A method for converting a pump for use in a flow stream containing a mixture of highly compressed gas, compressible liquid or supercritical fluid wherein the pump delivers a compressible fluid against a back-pressure regulator which in turn delivers a controllable flow rate downstream of the regulator without performing variable compressibility compensation adjustments on the pump. By using a pressurized source of compressible fluid combined with isocratic conditions, the delivery pressure from the pump is controlled at a rate higher than the downstream gradient with the back-pressure regulator, and an inexpensive pump may be used in place of specialized, expensive pumps and compressibility compensation systems for use in stems operating at or near supercritical fluid levels of compressible fluids. Significant capital and operating laboratory costs are saved through a simpler and cheaper system for accurately delivering compressible fluids into mobile phase flow streams.