Well, I was playing with the tester trying to see if I could spot the axis with the low spine to identify where the carbon fibre discontinuity was, to see if I could index my arrows. Long story short; no I couldn't
Investigation revealed that the reason was that the tester just didn't give repeatable results. It just wouldn't give results that were precise enough and repeatable enough to spot a low spot in the spine. The results were broadly correct but just not consistent.
Testing showed that the issue was that I had only put a load cell at one end. I was measuring the forces at one end then extrapolating the forces at the other end (basically by multiplying by 2). But if you don't press the arrow dead on in the middle then that strategy fails. Worse than that; because I was multiplying the forces by two I was doubling the errors. I'd already spotted that this was an issue and I'd tried to mitigate the problem by reducing the area of the central pillar but seemingly that wasn't enough.
So I bit the bullet and re-engineered the tester to use two 2kg load cells rather than one 5kg cell at one end.
And the result is, after a major re-engineering of the mechanics and a re-write of the software (drum roll
)...
... something that looks basically no different.
I struggled for a long time to get the new version to work; results were really erratic at first. It seemed like the load cell at one end was over-reading then topping out at 1kg and refusing to read higher. It looked for all the world like the mechanics at one end were bottoming out; that red herring took literally days to get past.
Eventually I disassembled the whole thing and found this:
See that bottom edge covered with what looks like hot glue? With the frayed wire sticking out? Yep, that's the problem.
That's where the resistance matrices live - near the area of the cell designed to bend and flex. It's also the area that
all the instructions and
all the data sheets tell you to be really careful with and not to damage
Data sheets which I had read, digested, and thought, "aaaaaah, it'll be fine." Of course it could have been damaged in the post but to be honest there are other more likely causes
(kicks hammers and sharp screwdrivers out of sight).
I swapped in a new load cell and life was suddenly a lot better. I had sensible, and more importantly, repeatable results.
Another week goes by and I inch closer to the simple utility of two nails in the wall and a rock on a string for a weight.