Materials, corrosion and risk
Water Activity
水分活度
Water activity is the ratio of the water-vapour pressure above a material at equilibrium to the vapour pressure above pure water at the same temperature.
- Abbreviations
- aw
- Aliases
- water activity value
QUANTITY, SYMBOL AND UNIT
Quantity / objectWater activity
Symbola_w
SI1dimensionless ratio: Commonly reported from 0 to 1
Definition
Water activity is the ratio of the water-vapour pressure above a material at equilibrium to the vapour pressure above pure water at the same temperature.
It is dimensionless and equals equilibrium relative humidity divided by 100 under the stated equilibrium conditions. Water activity describes the availability or escaping tendency of water in a material; it is not the same as the material's total moisture content.
Formula and variables
a_w = p / p₀ = ERH / 100 Conditions: Applies when the sample and surrounding headspace are at equilibrium and p and p₀ refer to the same temperature; ERH is equilibrium relative humidity expressed as a percent.
| Symbol | Meaning | Unit |
|---|---|---|
a_w | water activity | 1 |
p | water-vapour pressure above the sample at equilibrium | Pa |
p₀ | water-vapour pressure above pure water at the same temperature | Pa |
ERH | equilibrium relative humidity | % |
Relations and distinctions
Water activity and moisture content
Moisture content states how much water a material contains; water activity describes its equilibrium vapour-pressure relationship. Materials with similar moisture content can have different water activity.
Water activity and room RH
Water activity corresponds to equilibrium RH in a closed, temperature-controlled sample environment, not automatically to a transient room reading.
Engineering meaning
- Use a temperature-controlled equilibrium method when material stability or drying endpoint depends on water activity.
- Coordinate material water activity with ambient RH control when evaluating storage, drying or moisture migration.
Common misconceptions
MisconceptionA lower room RH immediately means the product has the same numerical water activity.
Accurate explanationThe material must exchange moisture and reach equilibrium at a controlled temperature before the ERH relationship can be applied.
Application boundaries
- The safe or acceptable water-activity limit depends on the material, organisms, process and applicable standard; no universal limit is given here.
- Equilibrium can take time, and sample temperature differences can materially change the measurement.
Supporting sources
-
U.S. Food and Drug Administration / SRC-FDA-WATER-ACTIVITY
Water Activity (aw) in Foods ↗
Inspection Technical Guide No. 39 / Definition, sorption behaviour and measurement sections
Supports: Water activity is a vapour-pressure ratio at equilibrium and equal temperature; Water activity equals equilibrium relative humidity divided by 100; Water activity is distinct from total moisture content and is temperature sensitive
Maintenance record
Water Activity
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026
- Term ID
- EKS-HUM-100