Water Science Interface
Interactive lab model
Water activity, moisture content, and ambient humidity
Adjust the parameters to see whether the sample absorbs moisture or loses it. At equilibrium, water activity aw corresponds to equilibrium relative humidity: aw = ERH / 100; moisture content depends on the material, temperature, and sorption isotherm.
aw = ERH / 100
Sorption isotherm
Storage risk
RH
Ambient RH
Relative saturation of water vapor in the air; it sets the external moisture pressure on the sample.
aw
Water activity aw
The water available for reactions, migration, and microbial use; at equilibrium, aw≈RH/100.
M
Moisture content M
The mass fraction of water in the sample; the same moisture content can map to different aw values in different materials.
Sorption isotherm and current state
The curve shows equilibrium moisture content at each aw under the current temperature. The dot is the current sample; the vertical line is the equilibrium aw implied by ambient RH.
aw 0.00-1.00
ERH = RH / 100
M %
Current sample aw
0.63
Estimated from moisture content and material curve
Ambient equilibrium aw
0.60
Equals RH / 100
Equilibrium moisture
9.9%
If exposed to the current environment over time
Moisture trend
Slow moisture loss
Sample aw is higher than ambient aw
storage risk
Medium-low risk
Stable zone
Faster reactions
Mold risk
Note: this page uses simplified sorption isotherms for educational visualization. Real aw-moisture relationships vary by variety, roast level, particle size, sugar/salt content, temperature hysteresis, and test method. For production control, use a calibrated water activity meter and sample-specific curves.
