Air permeability of paper

[artkul]

Hello everyone,

I am looking for a way to capture a variation in air permeability in light paper grades. So we apply the pressure difference on two sides of a paper sheet but how to capture non-uniformities in through-paper flow on 1 cm scale? Does anyone know a suatable experimental technique for that? I will really appreciate any suggestion!

 

[btrueblood]

Are you looking for some kind of ASTM or ANSI standard test method, or just a way to get it done, cheaply but with repeatability?

If the latter, it should be fairly easy to create the following setup: A clamp fixture which presses an o-ring seal tightly to the paper. The downstream side of the fixture vents to atmoshpere. The upstream side is plumbed to a ball valve. Upstream of the ball valve is a length of pipe then another ball valve. A pressure gage or transducer is plumbed to the wall of the pipe. The volume between the paper to second ball valve should be known (calculated or measured). The pipe is filled to a given pressure, and then the upstream ball valve is closed. Open the downstream ball valve, and record the pressure-vs.-time decay. Convert to units of your choice (typically atm-cc/sec/cm^2).

 

[artkul]

Thank you, btrueblood!
You are right! I am looking for a cheaper solution. There are ANSI standards to measure permeability but they are all designed for relatively large area and cannot capture 1 cm variations. The method you suggested is a way to go. Someone is already doing this actually but it works pretty slowly. One reason - you affect the moisture content of the sample in vicinity of the measuring point and have to wait until it is reconditioned or proceed at a point far from the previous. It is not a big deal for us, of course, since we just want to see how the paper structure interacts with jet flow, but we need to evaluate many papers samples and scanning even 10x10 cm paper would take a while.
I am not an expert in experimental fluid mechanics and my "dream" experiment would be clamping a 10x10 cm paper to a nozzle and looking at the paper from the front by some *tool* capturing in-plane flow variations through the sheet. I just wonder if such *tool* exists or not?..

 

[btrueblood]

I don't know why you couldn't scale the fixture above up into a multi-celled rig (crowd 100 O-rings and mating outlets onto the plates of a press, with seperate bleed-down chambers and pressure gages/transducers for each one) Okay, it's a bit spendy in pressure transducers and data acquisition channels, but it's a linear progression (speed vs. cost). Do you need an absolute measurement of the variation, or a relative measurement? A relative measurement of the flow might be possible using yarn tufts, or powder sprinkled on the paper.

 

[artkul]

Thank very much indeed! Yes we need a relative measurement only.
A multi-celled grid is an expensive solution. We calculated the price - we cannot afford it. (Those pressure sensors are bloody expensive!)
Using powder is an interesting idea indeed! I will try it. Could you please explain the other idea with yarn tufts in more detail?

 

[btrueblood]

Sorry, artkul. I don't have any real experience doing what I suggested, that was just a late-evening random thought. The powder idea may not work because the paper will tend to deform (bulge) as pressure is applied to it, and the powder won't stay at a uniform depth. You may be able to account for this effect. You should obtain a range of powders of varying densities and particle sizes, and then you may be able to calibrate what you see visually to a gross flow rate range.

Yarn tufts - if you can make them small enough, mounted on a (sparse) grid or screen placed against or near the paper. If the tufts are thin enough (and this might be impossible if the flow velocities are too low) you will see them "dancing in the breeze".

Another possibility is to use a smoke generator on the upstream side; whether the smoke will permeate through the paper or get filtered out is a question for the reader. You could try filling the "outlet" side with smoke, and then pressurizing the upstream side. You could also perhaps try putting a liquid over the paper (gee, would that affect the moisture content? Can't think of any liquid that won't be absorbed by the paper and watching the bubbles stream up.

You can look into Schlieren optical systems, but these will only realistically show you what is happening along a "slice" or plane perpendicular to the flow. Other more complicated optical techniques are out there, but will cost a lot more than a bunch of pressure transducers (you did look at the low-cost silicon 'ducers from Motorola, and sold by places like Digi-Key, right? And making a multiplexer switch is awfully easy these days, I bet you could make a multi-cell system for less than $1000).

 

[artkul]

Alright, I see. Well, I tried to apply powder on to a paper sheet, it is really hard to get an even distribution but bulging is not an issue, we prestress the paper in biaxial clamps. Now we will make a controlled pressure chambers and see how this works. We are also thinking about smoke we just need to isolate a smoke alarm.
Well, we checked with our suppliers... I assume that one can find a cheaper P transducer and other gadget but not here in Scandinavia where everything is so expensive.
btrueblood, I really appreciate your suggestions and will share the results when everything is ready.

 

[btrueblood]

Good luck, Artkul. One last thought: if you had a very smoky chamber upstream of the paper, and the smoke discolored the paper in a repeatable way (i.e. for every 1cc of smoke passing thru the paper the color of the paper darkens by some amount), you could probably run a test that simply passes a given amount of smoke through the paper, then uses digital photographs and image analysis to compare different regions of the paper. Of course, the paper is probably irreversibly changed by absorbing the smoke.