) – 1 atm ≈ 101.325 kPa
Torr or mmHg – Used in vacuum systems and medical devices
inH₂O or mmH₂O – For low-pressure differential measurements
kg/cm² – Used in some Asian countries
Each unit is selected based on the application and region. Conversions are often required for international compatibility and standardization in industrial instrumentation.
Overview of PSI, Bar, Pascal, mmHg, atm
Pressure can be expressed in many units. Here’s a quick breakdown:
PSI: Pounds per square inch — popular in the U.S.
Bar: Metric unit, 1 bar = 100 kPa
Pascal (Pa): SI unit; commonly used in scientific contexts
mmHg: Millimeters of mercury; used in medical and lab setups
atm: Standard atmosphere, ~1.013 bar
Conversions:
1 atm = 101,325 Pa = 1.01325 bar = 760 mmHg
1 PSI ≈ 6.895 kPa
1 bar ≈ 14.5 PSI
Choose your unit based on industry standards and system specs. Always label units clearly on control panels and calibration charts to avoid confusion.
Conversion Between Units with Examples
Pressure can be expressed in multiple units:
1 bar = 100,000 Pa = 14.5 PSI
1 PSI ≈ 6.89 kPa
1 atm = 101.325 kPa = 760 mmHg
For example:
5 bar = 500,000 Pa = ~72.5 PSI
100 PSI = ~6.89 bar
Always confirm units when installing a new gauge or reading a datasheet. Mixing units can lead to dangerous mistakes — like applying 100 PSI to a 100 bar gauge, which will barely move the needle and give false confidence.
Label units clearly on system drawings, and use conversion calculators or printed charts during commissioning or audits.
Which Units Are Used in Which Countries
Pressure units vary by country and industry:
PSI (pounds per square inch) – USA, Canada, UK (common in pneumatics and hydraulics)
Bar – Europe, Asia (industrial standard)
kPa or MPa – Australia, Japan, scientific applications
mmHg or Torr – Medical and vacuum systems
atm (atmosphere) – Legacy unit still used in academia
Standardization matters. Always label units clearly on gauges and datasheets. Mismatched units during installation or procurement cause costly mistakes and safety risks.
Industry-Specific Preferences for Units
Different industries and regions use different pressure units:
PSI: Common in North America, especially in pneumatics and hydraulics
Bar: Preferred in Europe and Asia for general industrial use
kPa/MPa: Widely used in engineering and Australia
mmHg or Torr: Used in vacuum systems and medical applications
kg/cm²: Still used in some older systems in Asia
For global equipment compatibility, many gauges include dual scales (e.g., bar + PSI). Engineers should always verify unit systems on datasheets, especially when importing or exporting machinery.
Unit Accuracy and Precision Implications
Accuracy and precision are often confused — but both matter in pressure measurements.
Accuracy is how close a reading is to the true value.
Precision is how repeatable the readings are.
For example, a gauge may consistently show 48 psi for a system that’s actually at 50 psi. That’s precise, but not accurate. Conversely, a gauge that swings between 47, 50, and 53 psi has low precision — even if one value is accurate.
Manufacturers rate gauge accuracy as a percentage of full scale — e.g., ±1% of a 100 psi gauge means ±1 psi error anywhere on the scale.
Higher precision is essential in lab work, gas metering, or calibration labs. In contrast, industrial applications might tolerate ±2–3% accuracy.
Digital gauges usually offer better accuracy and resolution, sometimes down to ±0.1%. For mechanical gauges, factors like temperature, vibration, and mounting angle can affect both precision and accuracy.
Selecting the right instrument depends on how much variation your process can tolerate. Don’t overspend on high-precision tools if ±5 psi won’t impact the outcome — but don’t under-specify for critical systems.
Conclusion
Pressure units like PSI, bar, kPa, and mmHg serve different industries based on history, region, and measurement needs. PSI is standard in the U.S., bar is common in Europe, kPa is used in engineering specs, and mmHg is preferred in medical contexts. Using inconsistent units within a system can lead to errors in process control, miscommunication, and safety hazards. Always match the unit with the instrument, documentation, and training materials. For international operations or scientific applications, use SI units (Pascals) to ensure clarity and interoperability. Choosing the right unit avoids confusion and keeps your team aligned, especially when working across departments, countries, or industries with varying conventions.