Principle Of Operation - Elcometer Protovale CoverMaster CM52 Operating Instructions Manual

2. PRINCIPLE OF OPERATION

The CM52 CoverMaster detects a reinforcing bar by briefly magnetising it, and then
detecting the induced magnetic field as it dies away. The strength of the induced field
depends mainly on the depth of the bar below the search head, and to a lesser extent on
the diameter of the bar.
Figure 3 Principle of operation
The search head contains two coils (Figure 3). When supplied with a pulse of current,
these coils create a magnetic field which magnetises the reinforcing bar and induces a
circulating 'eddy current' in the bar. After the end of the current pulse, the eddy current
dies away, creating a much weaker magnetic field as an 'echo' of the initial pulse. The
coils in the search head measure the strength of the induced field as it dies away, and
this signal is processed to give depth measurements plus an optional audible indication.
Note that the search head is half black and half white. The BLACK half is the 'active' part
of the head and contains the search coils. Therefore, unless the manual instructs you
otherwise, you should always bring the black face of the head towards the surface of
the concrete and the reinforcing bars.
The eddy-current echo from the reinforcing bar is not determined solely by the diameter
of the bar and its depth below the search coils. It also depends strongly on the
orientation of the search head with respect to the bar. When the long axis of the head is
parallel to the bar (Figure 4A) the signal is strongest; when it is at right-angles to the bar
(Figure 4B) the signal is weakest.
Figure 4 Alignment of search head
A) Strong signal - correct (B) Weak signal - WRONG
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