The image above represents water activity.

To compute for water activity, two essential parameters are needed and these parameters are **Partial Pressure of Water (P _{w})** and

**Equilibrium Vapour Pressure of Water at Same Temperature (V.P.**

_{w}).The formula for calculating water activity:

a_{w} = ^{Pw}/_{V.P.w}

Where:

a_{w} = Water Activity | Drying

P_{w} = Partial Pressure of Water

V.P._{w} = Equilibrium Vapour Pressure of Water at Same Temperature

Let’s solve an example;

Find the water activity when the partial pressure of water is 22 and the equilibrium vapour pressure of water at same temperature is 28.

This implies that;

P_{w} = Partial Pressure of Water = 22

V.P._{w} = Equilibrium Vapour Pressure of Water at Same Temperature = 28

a_{w} = ^{Pw}/_{V.P.w}

a_{w} = ^{22}/_{28}

a_{w} = 0.78

Therefore, the **water activity **is **0.78.**

**Calculating the Partial Pressure of Water when the Water Activity and the Equilibrium Vapour Pressure of Water at Same Temperature is Given.**

P_{w} = a_{w} x V.P._{w}

Where;

P_{w} = Partial Pressure of Water

a_{w} = Water Activity | Drying

V.P._{w} = Equilibrium Vapour Pressure of Water at Same Temperature

Let’s solve an example;

Find the partial pressure of water when the water activity is 24 and the equilibrium vapour pressure of water at same temperature is 14.

This implies that;

a_{w} = Water Activity | Drying = 24

V.P._{w} = Equilibrium Vapour Pressure of Water at Same Temperature = 14

P_{w} = a_{w} x V.P._{w}

P_{w} = 24 x 14

P_{w} = 336

Therefore, the **partial pressure of water **is **336.**

Continue reading How to Calculate and Solve for Water Activity | Ball Mill Sizing