The quality and precision of the operation of instruments, such as flowmeters, pressure gauges, altimeters, and valves are of decisive importance for all water and wastewater companies (WWCs), since they are the basis for measuring and providing data on the status of the water and wastewater installations and networks and thus play an important role in the decisions made by experts and managers. Such instruments are the only means of measuring and controlling the production of water, the pressure of water distribution networks and the production of other data required by all the WWCs. Therefore, one of the main goals and policies of the WWC officials is to procure and purchase of instrumentation with high precision, as well as verification of the correct operation of the instruments over time. If such goals are set and met, the information required for the installation is accurately measured and the decisions taken are based on the correct data. A specific process is therefore required for purchase of instruments with a proper quality as well as for periodical examination of the accuracy of the instruments installed, to ensure the proper operation. In this paper, based on experiments, the problems and the most appropriate solution to the problems of purchase, optimal operation and maintenance of instruments in the water and wastewater companies are presented.