James Rebarscope R-C-400 Operator's Manual page 25

If the unit shows out of range in the BAR SIZE screen, and the
display reading is 3000 or larger, this means the rebar is too close
to the surface. The first reading should then be taken with the
supplementary 1 5/8" spacer block, and the second with a 3/8"
spacer block. See pictures below. (The supplement spacer block is
used only to add space between the surface and the sensor probe.
It's size is not critical when performing the sizing function.)
Figure 15: 1 5/8" Block
Figure 16: 3/8" Block
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