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The most widely used criterion for hydraulic design of microirrigation submain units is to establish a maximum relative flow difference of 10% between more irrigated and less irrigated plants. This criterion allows an acceptable value of emission uniformity (EU) and leads to define a hydraulic criterion based on maximum relative pressure difference that depends on emitters discharge exponent, but it not depends on manufacturers coefficient of variation of the emitters, CV. In this paper the importance of manufacturers coefficient of variation of the emitters has been demonstrated. A computer program, made by one of the authors, has been used to test and analyze a microirrigation submain unit modifying manufacturers coefficient of variation of the emitters from 0 to 5%, that is the normalized limit for good quality emitters according to European standard that calls them like Category A emitters. From the obtained data some linear regression equations have been adjusted. These equations show that only very good quality emitters can meet maximum relative flow difference of 10% criterion. Finally, the establishment of a hydraulic design criterion of microirrigation submain units based on limit the maximum acceptable relative pressure difference between emitters or plants is not reliable, because flow difference is depending on relative pressure differences and also on manufacturers coefficient of variation of emitters.