Each measurement comes with certain variation and every measuring equipment has own
accuracy. Covermeter is one of such measuring equipments. British Standard 1881 Part 204 "Recommendations on the use of electromagnetic covermeters” states that the indicated cover to steel reinforcement by a calibrated covemeter should be accurate to with +/- 5% or +/-2mm, whichever is the greater, over the working range given by the manufacture. It also states that under the most favourable site conditions, indicated cover can be measured to an accuracy approaching that obtainable in the laboratory when the bar size is known. The accuracy of measurement likely to be obtained on the average site is within +/-15% or +/-5mm, whichever is the greater, for reinforcement at covers of less than 100mm.