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One more thing I just thought of regarding using dowels to support the test piece: Not only will the length shrink when measuring, but the deflection reading will be off too. As the plate deflects, the contact point on the dowel will move "in" and "down" the curve of the dowel. The "down" error won't be as bad as the "in" error but it will be there.

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But as the plate deflects or moves down with a straight edge wouldn't the length of the span lengthen? I think you might be over thinking this and that it would be in the margin of error, at least for our purposes. Wood is going to expan and contract even if you control the humidty there will be changes in your controlled room. Like Alan has said in the past, "If it falls in my range ..." You have to have a range and allow for a margin of error. Although the more precise you are then the smaller the margin of error. I'm sure you will have a more precise and smaller margin of error then me Andy. I'm just not going to get that involved in it. But I admire your persuit of precision in this. I probably will go with a straight edge though as that's easy enough to do but it will some roll to it to deburr it.

Good point. Using the right diameter dowel might actually compensate for a span increase.

I'm with you. I think the best argument against a dowel is measuring the span accurately. With any straightedge, you know that the point of contact will be the inboard edge. That's easy to measure to. With a dowel, the actual contact point not only varies dependent on load, it's hard to measure the span accurately no matter what.

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