Does anyone know of an accurate way to measure the amount a bow's back stretches when drawn? I'm thinking of a device/method which would give 0,1mm accuracy or better. According to my calculations my steel bows' backs stretch ~0,6% (0,48cm if bow is 80cm long). I've considered a few simple options which don't seem adequate:
A good tape measure would be accurate enough for measuring the total (or average) stretch of the back, but not good enough for measuring shorter sections. A pair of (vernier) calipers would be accurate, but the results would have to "calibrated" somehow, because the surface being measured is not flat. They might be useful for certain relative measurements, though.