Inlet pressure shutdown for inlet temperatures < 200C

On our GCMS 6890/5973, over the past year for several times, if the inlet temperature is set below 200C we dependably get the message “inlet pressure shutdown” All else is running fine. Why is this happening? We don’t see the connection. Has anyone seen a similar problem?

  • Inlet pressure and temperature should be completely independent.  Please make sure that your helium carrier gas supply pressure is between 60 and 100 psi.  Set the inlet mode to SPLIT, column flow 1.2 ml/min, Total flow 50 to 100 ml/min, gas saver off ... and try it.    Thermal expansion of the metal with heat may be closing off a leak, though. The leak test procedure is detailed in the 6890 Gas Chromatograph User Manual pages 318 and 319  

    Procedure: Leak testing an EPC split/splitless inlet
    There are numerous places in the inlet that can leak. This procedure lets you determine, in general, if there is an unacceptable leak in the inlet. If the inlet is leaking, you should use an electronic leak detector to pinpoint the component that is leaking.

    WARNING Be careful! The oven and/or inlet may be hot enough to cause burns.

    Materials needed:

    • No-hole ferrule
    • 7/16-inch wrench
    • Gloves (if the inlet is hot)
    • Septum nut wrench (part no. 19251-00100)
    • 9/16-inch wrench • 1/8-inch SWAGELOK cap
    • Bubble flow meter

    1. Complete the following preliminary steps:

    • If you have entered parameters that you do not want to lose, store them as a method.
    • Turn the oven off.
    • Cool the oven and inlet to room temperature.
    • Turn the inlet pressure off.
    • Remove the column, if one is installed, and plug the column fitting with the column nut and a no-hole ferrule.
    • Remove the old septum and replace it with a new one. For instructions, see “Changing septa”.
    • Inspect the O-ring and replace it if it is hard and brittle or cracked. See “Changing the O-ring” for instructions.

    2. Cap the septum purge fitting with a 1/8-inch SWAGELOK cap.
    3. Set the oven to its normal operating temperature.
    4. Configure the column as 0 length.
    5. Press [Front Inlet] or [Back Inlet] to open the inlet’s control table.

    • Set the inlet to its normal operating temperature.
    • Enter a pressure setpoint of 25 psi, or enter your normal operating pressure if it is greater. Make sure that the pressure at the gas supply is at least 10 psi higher than the inlet pressure.
    • Set the total flow to 60 mL/min.
    • Set the inlet to Split Mode. Wait a few moments for the pressure and flow to equilibrate. If pressure cannot be achieved, there is either a large leak or the supply pressure is to low.

    6. Turn either the pressure or the flow off. Because the septum purge and the column fittings are capped, gas should be trapped in the system and the pressure should remain fairly constant.
    7. Monitor the pressure for 10 minutes. A pressure drop of less than 0.5psig (0.05 psi/min or less) is acceptable. If the pressure drops much faster than the acceptable rate, see “Procedure: Correcting leaks” .

    Does it pass that test?

  • Thanks Paul... brilliant... after trying just about everything, thermal expansion of the metal with heat could explain it. Your suggestion might just be the answer.  Thanks for the detailed procedure. Will do ASAP and post results when done. 

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