Idea for a DIY airflow meter

[Dale Wambaugh]

With talk about windshirts and fabric breathability, I was thinking about more objective means of comparing fabric airflow qualities. I'm really thinking out loud here with a half-baked idea.

I'm familiar with a medical device called an incentive inspirometer, which is used for respiratory therapy.

My thought was to adapt this thing to a vacuum cleaner with some plumbing and stretch the fabric over a pipe on the intake end.

You would need to be careful to get the fabric stretched consistently and it would only be useful for side by side comparisons. There would be no real numerical data but you could compare against a known garment and get a very graphical readout with the sliding gauge in the meter. Of course it would vary with the vacuum cleaner and plumbing that is used, so my results couldn't be directly compared to yours. It should be better than blowing through the fabric stretched over your hand.

[Anonymous]

Sounds like it's worth a try. Looking forward to your results.

[David Thomas]

Another approach would be to stretch the fabric across a pipe, much as you describe, but in the middle of a pipe (between two sections of shop-vac tubing). Then see how much suction the vacuum can develop. It has some low vacuum with the pipe open, and it's highest flow with the pipe open. The vacuum measurement would be in between those two extremes, possibly different enough to be useful. If not, a smaller (or more likely) larger surface area of fabric would produce a measurable distance.

Then you just need to measure the vacuum, of which there are many ways. A range of differential pressure gauges is what I'd use in my garage. If stranded from home, vinyl tubing with water in it makes a manometer and measures in inches (or cm) of water column. Any altimeter is also a pressure/vacuum gauge, as is any barometer whether mercury filled, capsule-driven or solid state.

Unlaying both methods (flow and vacuum) is that the shop vac has a performance curve – distinct flows at distinct vacuums. Determining one determines the other. And the porosity of the fabric is the reverse – for a given differential pressure, it will pass a certain amount of air flow. So you are determining where one curve intersects the other.

[Jerry Adams]

amazon.com – $13 – 2 ounces – wind speed meter – LCD readout

[David Thomas]

Jerry: Spoil sport!

Good find.

[Dale Wambaugh]

Jerry is going to buy us one of these to play with:

[Nick Gatel]

Just buy an older model Houdini and be done with it. Nisley will keep you updated on any new break-through garments :)

[Dale Wambaugh]

I have the good Houdini. I like techy toys too.

[Jerry Adams]

I bought one of those $13 meters after I ran over and broke my previous meter, a Brunton ADC Wind.

The wind meter before that I set it on the ground to measure temperature, forgot about it, came back 10 minutes later and it was gone. That was after I leaked DEET all over it which dissolved it but it still sort of worked.

I have to get cheap ones because that's all I can afford.

[Nick Gatel]

"The wind meter before that I set it on the ground to measure temperature, forgot about it, came back 10 minutes later and it was gone."

Sounds like the wind blew it away.

[Delmar O’Donnell]

Dale you clever guy. I'll bet your idea can work — not only to give you a relative measure of fabric permeability, but you could probably approximate lab measurements of cfm as well. As long as your measurements are repeatable, you can probably covert to industry standards without spending for a fancy machine. More on that in a bit.

Your source of air — vacuum on "blow", air compressor turned to a specific psi, strong fan, whatever — just make sure it's repeatable.

Calibrate. Start with an average wind shirt (what's average? 5cfm? 10cfm?). Increase/decrease distance of air source and/or inspirometer until you're getting measurements in the middle of the range on your inspirometer — that would be around 2000.

Test several (a dozen?) jackets with *known* permeability in a common measurement (take what you can get: cfm/grams per meter/Ret/whatever). Let's assume cfm for now. The jackets have numbers of 5,10,4,20 cfm, etc… Record you inspirometer readings for same. Just spitballing some numbers, they might be 200, 3000,1500, 4000, etc, or whatever.

Once you have these two columns of numbers for a dozen jackets, you can predict the published cfm numbers (DVs) using your inspirometer numbers as independent variables (IVs) with a statistical regression. The regression will even give you a "fit" number telling you how well you are predicting the published numbers. If you are getting an r-squared of .90 or more, you have a darned accurate home testing machine. I'd be happy with an r-squared of .80 for a project like this, where gilt-edged accuracy is not the goal.

The regression will give you a simple formula (often just two variables, one to add and one to multiply), where you can take your inspirometer numbers, plug them into the regression formula, and it will yield estimates of cfm (or whatever metric you chose). Thus, a simple calculation will allow you to estimate permeability of new fabrics you encounter.

I did something similar once when estimating lumens for flashlights. I don't own a fancy laboratory integrating sphere, but I cobbled up some PVC pipe, using it with my existing light meter, which allowed consistent measurements of indirect light in a confined space. After testing maybe 18 lights with known/published lumen amounts, I was able to predict lights with and r-squared above .9, meaning that with some small amount of "slop" in the system (10%), I was getting impressively accurate lumen estimations without more than $3 invested for the PVC pipe.

Onward, man!

[Delmar O’Donnell]

Dale, if you have forgiven me for our verbal altercation in another thread, and if running a statistical regression is not your cuppa, you can send your data to me, and I'll run the regression on SPSS. Would only take minute or two.

[Dale Wambaugh]

No altercation, just spirited debate :)

Thanks for the offer. I'm very used to instrumentation from training in photographic processing, automobile repair, and electronics. This is a loose sort of thing, but I could make tests with samples of known fabrics to get some basic calibration.

The breath test gives a go-no go evaluation in a store. Some fabrics pass so little air that there is no point going further. I've seen many wind shells that were coated so heavily that they were really more like rain shells that weren't seam taped. Sierra Designs and Red Ledge come to mind. I've seen a lot of store brands like Eddie Bauer that were much the same.

There are many sports brand garments sold as "warm up" items that may have potential. I think the better running clothing is a relatively untapped source for UL hiking. Some objective testing would be handy.

[Roger Caffin]

Hi Dale and Delmar

Go for it – and we expect a nice article for BPL when you have built a couple and tested them!

Cheers

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