Humidity Meter Calibration Salt Test
Verify the accuracy of hand-held or process humidity meters using saturated salt solutions (LiCl at 11% RH and NaCl at 75% RH). This SOP ensures reliable relative humidity readings for critical industrial environments such as cleanrooms, storage, and HVAC systems.
To verify that hand-held and process humidity meters provide accurate relative humidity (RH) readings by comparing their output to known equilibrium values generated by saturated salt solutions. Accurate humidity measurement is essential for product quality, process control, and environmental monitoring in industrial maintenance, pharmaceutical storage, and critical manufacturing areas.
This procedure applies to all portable and installed humidity meters used for environmental monitoring in industrial maintenance and reliability programs. It covers the two-point salt slurry method using lithium chloride (LiCl) for low-humidity verification and sodium chloride (NaCl) for high-humidity verification. The procedure is performed in a controlled ambient temperature environment (20–25 °C).
- Lock out and tag out all equipment that the meter monitors if removal is required. Verify zero energy state before accessing sensor ports.
- Wear safety glasses and nitrile gloves when handling dry salt powders. Avoid inhalation of LiCl dust — use in a well-ventilated area or under a fume hood.
- Use only certified reagent-grade salts. Do not reuse salt slurries; cross‑contamination will invalidate the calibration.
- Saturated salt standards: lithium chloride (LiCl) and sodium chloride (NaCl) — certified ACS grade or equivalent
- Two glass jars (500 mL) with vapour‑tight lids
- Small glass dishes or beakers (50 mL) for salt slurry
- Distilled water
- Digital thermometer with ±0.1 °C accuracy
- Timer (minute resolution)
- Calibration log sheet (paper or digital)
- Reference hygrometer (optional, for cross‑check)