Woah, heaps of good stuff there, Jim. I'll take it in bits....
Are you guys assuming that all leakage is through these materials, and not around/past them?
In a compression test, the pressure will tend to push the pad open, while in a hang test, the negative pressure will tend to suck the pad tighter against the pad board (as well as compress the cardboard-felt-leather laminate). You seem to be assuming that this has negligible effect. I'm not so sure, though at present I have no capability to do any testing.
No, I'm expecting to see leakage through and around, and to be able to tell the difference. And I think I will be able to pull together the resources to test situations like the pad/board interface in both modes - suck and blow. Forming methodologies in my mind as we speak...
Now, would I be right in saying this. If a concertina is really leaky, the relatively small difference in leakage one way or the other through the pads is unlikely to affect the overall speed of closing or opening significantly. Whereas if the concertina is really tight, that (hopefully) small difference might be more noticeable. So it's people like Geoff, afflicted as they are by tight concertinas, who might be in a better position to detect a faster hang speed compared to closing speed?
Also, I'm not sure how you expect the permeability of gasket materials to affect/effect leakage. I believe the purpose of chamois and other gasketing materials (I have high-end instruments where the gasketing is leather with a smooth finished surface) surrounding the reed pans and atop the chamber partitions is to prevent leakage between and around the chambers. So the relevant measurement should be "sideways" through a significant "width'" of the leather, not simply through its thickness, and the measurement should also be made with the leather compressed, as it is when the pad board is pressed down by the fretted end when the end bolts are tightened, thus pressing the reed pan into the bellows end and against those gaskets end, while also compressing the strips atop the chamber partitions.
The test pictured above, with the leather sandwiched between the two sections of the test rig, conforms to that specification. The leather is clamped by about 10mm (3/8") all round the outer edge of the 150mm (6") hexagon. That's probably a bit more than would be clamped in a real concertina, but we can fiddle all that as we go once we see where it's leading us. The leakage I detected easily there, using the Magnehelic on full strength, was 0.4 of a cubic foot per hour, or about 0.2 LPM. We could afford 10 times that leakage and still have a "near perfect" concertina.
In particular, I suspect that far more significant than the permeability of the gasketing material is the tightness of the seal between the reed pan and the bellows frame. I've known instruments where the seal seems "tight" even though the reed pans need almost no pressure to seat them, while others need significant pressure to seat them properly into the bellows frame. Do the former still have residual leakage compared to the latter, or is there a relatively "loose" threshold beyond which there's not much change? We all know what happens when there's a visible gap between the reed pan and the bellows frame, no?
Measuring leakage inside the concertina presents greater challenges than leakages from inside to outside, but we should have a look at it once we've mastered external leakage testing. I don't even know how concertina makers and repairers approach the internal leakages. I'd be inclined to put a bright (but cool!) lamp in behind the reed pan and clamp the reed pan down with a sheet of glass or perspex. Has that any merit, or are there better ways to find out what's going on in the dark?
A possible exception to my above argument would be the valves, where leakage through their thickness -- especially when multiplied by the number of valves, if many are open for chordal work -- might possibly be significant. But this also raises the spectre of what happens when valves don't fully close/seal. And then there's the fact that a few of the highest reeds (at least in a treble English) normally don't have valves, so the amount of leakage past those reeds has been judged not to affect playing. Just how much is that, and how does it compare to the parameters you're trying to measure?
I think they are matters we can look into and put some numbers against. I've always been intrigued by the reeds with no valves. I guess it's not just a matter of "we can get away without them", but also a matter of "on these tiny reeds, valves would present a problem"?
It sounds like there are many issues that we could (and therefore should!) investigate, one I get the less sensitive flow meter and find a way to incorporate it into my test setup. I'll start an agenda list.
Now, we should essentially regard this, at this stage, as "pure" research - there may be no practical outcomes. But, who knows, finding out what flows where and why may lead to some better understandings and may lay our minds to rest on some issues that are just not worth worrying about!