raw · notes · ingested 2026-07-02
Low-cost vacuum gauge options for the water adsorption demonstrator (research note)
confidence: high
Source: https://www.lesker.com/newweb/gauges/gauges_technicalnotes_1.cfm
Consolidates: Lesker/SRS vacuum-metrology tech notes, Pirani water-vapor error (freeze-drying literature), Testo/Elitech/Supco/JB/Fieldpiece micron-gauge specs, MS5803-01BA + mercury-manometer + MPX/BMP alternatives. Credibility: High (authoritative metrology + maker datasheets; DIY-sensor numbers from datasheets).
The two jobs straddle where thermal gauges fail
- Purge (Job 1): reach a few hundred microns (<1 Torr) to strip non-condensable air. A thermal micron gauge is accurate here (±10% below 10 Torr).
- Operating point (Job 2): ~10–23 mbar (7.5–17.5 Torr), nearly pure water vapor. Pirani accuracy collapses to ±50% above 10 Torr and over-reads water vapor ~40–60% — do not trust a micron gauge here.
Saving grace — read operating pressure off the water thermocouple
Near 10–23 mbar the water saturation curve is ~0.75–1 mbar/°C, so pressure maps ~1:1 onto evaporator temperature. The thermocouple the experiment already logs is your operating-pressure readout (±1 °C ≈ ±1 mbar). No extra operating-point gauge strictly needed.
The range trap (pick the right micron gauge)
Many top out below the ~17,500-µm operating point and go off-scale:
- Off-scale: Fieldpiece SVG3 (9,999 µm), JB DV-41 / old Supco VG64 (12,000 µm).
- Covers both bands: Elitech VG-760 / VGW-mini (
$70–90, 0–19,000 µm) — the budget pick; Testo 552 ($300, 0–20,000, computes water sat-temp); NAVAC NMV1S / Fieldpiece MG44 (0–25,000 µm).
Cheap gas-independent operating-point readout
- MS5803-01BA (~$10–30 + Arduino/ESP32): 10–1300 mbar absolute, 0.012 mbar resolution, gas-independent → true water-vapor pressure. Floor is 10 mbar, so it can’t see the purge (use the micron gauge for that).
- Sealed mercury U-tube: cheap absolute reference, 23 mbar = 17.3 mm Hg, ~1 Torr by eye. Toxicity/spill caveat; can’t read hundreds of microns.
Reject
- Compound Bourdon gauge — the whole 0–25 Torr sits in the last inHg mark; ±10–15%, unreadable there.
- MPX5100A — ±2.5 kPa full-scale error is larger than the 2.3 kPa signal.
- BMP280 / BMP388 — 30 kPa (300 hPa) floor; can’t reach vacuum despite great resolution.
Bottom line
Cheapest complete answer: a ~$70–90 Elitech VG-760 micron gauge for the purge + infer operating pressure from the water thermocouple. Add an MS5803-01BA (~$15) if you want a direct gas-independent operating readout to cross-check.