How to Measure Plastic Permeability

Dry three desiccant/drying packets in an oven at about 200 F for about one hour.

Trace the mouths of each of three clean, dry small glasses or glass baby food jars on a piece of paper with a pencil. Measure the widest part of each circle to the nearest millimeter using a metric ruler. Write the respective measurement next to each circle.

Weigh each desiccant/drying packet after it cools. Use a scale accurate enough to weigh to 0.01g, about 0.03 oz. Weigh the packets quickly so they do not absorb water from the air. Write the weight of each packet next to one of the circles you traced, and then put the packet in that circle's glass or jar.

Place plastic food storage wrap over the top of one glass or jar containing a packet. Use two rubber bands to tightly seal the plastic wrap over the mouth of the glass or jar. The rubber bands should ̶0;snap̶1; if you pull them. Put the glass or jar in a large container or a garbage bag.

Use waxed paper to cover the mouth of another glass or jar containing a packet. Use paper towel to cover the mouth of the last glass or jar that contains a pack. Secure the material covering each glass or jar with two rubber bands. Place the glasses or jars in the large container or garbage bag.

Fill one small cup with water, and put it in the large container or garbage bag with the three covered glasses or jars. If you use a large container, put on its lid. If you use a garbage bag, tightly roll the bag's opening and seal it with tape or clips to prevent the water from evaporating. Leave the glasses or jars and small cup with water in the large container or garbage bag for at least three days.

Calculate the area of each of the three glasses or jars during the three-day waiting period. The permeability of a material depends on its size; the bigger its surface, the more water vapor or other gas can diffuse through it. Calculate the area of a circle using the formula pi x r-squared or 3.1415 x r-squared, in which "r" is the circle's radius. Divide the width of each circle by two. The result is the circle's radius, which is just the circle's diameter ̵1; its width ̵1; divided by two. Multiply the radius by itself: r x r. Then multiply the result by 3.1415, which is pi. The product is the circle's area. The area will be a number such as 19.6 square centimeters, though it could be larger depending on the glass or jar.

Open the large container or garbage bag after three days. If no water remains in the small cup, then put water in the cup, put the lid on the large container or tape or clip the garbage bag closed again, and wait some more time. When water remains in the small cup, take the cover off each glass or jar, and weigh each packet. Write the each packet's weight next to the first weight you wrote down for each packet. The second weight should be the same as or larger than the first weight.

Divide the weight gain for each packet by the area of each circle. The weight gain is how much a packet weighed after you took it out of the container or bag minus the weight of the packet before you put it in the container or bag. Write that number next to the respective circle. Divide that number by the area of the circle for that glass or jar. Write down and underline that number. The underlined number is the permeability of the cover material. Write down the cover material next to the underlined number.

Place the materials in order based on their permeability, largest to smallest. The paper towel should be the most permeable. The next highest permeability substance was probably the waxed paper, but the permeability of waxed paper depends on its manufacturer and may be almost as small as plastic food storage wrap's permeability. The least permeable material is probably the plastic wrap.