Abstract: A dilution technique for measuring the flow rate of a fluid stream includes injecting a tracer gas into a gas stream and then introducing the resulting tracer stock gas stream at a constant flow rate into the fluid stream. The concentration of tracer stock gas in a fluid stream sample is then obtained. A sample of the tracer stock gas stream is obtained at a point before this stream is introduced into the fluid stream. The resulting tracer stock gas stream sample is injected at a flow rate which can be varied, for example, by a peristaltic pump, into the gas stream. The concentration of a sample from the resulting tracer stock gas sample-in-gas stream sample is measured. By varying the flow rate of the tracer stock gas stream sample the two measured concentrations can be equalized. When the concentrations are equal, a relationship exists between the flow rates of the fluid stream, the tracer stock gas stream and the tracer stock gas stream sample.