Cover meter are devices that give information about concrete cover to steel reinforcement using magnetic fields. Magnetic instruments for locating reinforcing steel present within in concrete work on the principle that the steel affects the alternating magnetic field. When a hand-held search unit is moved along the concrete surface, a beep indicates that the unit is located directly above a reinforcing bar. These meters can also be used to estimate the depth of a bar if its size is known, or estimate the bar size if the depth of cover is known.

However, detailed calibration is needed to get satisfactory results. The bars may be located within 175 mm of the concrete mass but the method is not effective in heavily reinforced sections, sections with two or more adjacent bars or nearly adjacent layers of reinforcement. Inaccurate results may occur when the depth of concrete cover is equal to or close to the spacing of the reinforcing bars. During this test, steel or other metals must not be present close to the area under examination. Some results from tests must be calibrated by chipping of concrete to confirm concrete cover and bar size.

A number of pachometers are available that range in capability from merely indicating the presence of steel to those that may be calibrated to allow the experienced user a closer determination of depth and the size of reinforcing steel.