A Material's Porosity

sachi · Post by **sachi** » Fri Dec 29, 2006 2:20 pm

I'm doing my project on different properties of materials, and I'm stuck on how to measure the porosity of a fabric. I've looked around on google, and I can't seem to find anything. If anyone could give me tips on how I could describe the porosity of a material or measure it, that would be great. Thanks!

deleted-71447 · Post by **deleted-71447** » Fri Dec 29, 2006 5:32 pm

You might have already read that the porosity of a material is equal to the volume of space that can be occupied by air (or some other gas or liquid) divided by the total volume of solid and air space of the material. For incompressible materials like sand, the porosity is constant and is relatively easy to determine. For compressible materials (like fabric) the porosity decreases as compression increases, so there is not a single value of porosity.

A simple way to estimate porosity values for fabric would be to measure:
1) the total volume (let's call it "Vt").
2) the volume of solids in a sample of fabric (let's call it "Vs")

The volume of the pore space (Vp) will equal Vt-Vs, and the porosity of the fabric will equal Vp/Vt.

There are a variety of ways you could measure #1. You could stack several square pieces of fabric and calculate the total volume from the width x length x height of the stack. You could also stuff some fabric into an empty graduated cyllinder. As I alluded before, you will get different results depending on how much you compress the fabric and how you arrange the fabric.

For #2, you could submerge the fabric in a partially filled graduated cyllinder (make sure to squeeze and agitate the fabric under water to get rid of as many air bubbles as possible). The increase in volume after adding the fabric is approximately equal to the volume of solids in the fabric.

I hope that helps.

sachi · Post by **sachi** » Fri Dec 29, 2006 11:57 pm

If I don't have access to a graduated cylinder, is there any other way in which I could just describe the porosity? For example, use a scale from one to five describing how porous something is. Could you give me some examples of porous and non-porous materials?

Thanks.

sachi · Post by **sachi** » Sat Dec 30, 2006 12:16 am

Also, you mentioned before that how I compressed the fabric would change the results, but wouldn't there be a huge possibility that my results would be very inaccurate due to this? After all, I couldn't compress the fabric a lot and make it have less volume, or I could not compress it at all, and it would have much more volume.

Once again, thanks!

deleted-71447 · Post by **deleted-71447** » Sat Dec 30, 2006 3:31 am

In response to your first post, you don't really need a graduated cylinder. You can measure the change in water level and then calculate the change in volume from the dimensions of the container. The experts here can help with that if needed. There are also lots of other ways you can measure the volume of fabric, such as the other method I described.

The first paragraph of this article has some information about porosity of some typical geologic materials.
http://en.wikipedia.org/wiki/Porosity

Maybe I don't understand the nature of your project, but I don't see the point of assigning a value of porosity in an arbitrary range of 1 - 5. You might as well make a guess of the actual value of porosity, which ranges from 0 - 1, and has some physical meaning.

In response to your second post, if your measurements are accurate, then your estimate of porosity will also be accurate. To overcome the issue of compressible materials having variable porosity, you could give a range of values that are representative of conditions that are relevant to your project, or you could choose a certain condition (e.g. a loosely folded cotton t-shirt) and give the result for that particular condition.