raw · notes · ingested 2026-07-02
Low-cost vacuum pump options for the water adsorption demonstrator (research note)
confidence: high
Source: https://en.wikipedia.org/wiki/Vapour_pressure_of_water
Consolidates: water vapour-pressure table (Wikipedia), Leybold pump-down, HVAC single-vs-two-stage guides, VEVOR/Harbor Freight/JB specs, fridge-compressor DIY forums, aspirator/venturi vendor data, gas-ballast guidance (VACUUBRAND/HVACoLink). Credibility: High (authoritative physics + vendor specs; forum sources for the DIY routes).
The spec (what you actually need)
Operating pressure = water saturation pressure at the target temperature:
| Target °C | Torr | mbar | microns |
|---|---|---|---|
| 20 | 17.5 | 23.4 | 17,500 |
| 10 | 9.2 | 12.3 | 9,200 |
| 0 | 4.6 | 6.1 | 4,600 |
To chill toward 10 °C, reach ~10–12 mbar; over-pump to a few hundred microns first to purge non-condensable air and degas the water (>10× margin). At a few litres, pump-down speed is trivial (seconds) — the binding constraint is ultimate pressure and surviving water vapor.
Recommended — budget/used HVAC rotary-vane pump + gas ballast
~$60–100 used, ~$120–130 new. VEVOR VP245 (5 CFM, two-stage, 25 µm, $130); Harbor Freight ICON 58141 (5.9 CFM, ~18 µm, $130–170); used JB Eliminator 6VE ($120, rebuildable — preferred brand). Rated 15–150 microns, i.e. 100×+ deeper than needed. Single-stage suffices here: even if it stalls at ~300 microns under vapor load, that’s far below the 6,000–17,000-micron operating point. Gas ballast (open early, close ~3,000 µm) or an upstream cold trap/desiccant protects the oil from water vapor — the one feature that actually matters for a water rig.
Cheapest that works — two fridge compressors in series
Free–$30 salvaged. One compressor bottoms at ~20 Torr (compression ratio ~10–30) — marginal; two in series clear 17 Torr comfortably. Intake = suction/low-side port; keep oil topped up / add an oil trap (units mist oil out the exhaust).
Most water-appropriate — cold-water aspirator
~$15–40, oil-free (no contamination of a water rig). Floor is capped by water vapor pressure (~24 Torr at 25 °C), so needs cold (~10–15 °C) supply water to reach ~15–20 Torr; wastes water unless recirculated with a chilled reservoir. Needs mains pressure >~11 psi.
Don’t bother (physically short of 17 Torr)
Mityvac/hand pumps (~125 Torr), diaphragm pumps (50–100 Torr), single-stage venturis (60+ Torr). Their inHg specs look close (25–27.5 inHg) but are 3–7× short in absolute terms.