Cover meters are electromagnetic in operation. Electric currents in a coil winding in the search head generate a magnetic field which propagates through the concrete and will interact with any buried metal present, such as reinforcing steel.

The interaction will be due to either or both of two physical properties of the steel:

1. its magnetic permeability
2. its electrical conductivity

For a full description of technical explanation of cover meter testing, see Wikipedia

In practice, by making certain assumptions about the bar, and specifically by assuming that only one bar is present within the primary magnetic field, the cover meter can be calibrated to convert signal strength to distance and hence to indicate the depth of cover.

If there is more than one bar (or even scaffolding!) within the range of the primary field the instrument will receive a greater signal and indicate a shallower cover than the true cover. The skilled operator will always carefully map out the position and orientation of the steel, breaking out some steel if necessary, to ensure that accurate results are obtained.

Care must always be taken when dealing with multiple bars and the effects of adjacent steel such as window frames or scaffolding as mentioned earlier.

The latest cover meters now provide an image of the reinforcement under the concrete surface.

We use cover meter testing alongside our other concrete testing methods; the benefit of the cover meter is that it is completely non-destructive and very reliable as a first indicator.