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Against a perfectly stable clock source, if the crystal is 10ppm slow/fast, it "loses"/"gains" 1sec 10sec every million seconds, or 1used 10used every second. To measure the relative drift between two motes, you can have them toggle a pin at regular intervals and observe the edges on a scope (when no communication or synchronization is taking place). If you zoom in enough, you'll see the edge drift one against the other, as shown in the following video:
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