Measuring water quality and preventing drawbacks caused by deteriorated water quality in tap water fluid power systems is a unique problem. Tap water is a suitable environment for waterborne microorganisms. It also contains dissolved and undissolved organic and inorganic matter. Wear particles in the tap water fluid power systems are a separate problem, however closely linked to problems above mentioned. Contamination and the quality of the pressure medium in the system is a function of local characteristics of tap water, operating parameters, system and component design and contamination introduced to the system.To study effects of water quality on tap water fluid power systems, and to evaluate methods for measuring water quality and particle counting, a pilot scale hydraulic system was constructed. The pilot scale system emulates typical operation of a commercial tap water fluid power system. The level of contamination in the pilot system is measured using traditional methods of water quality control and condition monitoring.Experiments showed microbial growth in tap water fluid power systems. The amount of viable bacteria was found to vary throughout the system, being substantial in some points and negligible in some others. Substantial mineral matter deposits were found only in certain points of the system. Applying traditional methods of water quality control and condition monitoring was found to be partially successful.