This paper reports on the effect of an enhanced test method for the recently drafted CEN method for measuring the depth of carbonation of hardened concrete which was aimed at reducing the known variability of the test. Variations in environmental factors that may have led to a poor precision have been identified, for example the assumption that natural ventilation will provide the required level of carbon dioxide and relative humidity has been shown to be incorrect. This lead to the development of an environmental control system that actively controls the storage atmosphere. In addition to this, a two stage normalization process of test data is proposed comprising the use of two additional mixes along with the mix being tested. Results from the assessment of the repeatability of a limited series of 10 mixes carried out by one operator of a 37 N/mm2 PC/30% PFA concrete mix indicated that a significant improvement could be achieved following this type of control and normalization.