Abstract: A method of combining a first fluid from a first source (1) with a second fluid from a second source (16) in a de-sired ratio, by measuring the flow rate and pressure of the first fluid (at 12 and 14) prior to combining (at 10), and determining from this the desired flow rate and pressure at which the second fluid should be added to achieve the desired ratio. The pressure and flow rate of the second fluid is measured (at 6 and 8) and provides a feedback loop to ensure that the second fluid is being added at the desired flow rate and pressure to achieve the desired ratio. The pressure and flow rate of the second fluid may be controlled by multiple pumps 2 of different power.