Vacuum Pressure Measurement
From Rough Vacuum to Ultra-High Vacuum
Understand Vacuum UnitsVacuum—pressure below atmospheric—is essential in semiconductor manufacturing, scientific research, food packaging, and many industrial processes. Measuring vacuum requires different units and techniques than positive pressure, with ranges spanning from slight vacuum to pressures approaching absolute zero.
Vacuum Ranges
| Range | Pressure (torr) | Pressure (mbar) | Pressure (Pa) | Applications |
|---|---|---|---|---|
| Rough/Low vacuum | 760-1 | 1013-1.3 | 101,325-133 | Vacuum packaging, degassing |
| Medium vacuum | 1-10⁻³ | 1.3-10⁻³ | 133-0.1 | Vacuum drying, freeze drying |
| High vacuum (HV) | 10⁻³-10⁻⁷ | 10⁻³-10⁻⁷ | 0.1-10⁻⁵ | Thin film coating, electron microscopes |
| Ultra-high vacuum (UHV) | 10⁻⁷-10⁻¹² | 10⁻⁷-10⁻¹² | 10⁻⁵-10⁻¹⁰ | Particle accelerators, semiconductor fab |
Common Vacuum Units
Torr (mmHg)
Named after Torricelli, 1 torr equals 1 mmHg—the pressure that supports a 1mm column of mercury. Widely used in North America, especially in scientific applications.
- Standard atmosphere: 760 torr
- Rough vacuum: 1-760 torr
- High vacuum: millitorr (mTorr) range
Millibar (mbar) / Hectopascal (hPa)
Common in Europe and international standards. 1 mbar = 1 hPa. Convenient because 1000 mbar ≈ 1 atmosphere.
- Standard atmosphere: 1013.25 mbar
- 1 mbar = 0.75 torr
Pascal (Pa)
The SI unit. Standard atmosphere is 101,325 Pa. For vacuum work, milliPascal (mPa) or microPascal (µPa) may be used.
Conversion Table
| Torr | mbar | Pa | psi (below atm) |
|---|---|---|---|
| 760 | 1013 | 101,325 | 0 (atmospheric) |
| 100 | 133 | 13,332 | 12.8 below atm |
| 10 | 13.3 | 1,333 | 14.5 below atm |
| 1 | 1.33 | 133 | 14.67 below atm |
| 0.1 | 0.133 | 13.3 | — |
| 0.001 | 0.00133 | 0.133 | — |
| 10⁻⁶ | 1.33×10⁻⁶ | 1.33×10⁻⁴ | — |
Conversion Formulas
- Torr to mbar: Multiply by 1.333
- mbar to torr: Multiply by 0.75
- Torr to Pa: Multiply by 133.3
- mbar to Pa: Multiply by 100
Absolute vs Gauge Pressure
Absolute Pressure
Measured relative to perfect vacuum (zero). Scientific vacuum measurements use absolute pressure. Example: 10 torr absolute.
Gauge Pressure
Measured relative to atmospheric pressure. In vacuum, gauge reads negative. Example: -25 inHg gauge (inches of mercury below atmospheric).
Converting Between Them
- Vacuum absolute = Atmospheric - Vacuum gauge
- -25 inHg gauge = 29.92 - 25 = 4.92 inHa absolute
- Or approximately: 760 - (25 × 25.4) = 125 torr absolute
Measuring Vacuum
By Range
| Vacuum Range | Measurement Method |
|---|---|
| Rough (>1 mbar) | Bourdon gauge, capacitance manometer |
| Medium (1-10⁻³ mbar) | Capacitance manometer, Pirani gauge |
| High (10⁻³-10⁻⁷ mbar) | Pirani gauge, cold cathode gauge |
| Ultra-high (<10⁻⁷ mbar) | Hot cathode ionization gauge |
Industry Applications
Semiconductor Manufacturing
Chip fabrication requires ultra-high vacuum (10⁻⁹ mbar) to prevent contamination during deposition processes.
Food Packaging
Vacuum packaging typically uses 1-100 mbar to remove oxygen and extend shelf life.
Scientific Research
Particle accelerators and surface science experiments operate at extreme vacuum (10⁻¹⁰ to 10⁻¹² mbar).
Conclusion
Vacuum pressure is measured in torr, mbar, or pascal, with different ranges requiring different measurement techniques. Understanding that 1 torr ≈ 1.33 mbar ≈ 133 Pa helps navigate between unit systems. From food packaging at a few hundred mbar to semiconductor fabs at 10⁻⁹ mbar, vacuum technology spans over 15 orders of magnitude of pressure.