 The low pressure mixture is analyzed for oxygen content and the oxygen and helium flows adjusted accordingly.On the high pressure side of the compressor a regulator is used to reduce pressure of a sample flow and the trimix is analyzed (preferably for both helium and oxygen) so that the fine adjustment to the intake gas flows can be made.The benefit of such a system is that the helium delivery tank pressure need not be as high as that used in the partial pressure method of blending and residual gas can be 'topped up' to best mix after the dive.