How the conversions work
Everything converts through the pascal, \(1\ \text{Pa} = 1\ \text{N/m}^2\). The defining relations are
| Unit | Pascals | Definition |
|---|---|---|
| bar | 100 000 | Exact |
| mbar (= hPa) | 100 | Exact |
| atm | 101 325 | Exact (standard atmosphere) |
| Torr | 133.322 368 4… | Exact: 101 325 / 760 |
| mmHg (conventional) | 133.322 387 415 | Exact: 13.5951 g/cm³ × 9.806 65 m/s² × 1 mm |
| psi | 6 894.757 293 | 1 lbf/in² |
| inHg (0 °C) | 3 386.389 | Conventional, 25.4 mmHg |
Handy anchors for vacuum work: 1 Torr ≈ 1.333 mbar, 1 mbar ≈ 0.750 Torr, and 1 mTorr ≈ 0.1333 Pa. A Torr and a millimetre of mercury differ by about one part in seven million, which is below the accuracy of any vacuum gauge. The two are interchangeable in practice, though only the Torr is defined exactly from the atmosphere.
Vacuum ranges
The ranges below are the ones most often used in vacuum-industry literature. ISO 3529-1, DIN 28400-1, and the AVS draw some boundaries differently. Treat them as conventions for talking about a system, not as physical thresholds.
| Range | Pressure (mbar) | Gas behavior | Common gauges |
|---|---|---|---|
| Rough (low) vacuum | 1013 – 1 | Viscous flow; gas behaves as a fluid | Capacitance diaphragm, piezoresistive, mechanical dial |
| Medium (fine) vacuum | 1 – 10⁻³ | Transition from viscous to molecular flow | Pirani (thermal conductivity), capacitance diaphragm |
| High vacuum | 10⁻³ – 10⁻⁷ | Molecular flow; wall outgassing dominates the gas load | Cold-cathode (inverted magnetron), hot-cathode ionization |
| Ultra-high vacuum | 10⁻⁷ – 10⁻¹² | Surfaces stay clean for hours; mostly H₂ remains after bakeout | Bayard–Alpert or extractor ionization gauge, RGA |
| Extreme-high vacuum | < 10⁻¹² | Gauge x-ray limits and outgassing of the gauge itself matter | Extractor and specialized ionization gauges |
Two points matter when reading a gauge. Thermal-conductivity (Pirani) and ionization gauges are gas-dependent and are normally calibrated for N₂, so in helium or argon they can be off by a large factor. Capacitance diaphragm gauges measure force per area directly and read the same for any gas.
Worked example
A turbo-pumped chamber reads 5 × 10⁻⁶ Torr. That is 5 × 10⁻⁶ × 133.322 = 6.67 × 10⁻⁴ Pa, or 6.67 × 10⁻⁶ mbar, which falls in the high-vacuum range. At 20 °C the N₂ mean free path is about 6.48 mm·Pa ÷ 6.67 × 10⁻⁴ Pa ≈ 9.7 m, much longer than the chamber. Molecules cross the chamber without hitting each other, so the flow is molecular.
Assumptions and limits
- All units are absolute pressures. Gauge pressure (psig, barg) is measured relative to local atmosphere. Add the ambient pressure, about 14.7 psi or 1013 mbar at sea level, before converting.
- Mercury-column units use the conventional density and standard gravity. Real manometer readings need temperature and local-gravity corrections.
- Mean free path and number density assume an ideal gas at 20 °C. The N₂ mean free path uses a hard-sphere diameter of 3.75 Å.
References
- A. Thompson and B. N. Taylor, Guide for the Use of the International System of Units (SI), NIST Special Publication 811 (2008), Appendix B (conversion factors).
- J. F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley (2003).
- K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH (2016).