The Vacuum Stack
Block 2 — Gauges
You cannot pump what you cannot measure, and no single gauge reads the whole range. Vacuum measurement spans more than seventeen decades. You always combine several. This block walks the families and where each one lives.
Source rating, used throughout:
- [A] primary: standard (ISO, AVS), peer-reviewed paper, lab or CERN/USPAS.
- [B] manufacturer: technically sound, commercial interest. Cross-check.
- [C] secondary: trade page or recopy. Flagged. Never the only source.
Pressure in Torr (1 Torr is about 1.33 mbar).
0. Why several gauges
Measurement runs from atmosphere (~1e5 Pa) down to ~1e-12 Pa. No gauge covers it all. You always combine technologies. [A] Cornell/USPAS, Yulin Li, Vacuum Science, gauges session, https://uspas.fnal.gov/materials/13Duke/USPASVacuumSession2Gauges.pdf
Three broad classes:
- thermal: Pirani, thermocouple. Read pressure through gas heat conduction.
- ionization: hot cathode (Bayard-Alpert, extractor), cold cathode (Penning). Read pressure through the ion current created.
- mechanical / reference: capacitance manometer (Baratron), spinning rotor.
One rule cuts across all of it. Every ionization gauge depends on gas composition. Relative sensitivity (nitrogen = 1) runs about 5 for acetone vapor and 0.18 for helium. An ion gauge reads wrong if you do not correct for the gas. [B] Kurt J. Lesker, pressure measurement notes (citing a JVST report), https://www.lesker.com/newweb/gauges/gauges_technicalnotes_1.cfm
1. Mechanical and reference gauges
Capacitance manometer (Baratron). From atmosphere to ~1 mbar, lower on heated high-end models. A membrane, often Al2O3 ceramic, deflects, and the capacitance change gives pressure. It is independent of the gas, which makes it the precision reference at low and medium range. [B] InstrumentationTools (Al2O3 membrane), https://instrumentationtools.com/vacuum-gauges-working-principle/
Spinning rotor (SRG). A magnetically levitated ball whose deceleration reads pressure. A transfer reference for calibration in high vacuum. [A] Cornell/USPAS.
2. Thermal conductivity gauges (rough / medium)
Pirani. Useful from ~67 Torr to ~1e-4 Torr. Below ~5e-4 Torr it has only one significant digit. A heated filament loses heat to the gas; more gas, more cooling. It depends on the gas, so it needs calibration per gas. Small, simple, wide range, robust, cheap. [A] PMC/NIH, composite MEMS Pirani (classification, range can extend high with MEMS design), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9921686/ ; limit ~5e-4 Torr, [A] Wikipedia (standard physical limit), https://en.wikipedia.org/wiki/Pirani_gauge
Thermocouple. Same heat-loss principle, read by a thermocouple. Rough and medium range. [C] VacAero, vacuum gauges (overview, classification correct, cross-check), https://vacaero.com/information-resources/vac-aero-training/248209-vacuum-gauges.html
3. Hot-cathode ionization (HV / UHV / XHV)
A hot filament emits electrons that ionize the gas. The positive ion current collected is proportional to pressure. [A] Cornell/USPAS; [B] Lesker.
Bayard-Alpert (BAG). The UHV reference. Standard version ~1e-3 to ~1e-10 Torr. The nude UHV version, grid as an open basket mounted straight on a flange, reaches ~1e-11 to 1e-4 Torr. [B] Lesker, pressure measurement notes, https://www.lesker.com/newweb/gauges/gauges_technicalnotes_1.cfm Limits: gas-dependent, frequent calibration, the hot filament outgasses and has a finite life. Lab spec for UHV: gauges bakeable to 450°C, replaceable and dual filaments, electron-bombardment degas. [A] SLAC vacuum spec (public), https://slac.stanford.edu/pubs/slactns/tn01/slac-tn-86-006.pdf
Extractor (e.g. IE514). Reaches below the standard BAG into XHV by cutting spurious currents (soft X-rays, ESD). [A] Cornell/USPAS lists it among XHV gauges; product anchor [B] Inficon IE514/IE414, https://www.inficon.com/en/products/vacuum-gauge-and-controller
Schulz-Phelps. A hot-cathode gauge for the high-pressure end, near 1e-1 Torr, where the standard BAG saturates. [B] Lesker.
4. Cold-cathode ionization (HV / UHV)
Penning (and inverted magnetron). About 1e-2 to 1e-9 Torr on modern designs. A cold-cathode discharge sustained by a magnetic field, no hot filament. The discharge electrons ionize the gas. Rugged, long-lived, nothing to burn out, so it suits industry. But it can be slow to strike at very low pressure and reads less linearly than the hot cathode. [B] InstrumentationTools (1e-2 to 1e-9), https://instrumentationtools.com/vacuum-gauges-working-principle/ ; [A] Cornell/USPAS.
5. Field abacus — gauges at a glance
| Gauge | Range (Torr) | Principle | Gas-dependent | Use |
|---|---|---|---|---|
| Capacitance (Baratron) | 1500 to ~1e-4 | mechanical, membrane | no | precision reference |
| Pirani | ~67 to ~1e-4 | thermal conduction | yes | rough/medium, robust |
| Thermocouple | ~10 to ~1e-3 | thermal conduction | yes | rough, simple |
| Penning (cold cathode) | ~1e-2 to ~1e-9 | ionization, B field | yes | HV/UHV, industrial |
| Bayard-Alpert (hot cathode) | ~1e-3 to ~1e-10 (nude 1e-11) | thermionic ionization | yes | UHV reference |
| Extractor | ~1e-4 to < 1e-11 | low-noise ionization | yes | XHV |
| Spinning rotor | high vacuum | mechanical, levitated rotor | nearly no | calibration |
Ranges are typical orders of magnitude; a specific model can differ. Sources: Cornell/USPAS [A], Inficon/Lesker [B], MEMS Pirani [A] PMC.
6. Field rules
- Cross a (relative, gas-dependent) ion gauge against a (absolute, gas-independent) capacitance manometer whenever the true value matters.
- In UHV, measure where it counts. There can be a large pressure drop between the point of interest and the gauge (see Block 1, the HeatWave cathode).
- Always correct by the sensitivity factor of the dominant gas.
- A hot filament outgasses and has a finite life. A cold cathode is rugged but reads less finely.
Method note
Every figure is rated A/B/C. Priority goes to primary sources, here the Cornell/ USPAS course and the public SLAC spec. Manufacturer pages are flagged as commercial. Where a key figure rests on a single secondary source, it is marked. No internal lab data is used.
Next block: Seals & Materials, the wall and the triple junction.