When managing environmental or product controls, you must distinguish between three types of humidity, as each behaves differently.
- Absolute Humidity (AH): The actual weight of water vapour per unit of air (e.g., kg/kg). It is entirely unaffected by temperature.
- Relative Humidity (RH): The ratio of water vapour in the air compared to the maximum it can hold at that specific temperature. Expressed as a percentage, it must include a temperature reference (e.g., 50% RH at 21°C).
- Equilibrium Relative Humidity (ERH): The RH of the 3mm boundary layer of air immediately surrounding a product, dictated by the product’s own moisture content. Like RH, it is expressed as a percentage at a specific temperature.
Relative Humidity & Human Comfort
RH directly impacts human health and comfort by affecting how moisture evaporates from our skin:
- Low RH: Accelerates evaporation, causing cooling (making us feel colder than it is), dry skin, and a higher risk of infection.
- High RH: Slows evaporation, preventing the body from cooling down and making us feel hotter.
While the human body can adapt to uncomfortable RH levels for short periods, extreme exposure over time impacts health and well-being.
Equilibrium Relative Humidity & Product Quality
Hygroscopic products (both organic and inorganic) constantly exchange moisture with the surrounding air based on vapour pressure differences. They will absorb or release moisture until the boundary layer (ERH) matches the ambient air (RH).
- Unventilated Environments: The product dominates. It will absorb or desorb moisture to dictate the RH of the surrounding air until equilibrium is reached.
- Ventilated Environments: The supplied air dominates. The product will continuously gain or lose moisture to match the air. Therefore, the supplied air’s temperature and RH must precisely match the product’s required ERH.
The Bottom Line: Because some products have a very narrow tolerance for moisture change, tight environmental control is critical. Managing RH, moisture transfer, and ERH directly protects your product quality—and your profits.
For more information on how ERH can optimize your product processing, please contact us.