Abstract: A system for calibrating the flow rate of a pump, and especially designed for a pump which is normally used in a system to blend highly vaporous and odorous fluids, in which the blending fluid discharged from the pump is diverted from its normal flow stream into a closed calibration system which has a sight glass for receiving the fluid discharged by the pump. After a predetermined number of cycles by the pump, the amount of fluid within the sight glass is determined and then the calibration system is purged by a displacement gas so that the offensive material is returned back to its original source to be reused in its normal fashion.