The paper refers to the study of water meters and the impact of some external factors on the variability of water meters metrological parameters during their operation period. The study was conducted in three groups, depending on cause and the moment of dismounting – before expiry of operation period, at the end of operation period and to metrological expertise. In this paper, are presented results of measuring errors investigations, which show variability of errors during operation time. In the group of water meters with exceeded errors, analyses were conducted on the damage types and the factors affecting the deterioration of metrological accuracy. Research is aimed to define the conditions required in order to maintain the metrological accuracy during operation period.