4 Replies Latest reply on Sep 16, 2017 2:52 AM by james_jenkins

    inlet pressure problem 7890A


      Having a problem with inlet pressure/septa purge not holding pressure or stabilizing flow.


      I have a split inlet, 2:1, pressure set at 92, typically holds at 47.  But total flow jumps around and never stabilizes. 


      Leak check Service:  Total flow continually wildly fluctuates (200-350) and will not stabilize, column indicates stable flow of 7.6


      Diagnostic: Pressure stays at 47, test pressure is 10, Inlet, Septa Purge, and Columns flows all stable, but Total fluctuates constantly fluctuates. Inlet Flow Sensor has alot of noise from 200-500 constantly.  Back inlet flow control set at 73, Fluctuates 200-400.


      I have replaced and cleaned out Everything in maintenance, septa, split liner, even changed column, purge trap. I even cleaned out the septa purge line from inlet to sensor and outlet, that line is open.

        • Re: inlet pressure problem 7890A

          The split ratio is likely too low to maintain a consistent pressure. Typical split ratios are 10:1 or higher. If the sample only needs to be diluted 3-fold, it can likely simply be diluted before being injected and you can run a splitless injection. Otherwise, use a higher concentration and split at a higher ratio in the GC.


          What can also affect this issue is whether you're running in Constant Pressure or Constant Flow mode. Which mode are you running in? Is there a temperature ramp in the method? What's the initial oven temperature?

          • Re: inlet pressure problem 7890A

            What o-rings are you using on the inlet liners?  I have seen people accidentally use the Flip Top o-rings meant for a specific 6890 inlet weldment on a 6890 or 7890 and the inlet top cannot seal correctly leading to pressure and flow issues.


            Also, have you checked the split vent traps in their holders under the cover on the top back of the GC?  The inlet flows you're mentioning are pretty high and since it's happening on the front and back inlets it seems like a problem with maybe a consumable recently installed.

            • Re: inlet pressure problem 7890A

              I agree with valentinrusu's comment regarding the split ratio; a 2:1 split is low, especially if you have a sampler like a purge and trap in line.  Has the system worked before with these conditions and did it just stop working?  If the system used to work, what (if anything) was changed before the problem started?


              What is the pressure on the gas tank that is supplying the inlets?

              • Re: inlet pressure problem 7890A

                I went ahead and flagged the “Correct Answer” on this post.  If you still need help though, please let us know and we would be happy to continue working with you.