About Reinforcement Location.

For many years the commonly used method was that of "Magnetic Reluctance".
A U-shaped core (or yoke), of iron laminations or ferrite, carries two windings (see figure 2a). As in induction balance machines, the coupling (mutual inductance) of the two windings is measured, using a "bridge" circuit to monitor changes. The physical quantity being inferred is the "reluctance" of the magnetic path, which is determined predominantly by the magnetic properties of the core, and only affected to a lesser extent by the presence of nearby magnetic steel. Since the magnetic properties of the core are inevitably temperature-sensitive (and even affected by other magnetic fields, including the Earth's), zero-drift is as much of a problem as in any other "balanced" technique.
The Pulse-Induction technique (see figure 2b) does not use any magnetic core inside the search head, and so is completely immune from these effects.

Figure 2: Magnetic Reluctance and Electrical Conductivity Some more recent rebar locators and cover meters have used methods which are electromagnetic in nature (rather than purely magnetic), but the coil configuration dictated by the detection technique is invariably far short of optimum for practical bar location:- either the field is too widespread and diffuse, which makes resolution of closely-spaced bars impossible (see figure 3b); or else (if more compact) the field is non-directional, and cannot allow distinction between horizontal and vertical bars (figure 3c).
Since a Pulse-Induction coil can (in principle) be any shape required, the "shape" and extent of the field can be optimised for both bar-resolution and bar-orientation, with total zero-point stability (figure 3d).


Figure 3: Coil Configurations