Aha! As I think bernard said when he first mentioned it, it takes only the tiniest gap in the magnetic path to have an enormous effect - the two parts of the magnetic core must be, and remain, in contact.That was it John, even a fraction of a mm causes a massive drop in the reading! The extra bit of sheath, rope, etc... in the sensor must of pushed open the clamp, only by 10th or so of a mm, but that's all it took.
Indeed. Hopefully the laws of physics will start showing themselves properly with the clamp properly closed, and he'll get the answer that we would all expect!!
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