Using a spirometer (a lung capacity tester, and inexpensive plastic column or gauge used by patients with asthma, etc) to estimate breathability of fabric wasn't my idea; it was someone else's here on BPL. (Whose?) I thought the idea was interesting, but never had fabrics of known CFM to test it on. Somewhere I'd written down that a light cotton t-shirt was approximately 400 CFM.
Then, in a concurrent thread (on the Alpine Houdini), Richard Nisley stated:
"I invented a $.01 windshirt tester (paper coffee filter air permeability reference) that is MUCH more accurate than just blowing through fabric to determine if it is easy or hard….a double layer paper coffee filter will provide the near optimal 35 CFM air permeability…a single layer paper coffee filter will provide ~70 CFM."
So with a lavish dataset of three data points, I decided to give the spirometer idea a try. My Spirometer has a detachable mouthpiece, so I placed the fabric tightly between the mouthpiece and the gauge, and gave it a blow. (For the record, I reliably blow around 680 Liters per minute.) The T-shirt yielded 550 L/min, the single coffee filter yielded 120 L/min–at the expense of my eardrums blowing out and hitting the adjacent walls–and the double coffee filter yielded nothing at all, as I could not blow hard enough to move the gauge.
OK, so now down to TWO data points. Still, enough to make a really really crummy regression estimate. Which turned out to be:
CFM = .767 times Liters/min, minus 22.
So, based on that, here are some ballpark estimates:
Paradox baselayer, estimated CFM 463.
Thin Marmot (Polartec 100?) estimated CFM of 455.
Patagonia Synchilla (like Polartec 200?) estimated CFM of 370.
Heavy cotton sweatshirt, estimated CFM of 248.
Nylon Ex Officio shirt, below 70, because the gauge didn't move.
Current Koppen wind shirt…unknown, at the tailor.
OK, so this is some real meatball science, if you want to be really generous and call it science, and it's likely wildly inaccurate due to the two data points used, both of which have lots of slop in the measurement.
BUT, it does promise that you might be able to take a spirometer to your favorite gear store and get at least a ballpark idea of the breathability of various highly breathable fabrics.
I just wish I had about 6 fabrics of known CFM for testing, I could make the prediction so much better.
And sadly, it looks like testing in the range of a good windshirt is off the scale for the setup/method I used here (on the low end).
HOWEVER, there is the possibility that the fabric could be placed at the EXHAUST port, rather than between the mouthpiece and the gauge, and a different calculation made. On my spirometer, air exhausts two places: 1) out the top of the gauge, and 2) out the back of the mouthpiece. If you were to block the exhaust port out the back of the gauge with the test fabric (or perhaps the top of the gauge would work better?) all excess air would go through the remaining open port. So (in theory) you'd be able to test fabrics of much lower permeability…without having to find your eardrums on the adjacent walls, and re-insert them into your body, as I have had to do several times now.
But I have not experimented with the back- or top-block method yet. I would like to fire up the air compressor and try this again with a known air pressure — that would yield a reliable air source, much more reliable than my lungs — but with the downside of the local REI store not appreciating the noise my air compressor makes, or the holes it might blow into garments for sale.
