The use of Plastics as a material for the construction of a plant has forced the need to verify currently available information regarding the value of the resistance coefficients. They are the necessary knowledge needed for calculating the mechanical energy losses that arise in the flow. These coefficients are important in the calculation of hydraulic dimensioned of installation. They affect the final result in a significant manner, because their incorrect chosen can lead to serious consequences of errors. Unfortunately, the values supplied by the manufacturers, standards or other literature are compared with the values obtained by experimental tests. The article presents the results of experimental studies designed to determine the values of coefficients these in elbows 90° in three specific diameters derived from the four selected systems manufacturers. The resulting was compared with the values that are given by the manufacturer, designated according to PN-76-M-34034 norm and available in the literature.