Take three testpieces, each measuring 20cm x 20cm, from the sample fabric to be tested. Place the tank containing water at approximately 23 in the constant temperature apparatus, in which air at 302 circulates. As shown in the figure, attach a testpiece to the testpiece supporting frame with a rubber band so that the back surface of the testpiece faces outside the supporting frame. Fix this testpiece supporting frame so that it floats in the tank water (placed in the constant temperature apparatus). Next put the potassium acetate solution* kept at approximately 23 into the water vapor permeable cup until it reaches the cup's 2/3 volume level. Attach the auxiliary film for measuring water vapor permeability to the cup with a rubber band to prepare a test body. First measure the mass a0 (mg) of the test body with its film equipped side facing upward. Immediately after this, turn the test body over and attach it to the testpiece supporting frame. After 15 minutes, remove the test body, reverse it and measure the mass a1 (mg).

Where P = water vapor permeability (g/ hr.)     
      S = water vapor permeable area () 

Repeat the above procedure three times and take the average of the calculated values.