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Terry McGee posted a topic in Instrument Construction & RepairRecent discussions on leathers and leakage prompted me to think about doing some tests. I was particularly interested in questions like why we use chamois leather as a gasket when we know it isn't airtight. Does clamping it between two flat surfaces solve all its problems? And I thought I'd make it more fun for me and you by having you looking over my shoulder, putting forward helpful hints, coming up with bright ideas, making cruel and unnecessary jibes .... I thought I'd start by making up a general purpose test rig, which I could also use to test bellows, reed pans etc for leakage. One piece was like the end cap of a concertina, except no button holes or vents, other than a hole in the middle to attach to the leakage detector. The other piece was just a flat plate of the same concertina-like dimensions. Put them together and you have a thin, airtight, empty, hexagonal box. Both were made from 12mm (1/2") MDF, sometimes called craftwood. Smooth, dense, flat - just the thing. So I make up my two pieces and clamp them together with 6 big spring clamps where the 6 bolts of a concertina end would normally go. First test is to confirm that they form an air-tight cavity. Plug in the Magnehelic leakage detector, et voila - hmmm, leaks like a sieve. Indeed, the Magnehelic dropped from its setup point of 8 when open to atmosphere, to about 7.8. My test rig was essentially transparent to it! A failure of stupendous proportions! What an excellent start to our journey! Aha, I thought, clamping isn't enough - air is getting out through the gap between the two pieces. I can prove that by wrapping it around in duct tape. Hmmm, absolutely no difference. About now the true horror is sneaking in. MDF might be dense, smooth and flat - but it ain't airtight! Hey, this science stuff is cool - we've learned something already! But can I prove it, and what am I going to use for a test chamber? The answer is clearly: - let's render it airtight by doping it. So, I've lacquered all surfaces of both sections with a layer of PVA glue. That's going to take some time to dry, and it's getting late down here. We'll pick up the story tomorrow. Terry
Concertina Pneumatics Lab
Terry McGee posted a topic in Instrument Construction & RepairYou may remember me talking about getting some bits together to research concertina pneumatic issues. Well, the slow boat from China has come in, and the even slower boat that is me seems to have got its act together. Here's the result: An explanation of what it it comprises is at http://www.mcgee-flutes.com/Concertina/Concertina%20pneumatics%20lab.htm An interesting little challenge that came up too.