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Can anyone help me with my application?  

Here are the components and order that the DI water goes through:  

I have incoming DI water (55 psi, 70 deg F)  going through 1/2" teflon tubing that is regulated at 30 psi. Then there is a needle valve (H2O In valve).  From the valve, it goes through the part and there are pressure transducers before the part and after the part measuring the pressure drop.  Then there is another needle valve (H2O Out valve) and it goes to a rotameter.   From the rotameter, it drains to a sink.  The process includes adjusting the "H2O In Valve" to a certain flow rate (ranges from 0.8 to 4.5 LPM) reading at the rotameter and then adjusting the "H2O Out Valve" creating a back pressure on the pressure transducer (after the part) to 150 mmHg.  My goal is to replace the rotameter with a digital gage to improve the repeatability (at least 3% accuracy).   So far I have replaced the rotameter with a McMillian digital gage (3/8" OD fitting)  and it produced a back pressure at the higher flow of 4.5 LPM (too high of a pressure at the pressure transducer after the part).  Also, does the pressure drop of the gage that I purchase apply in this situation if I am measuring the pressure differential upstream?  Can anyone recommend a digital gage that can work with these flows and pressure and not produce a back pressure?  In addition, if there is an automatic flow controller out there where I would set the flow rate and I would only have to adjust to the correct pressure, I would be open to suggestions for that too.     

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