Enter the constant k, area, partial pressures at points 1 and 2, and the thickness into the calculator to determine the water vapor transmission rate.

Water Vapor Transmission Rate Formula

The following equation is used to calculate the Water Vapor Transmission Rate.

WVTR = k*a*(P1-P2) / d
  • Where WVTR is the Water Vapor Transmission Rate (g/100in^2/day)
  • k is the experimentally derived constant base on porosity
  • a is the area (in^2)
  • P1 is the partial pressure at inlet
  • P2 is the partial pressure at outlet
  • d is the thickness of the membrane.

What is a Water Vapor Transmission Rate?

Definition:

A water vapor transmission rate, often called a moisture vapor transmission rate, is a measure of the rate of the passage of water through a membrane per unit time.

How to Calculate Water Vapor Transmission Rate?

Example Problem:

The following example outlines the steps and information needed to calculate the Water Vapor Transmission Rate.

First, determine the constant K based on the porosity of the material. In this example, this is given as 3.15.

Next, determine the area of the flow. This is calculated to be 10in^2.

Next, determine the partial pressure at inlet. This is measured to be 10.

Next, determine the partial pressure at outlet. This is measured to be 5.

Next, determine the thickness of the membrane. This is .50 in.

Finally, calculate the water vapor transmission rate using the formula above:

WVTR = k*a*(P1-P2) / d

WVTR = 3.15*10*(10-5) / .5

WVTR = 315