SEFRAM MW9520 Manuel D'utilisation page 54

(3) Other application:
(a) Detection of a metallic water supply and heating
pipe:
The pipe must be conductive, and so metallic.
(for example galvanized steel)
The pipe must not be earthed, and there is a
relatively high resistance between itself and
ground. (otherwise, the detection distance is
very short)
Connect "+" (red) terminal of MW9520-E to the
pipe and "COM" (black) terminal of MW9520-E
must be earthed.
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